# New cheap BMS poll



## dimitri (May 16, 2008)

Greetings fellow DIY EV enthusiasts,

I'd like to summarize some data I gathered from recent posts about LiFePo4 BMS. Although I don't agree with all ideas and my experience does not confirm some of these ideas, I still consider them potentially valid and worth exploring. Being a capitalist at the core, I am also interested in potential business opportunity.

Right from the start, let me make it clear, I am only exploring cheapest possible BMS with minimum components and functionality, no digital data, no displays, no computer hookups, simple NO/NC control logic for 2 main events – LVC and HVC, from any cell in a series connected string, no shunting / balancing of any kind. If you want something fancier, please contact other people, like Etischer or EV Components, or other BMS designers/makers.

What I hear people say is that balancing is risky and not necessary, so let's drop it from BMS. I hear that LVC and HVC on cell level is important, but only if you have means to tie it to charger and controller, so action can be taken. I hear that simple voltmeter or E-meter or controller’s gauge driver can be used for instrumentation and approximate SoC gauging, so BMS doesn’t have to do it. I hear that some people don’t want to pay premium for BMS since they can just use extra money to replace killed cells.



Let me try to list all requirements in order of importance:
Low cost, up to $10 per cell, plus $40 head end controller.
Low risk of draining the cell, no current path over 50mA
LVC and HVC levels preset to common popular levels ( TS , SE , etc ), no trimpots.
Single wire isolated interface for both LVC and HVC on same single wire.
Easy to install automotive style Quick Disconnect terminals for signal wire.
Signal circuit is Normally Closed, so any fault from any cell or component will open circuit and cause an alarm.
Simple analog circuit with minimum components, high noise resistance, no digital stuff.
Head end BMS controller will provide NC/NO pairs to control charger ( turn off on HVC ), motor controller throttle ( reduce or cut on LVC), dash light and/or buzzer for both LVC and HVC.
Head end as simple and as universal as possible for any charger/controller combo, may require additional relays and/or circuits to interface with specific charger/controller.
No shunting or balancing, simple on/off signal when first cell reports HVC or LVC.
2-3 mounting options. Small PCB on each cell with wires or rigid ring terminals or single PCB for a group of cells. Perhaps a combo PCB which can be cut in pieces or used as a whole.
Single LED to indicate a cell between LVC and HVC, look for cell with LED off to find the offending cell for troubleshooting or repair.
Initial manual pack balancing is recommended and occasional ongoing balancing may be needed in some cases of cells SoC drifting apart, which would be done manually, outside of BMS functions.
So, having said all above, please provide your feedback. Will you be interested in purchasing such simple BMS? How many cells in your pack? Do you prefer individual cell PCBs or a group PCB? If group, how many cells per group? What instrumentation would you use to gauge pack SoC? What charger / controller combo would you use with this BMS?

Any other ideas or suggestions? Please check against conflicts with existing requirements, especially first 4 requirements.

I’m not even sure if this is all possible in one product, especially low cost part, since it would require massive bulk orders of PCBs and parts to keep the price low, so I’d like to feel if there is demand for such simplistic product.

I have some ideas how to pull this off and will be making a prototype soon.

Let’s try to keep this thread brief and on subject, there are other threads for off the wall ideas and long winded discussions.

Thanks


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## DIYguy (Sep 18, 2008)

dimitri said:


> there are other threads for off the wall ideas and long winded discussions.
> 
> Thanks


teeheeheee


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## cycleguy (Oct 7, 2009)

I'm interested in a very simple BMS with LVC and HVC with the following features.

1. Individual cell monitoring up to 32 cells

2. The ability to shut off the charger once the first cell reaches 4V 
(adjustable)

3. An LVC at 2.5V (adjustable) for any cell, with a built in 30sec. delay to 
prevent LVC during voltage sag under accel.

4. A warning light output signal to indicate when a cell cell drops below 
2.8V (like a low fuel warning light)

5. Prefer a single unit over cell mounts.


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## EV-propulsion.com (Jun 1, 2009)

Yes I second that. Will need the capability to operate a relay at the high and low ends. Also, with the high voltage motors and controllers available, must be able to monitor 60+ cells.
_*Build it and they will come.*_
_M_ike
www.EV-propulsion.com


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## tomofreno (Mar 3, 2009)

Regarding (2) I think 50mA is too high, prefer 10mA, or if 50mA, something that warns of the failure. I like 3,4,5,6,7,10 and 12. I don't require controller interface, just light and buzzer - and I can supply these. Yes I would be interested in purchasing something like this, but I will also be watching other developments, so no guarantee. A simple analog circuit is very appealing though. Pack consists of 36 cells. Prefer board per cell. TBS gauge for soc. Curtis 1238-7501 controller HPGC AC50 motor. It would also be nice if it was designed to withstand a cell failing open, resulting in rest of pack voltage across the board. Should result in low power dissipation with this voltage. Well thought out Dimitri.


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## Woodsmith (Jun 5, 2008)

I don't know what I need or want really.

What I fancy the idea of is a nice digital display on the dash that tells me everythign about the pack, how it is running and how it is charging. It would be great if it showed me in a diagram the charging progress of each cell.
However, I can imagine that that would be costly and complicated and so I would just like something that I can afford that will keep my batteries alive and running at their best.

I am looking at 40 x 200ah or 50 x 160ah cells tro achieve the range I need.
I am able to assemble a circuit board from a kit of parts or from a shopping list.
I don't have a 'scope to measure anything nor the knowledge to design or modify an 'off the shelf' unit.


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## dimitri (May 16, 2008)

Hmm, 189 views and only 21 poll answers, oh well...

Thanks for feedback guys, let me try to digest some of the info.

Cycleguy, when you say adjustable voltage, do you mean trimpots on boards or ability to select static values from a common list at the time of order? Trimpots add cost and require time consuming calibration, while any given cell type like TS or SE have well known voltage levels that most people agree on. 2 trimpots will add $2-$3 to each board, those little suckers are expensive and I am trying to shave cost where possible. Some people also don't trust trimpots stability in volatile environment like a moving car, static resistors will be more stable. Please provide more details on this? Perhaps trimpots can be made optional at additional cost?

I'm not sure about LVC delay, I think it will only help you destroy more cells. In my experience, when LVC comes on, you better stop driving since you are seconds away from a depleted cell. Of course proper selection of LVC voltage is the key, considering sag. For example, my LVC for TS cells is at 2.6V and it proved to be good, it doesn't get in a way of 3C accelerations, yet it alarmed me just in time to prevent cell damage when I was half a mile away from home. Perhaps additional functions can be implemented on head end if needed, but cell boards will simply report LVC as it happens.

Your idea of warning light is essentially an extra signaling function, making it a 3 signals BMS instead of 2 signals, which conflicts with my requirements 1 and 4. I think SoC gauge should be doing the warning, not BMS. If you want SoC as part of BMS, then its outside of scope of cheap BMS.

Tom, when I said 50mA, I meant a max current in case of semiconductor failure like what you experienced with VBs. Normal operation will take about 10-15 mA. Every stage will be protected by resistor. I also plan to have a fuse on each PCB to protect from high voltage situation as you mentioned.

Thanks again for feedback, keep it coming as I am working on prototype and welcome fresh ideas.


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## ahambone (Jan 13, 2009)

You might get more interesting feedback if you polled people who already have LiFePO4 packs and ask them what kind of BMS they have. 

Cheers,
--Adam


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## petenyma (Apr 2, 2008)

I have 38 SE 100AH cells. 

I am using the Chinoz PFC charger and it does have a BMS input that will stop charging if 12v on the BMS input go's away. 

Zilla controller. 

I am certainly interested in a solution that will monitor LC and HV for each cell.

I would prefer individual boards for each battery.

Looking forward to an affordable solution!
Pete


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## dexion (Aug 22, 2009)

dimitri said:


> Hmm, 189 views and only 21 poll answers, oh well...
> 
> Thanks for feedback guys, let me try to digest some of the info.
> 
> ...


I dont use a bms on my bike or my car, BUT I am more involved in watching the batteries than im sure many want to be. I work to my weakest cell both charging and discharging and I have seen ZERO drift on my bike over the past year and about 100 cycles. My car is too new and Im still working the bugs out of the batteries yet but I plan to treat it the same as my bike. I use a small volt meter on my weakest cell. Here are some observations:

1. LVC is very important. IT is what it is, have a light turn on when your close but cut the pull from the batteries when it is reached and ultimately cut off the controller when a limit is met. 2.7 warning light 2.5 pulse controller 2.45 cut etc for ts cells.
2. HVC isnt much of a problem if you work to your weakest cell for me anyhow BUT, if you want the hv protection lets make it (for the obsessives like me) usable.
Make it adjustable per cell. SO:
cell a is the lowest capacity. It will kick in for cell a at whatever your max voltage is for the chem you are using.
cell b is the highest capacity. It will kick in at x volts (lower with some formula you could work out than the max voltage) adjustable for what you set it to. So you work out your final needed voltage to have it run out when cell a runs out.
cell c rinse and repeat. I can see some cases where this wouldnt work if the pack is really far apart. Have the charger turn off when all the lights on the bms are lit.

This however seems expensive and perhaps it makes more sense to just buy more batteries with the money you are going to use for the bms and either store them or make your pack bigger so you dont go down as far on your trips.

That way you can ballance at the top and the bottom**** NOTE I don't think this is really needed/too complicated for everyone (since it really amounts to working to your weakest cell anyhow) but if you must have a bms or dont want to bother with the cells then this setup may allow you to set it and forget it (the set it will take some time the first time but from then you may be able to forget it for years.) 

About the only benefit (aside from the warm fuzzy feeling you get from a straight line at the top and the bottom) would be a possible longer life of the cells in the middle/top of the capacity range. These would be charged less than 100% so they may last longer than the ones that are charged 100%. But then again they wont be anyhow if you just work to the weakest cell** Note in my cases both of my weakest cells fill up and empty first on the bike and the car so I may have an easier time than others with this.


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## cycleguy (Oct 7, 2009)

Dimitri, adjustable LVC would be nice but not necessary. 
I'm not nearly as interested in pack SOC as I am in individual cell SOC. I want to know when the lowest cell is nearing 100% DOD. A pack is only as good as it's weakest cell unless you don't mind killing a few now and then.

The LVC delay is critical for me, under heavy acceleration I can reach up to 10C discharge with voltage sag down to 2V even on a full charge. The voltage recovers quickly as soon as the load is reduced, the last thing I want is a cut out during heavy acceleration. So, two stages would work for me, the first stage would be a warning light indicating the weakest cell is below 2.8V static and a hard cut-off at 2.5V static. 
I guess if this adds too much cost then a simple warning light for a cell at 2.5V or less with no cut off and leave it to the operator to have the discipline to shut things down, which would not be a good idea.


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## Jan (Oct 5, 2009)

If money wasn't an issue, I would prefer a networ interface. Just like I would like to have on the controller and charger. With a simple XML interface: A HTTP post with a request would return a nicely readible XML message with al the actual status information of the batteries. And the controller, motor and charger... So I could write my own software to present this fancey on the dashboard display. Wich is actually just the flat screen of the central computer...

But money is an issue, so I would settle with the most simple BMS.


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## Harold in CR (Sep 8, 2008)

It could be that a lot of guys are like me. Don't KNOW what they need, so, wait until someone produces a quality-reliable system, then buys it. 

I have NO idea about electronics, but, can fab about anything, with little breakdown, later.

I still think voltblochers are a good product. Just had some bad rap with assembly- faulty parts ???


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## MN Driver (Sep 29, 2009)

I guess one thing that I rarely see but I think would be helpful, especially if pulling around 3C for a long time or more than 3C intermittently would be to have some sort of cell temperature buzzer.

Going too high or low in voltage(the voltage levels of detectable damage is debatable) are the most obvious ways that cells can be damaged but an aged cell that can't produce the power the way it should without stress or current flow in excess of manufacturer specs will produce more heat than a cell within limits that is healthy. There were tests on A123 cells by FMA that showed how heat kills cycle life and they put the limit at 140 degrees F, which is 60 degrees celcius for an internal cell temperature limit before the cycle life is drastically reduced. Thunder Sky cells are rated at 55 degrees celcius but I don't know how insulative the cases are on these cells and how long it would take for us to see the temperature or specs on how long they take to get hot or if the cells develop any hot spots, especially toward the center versus the outside. I'd like to have some sort of alarm go off at about 45 degrees celcius(113 degrees F) to let me know to back off and that high internal cell temps might be occuring. Cyclemark's cell that smoked and the ones that developed high pressure bulging makes me wonder if having some sort of alarm for cell temps could have saved that cell or possible some of the others, maybe that isn't the situation but I'm not sure what mechanisms are present other than cell temperature and lower voltage that comes with sagging that affect high C draw pack degradation but I know those two are likely culprits.

Per a quoted Thunder Sky email in the Thunder Sky experiences thread, it said that 3C can be used continuously as long as temp remains in spec, this confirms my thoughts on this along with the FMA reference on A123 cells, which makes it something I'd want to monitor.

Does it apply to everyone? I don't think so. Does it need to be implemented in this BMS system? Probably not, as it could easily be something seperate. Either way I think it is an important consideration.


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## JRP3 (Mar 7, 2008)

Can you guess what I voted   However, a simple, cheap, HLVC warning system would be interesting. Don't give into feature creep until you make these and people start using them, they may see they don't need all the extras. Don't do adjustable H and L, set it up safely for the cell chemistry to keep it simple and cheap, and so that people can't dial in a value that takes them too close to failure. Charger shut off at first HV, and warning at first LV. Done. Remote mounting optional.


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## jorhyne (Aug 20, 2008)

JRP3 said:


> Can you guess what I voted   However, a simple, cheap, HLVC warning system would be interesting. Don't give into feature creep until you make these and people start using them, they may see they don't need all the extras. Don't do adjustable H and L, set it up safely for the cell chemistry to keep it simple and cheap, and so that people can't dial in a value that takes them too close to failure. Charger shut off at first HV, and warning at first LV. Done. Remote mounting optional.


Could not have said it better myself.


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## tomofreno (Mar 3, 2009)

> I also plan to have a fuse on each PCB to protect from high voltage situation as you mentioned.


 I'll ask the obvious just to be sure, you plan some way to warn a fuse has been blown correct? If you have such a fuse, maybe adding shunt capability with a larger resistor to limit current and power dissipation to a safer level, say 300mA, would be useful. Cedric and others claim this is enough to keep cells in balance. Adds more cost of course.


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## Dalardan (Jul 4, 2008)

Nice poll.

One idea could be just to add 1 individual cell SoC meter on the weakest battery. The customer would have to identify his weakest cell first then place the meter on it. It couls even be independant from the LVC/HVC BMS. This would keep costs down and add the equivalent of a "fuel gauge". Thinks it's worth it?

Dalardan


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## dimitri (May 16, 2008)

tomofreno said:


> I'll ask the obvious just to be sure, you plan some way to warn a fuse has been blown correct? If you have such a fuse, maybe adding shunt capability with a larger resistor to limit current and power dissipation to a safer level, say 300mA, would be useful. Cedric and others claim this is enough to keep cells in balance. Adds more cost of course.


Tom,

the problem with lower shunting level is that chargers can't lower their CV current enough to be less than shunting capacity, so you end up overrunning shunts and create more problems that you solve. Having a fuse doesn't really help since fuse would have to be higher value than shunting and if shunt fails open then it will bleed the cell happily thru the fuse. 

I suspect most people who blame shunts just don't have them properly matched up with their charger, VBs were designed to shunt as much as practically possible on a small board.

I agree that shunts add a certain level of risk to BMS, so you have to pick lesser evil , but you have to look holistically at the entire setup, it doesn't help if you cut corners on one side and then blame the other side for it.

Since we don't have ability to throttle back commonly used chargers, I think its lesser evil to simply shut down the charger when first cell hits HVC and rely on manual initial balancing and perhaps occasional manual balancing maintenance if needed.


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## tomofreno (Mar 3, 2009)

> Since we don't have ability to throttle back commonly used chargers, I think its lesser evil to simply shut down the charger when first cell hits HVC and rely on manual initial balancing and perhaps occasional manual balancing maintenance if needed.


 Yeah, that seems to be the case.


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## dimitri (May 16, 2008)

tomofreno said:


> I'll ask the obvious just to be sure, you plan some way to warn a fuse has been blown correct?


My idea of a single wire Normally Closed signaling loop is such that ANY failure ( blown fuse, loose wire, dead cell, LVC or HVC on any cell , blown component ) would result in open circuit and produce the alarm event. Head end control board will decide what to do with this alarm event, sound a siren, drop a charger, drop or reduce throttle, eject the driver from the seat, up to EV designer's imagination


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## tomofreno (Mar 3, 2009)

> My idea of a single wire Normally Closed signaling loop is such that ANY failure ( blown fuse, loose wire, dead cell, LVC or HVC on any cell , blown component ) would result in open circuit and produce the alarm event.


 Would be great!


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## tomofreno (Mar 3, 2009)

> One idea could be just to add 1 individual cell SoC meter on the weakest battery.


I think I already have this capability. The charger supplies current through the shunt so I can read DC current into the cells during charging on my TBS gauge. Then I can record time and know how much Ah charge I put into the cells. After I have gone through a couple charge/discharge cycles monitoring cell voltages, I should be able to identify the lowest capacity cell as the one with the largest voltage change during charging, and know it's capacity from the Ah data. Then I just enter this Ah capacity on the TBS gauge for the pack capacity, so it reads soc based on the capacity of this cell (the TBS monitors discharge current through the shunt and integrates over time to track Ah of charge discharged). I would then not be in any danger of over-discharging this cell, nor any others if the cells remain fairly well balanced, if I stop driving by 20% soc. This should be quite safe if I monitor cell voltages at regular periods to see if capacities/and or balance have changed. If another cell should become the lowest capacity, I just enter its capacity in the TBS. If they become too imbalanced, diminishing pack capacity, I re-balance them by charging lower voltage cells with a ps. I install Dimitri's HLVC system as a safety backup to ensure I don't over charge/discharge if balancing changes more than I expected between my monitoring periods.

Tom


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## etischer (Jun 16, 2008)

dimitri said:


> Tom,
> 
> the problem with lower shunting level is that chargers can't lower their CV current enough to be less than shunting capacity, so you end up overrunning shunts and create more problems that you solve.


High Voltage Cutoff is what protects the cell. 

Whether you shunt balance or not, the HVC protection is there to shut off the charger. 

Whether your shunts are over run by charge current or not, the HVC is still protecting each cell.


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## dimitri (May 16, 2008)

etischer said:


> High Voltage Cutoff is what protects the cell.
> 
> Whether you shunt balance or not, the HVC protection is there to shut off the charger.
> 
> Whether your shunts are over run by charge current or not, the HVC is still protecting each cell.


HVC is just a signal, it doesn't help if your charger is dumb enough not to be able to react on it.

HVC from first cell can be interpreted in 2 ways, either throttle the current to less than shunting and let the rest of the pack safely catch up, or shut down the charger and forget the balancing. I don't see how you can have both with a dumb charger. 

How many commercial chargers do you know which are capable of throttle down on HVC signal? How much do those cost?


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## dimitri (May 16, 2008)

Someone mentioned BMS from EV Power in Australia. After reading their Web site I have to admit that their product is almost exactly what I had in mind, although its more expensive and some data posted on their pages seems to be contradicting, but perhaps its just due to various versions and ongoing development. It would be nice if someone with real experience with these modules came in here and gave us an independent review. Anyone?

They state support for SE cells, but operating range of 2.5V - 4.1V does not match SE range, which would cause problems. This means separate product selection for SE should be offered, but its nowhere to be found.

I like physical design of the unit, but it appears to be a single PCB and I am not sure that pressure of fully torqued terminal bolt will not crack and destroy fiberglass layers of PCB. Its possible they use special PCB designed to withstand pressure, don't know. 

They claim simple NC signaling loop, and in another page they refer to some proprietary heartbeat protocol, so which is is? 

They have some shunting ability to slow down first cells to reach 3.65V and let others catch up, although with 0.6 Amp shunting it will only take a minute for first cell to signal HVC and shut off the charger, so I am not sure how useful their shunting is, but it seems to be designed with safety in mind even if its not so useful. This could work well if your charger only pushes 1 Amp in CV mode, but I am not aware of such chargers for large cells.

Again, on paper and pics looks like real nice product although not so cheap. They are looking for US and European distributors, so I wonder why none of many US EV shops jumped on this yet? Maybe someone did? Anyone?

Even though I have plenty of design ideas, I doubt I can compete with this nice product without cutting corners, so if someone is waiting for me to start selling BMS, I have not decided if I want to step into this market.

BTW, here are useful links, enjoy and please come back with comments.

http://www.ev-power.com.au/-Thundersky-Battery-Balancing-System-.html

http://www.evworks.com.au/index.php?product=BMS-CM160-V6

http://www.evworks.com.au/store/datasheets/EVPower_CM_datasheet.pdf


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## dimitri (May 16, 2008)

etischer said:


> High Voltage Cutoff is what protects the cell.
> 
> Whether you shunt balance or not, the HVC protection is there to shut off the charger.
> 
> Whether your shunts are over run by charge current or not, the HVC is still protecting each cell.


Eric, I think I misunderstood your statement at first, sorry about that. You consider shunting point and HVC as 2 different voltages, just like abovementioned EV Power system, right? So, shunting starts first, then after some time ( depending on how much current is pushed by the charger and how much this current is over shunting ability of the modules ) HVC hits and charger is forced to turn off. In my mind I always had shunting and HVC to be the same voltage point, which relies on charger to shut off on its own when max pack voltage is reached while modules must shunt ALL current to prevent overcharge, this is how VB was designed. Just 2 different ways to do it. One is designed to make sure most cells have a chance to reach HVC, but requires proper match with the charger ( this is what my EV does, BTW , and it seems to work well for me ), the other is focused on safety shutoff, even if that means some cells never get to catch up all the way. After all recent discussions I definitely see merits to this approach, although it would require additional voltage sensing components to distinguish shunting voltage from HVC voltage, which adds cost.


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## Dolphyn (Nov 17, 2009)

The NC signalling loop idea has got me thinking, so I'll toss out one more idea for battery management and monitoring.

It seems vitally important to monitor the "low-cell voltage". That is, I always want to know the condition of the lowest cell. Instead of (or in addition to) a "pack state-of-charge", I want an indication of the weakest cell's state of charge. During actual driving, that could be more important than knowing _which_ cell is weak!

One approach would be to have multiple NC signalling loops, say, one each for 2.5 volts, 2.7 volts, 2.9 volts, 3.1 volts, and 3.3 volts. Each could be wired to an LED on the dashboard of the vehicle, so that in this example "three LED's on" means that every cell is at least 2.9 volts. This would provide instant visual feedback to the driver if any cell (or the whole pack) is getting weak.

Another approach would be to have a BMS output that simply matches the lowest-cell voltage, and wire that output to a voltmeter on the dashboard.

Alternatively, one could do something like the "low cell voltage output" described here:


liionbms.com said:


> Low Cell Voltage Output: Analog voltage that remains at 5 V as long as the battery is able to provide current, and decreases down to 0 V as the lowest charged cell's voltage is reduced from 2.9 V down to 2.1 V.


I think their idea is that it would be wired to the throttle somehow, to reduce the power available to the driver, but it would be equally useful to provide a visual queue. (Note, I've asked evcomponents.com and eeveemotors.com about that BMS, and it doesn't actually seem to be available.)

Anyway, regarding EV Power ...


dimitri said:


> They claim simple NC signaling loop, and in another page they refer to some proprietary heartbeat protocol, so which is is?


I think both are correct! If you buy the Master Unit, the heartbeat protocol is used, but otherwise the loop is used as a simple NC signalling loop.


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## dimitri (May 16, 2008)

Dolphyn said:


> One approach would be to have multiple NC signalling loops, say, one each for 2.5 volts, 2.7 volts, 2.9 volts, 3.1 volts, and 3.3 volts. Each could be wired to an LED on the dashboard of the vehicle, so that in this example "three LED's on" means that every cell is at least 2.9 volts. This would provide instant visual feedback to the driver if any cell (or the whole pack) is getting weak.


This will require additional wire for each signal, which creates spaghetty wiring mess, conflicts with requirement of easy install. It also adds voltage sensors for each level, which adds cost.




> Anyway, regarding EV Power ...
> 
> I think both are correct! If you buy the Master Unit, the heartbeat protocol is used, but otherwise the loop is used as a simple NC signalling loop.


You can't use one wire for both without major complexity of design and I believe their modules are just as simple as what I listed in my original list. One could say that I came up with my list after reading EV Power specs  , but its not true, I came up with the list by listening to feedback from many forum members here and my own LFP experiences.


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## Dolphyn (Nov 17, 2009)

I'll admit after reading the site again, I'm not entirely sure what purpose the heartbeat serves. Maybe I'm missing something, but it doesn't seem like the control unit does anything that would require any sophisticated feedback from the modules.


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## karlos (Jun 30, 2008)

"Another approach would be to have a BMS output that simply matches the lowest-cell voltage, and wire that output to a voltmeter on the dashboard."

I like this idea Dolphyn, it would always give you a worst case scenario and remove risk of hidden battery damage.

My contribution to this discussion goes like this. Maybe a resolution to 'creep' in battery imbalance and need for correction is to have each cell charged by it's own charger so each time they are charged, any imbalance is corrected? They could be cheap mass produced units that are assembled on a uni-rail and located close to each battery bank location.
Does this idea have any merit?

Thanks to all contributors to this discussion, it has been extremely interesting and informative!


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## dimitri (May 16, 2008)

karlos said:


> "Another approach would be to have a BMS output that simply matches the lowest-cell voltage, and wire that output to a voltmeter on the dashboard."
> 
> I like this idea Dolphyn, it would always give you a worst case scenario and remove risk of hidden battery damage.
> 
> ...


Single cell chargers are usually small and not capable of serious currents. Also, with 30-60 cells in a typical pack, it becomes expensive quickly. Chargers would have to be isolated and capable of 10-20-30 amps, which is a pretty tall order for a small and high quality charger, so I doubt it's cost effective approach. Wiring is also messy with many chargers.

Also, it doesn't address the LVC issue, which is more important...


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## karlos (Jun 30, 2008)

Thanks Dimitri for the reply. I understand your point about the amps required for individual chargers.
All the best in your efforts to create a practical solution. I wonder what they have come up with for the Leaf?


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## etischer (Jun 16, 2008)

The charge current should begin to taper once the charger switches from Constant Current to Constant Voltage. Balancing should occur during the CV stage when the current has been reduced. If at any time the voltage is above HVC the charger should shut off. 

I was thinking of using a relay to switch between two pots, one for full current, one for balancing current. It can also be done by pausing and resuming charge current, I believe this is how the Rudman regulators interface with the Manzanita charger. 



dimitri said:


> HVC is just a signal, it doesn't help if your charger is dumb enough not to be able to react on it.
> 
> HVC from first cell can be interpreted in 2 ways, either throttle the current to less than shunting and let the rest of the pack safely catch up, or shut down the charger and forget the balancing. I don't see how you can have both with a dumb charger.
> 
> How many commercial chargers do you know which are capable of throttle down on HVC signal? How much do those cost?


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## etischer (Jun 16, 2008)

Yep, you got it =)

Each regulator (1 regulator per cell) has a configurable shunt trigger voltage where it will start to dump excess current. STV can be set via serial communications. 

The voltage for each cell is also reported back to the hub over serial communication for triggering HVC and LVC. 

Whether you decide to enable shunt balancing or not, the HVC will still stop the charger if any cell is above the HVC. We will also stop the charger if any shunt is too hot. Heatsink and cell temperatures will also be reported back to hub. 





dimitri said:


> Eric, I think I misunderstood your statement at first, sorry about that. You consider shunting point and HVC as 2 different voltages, just like abovementioned EV Power system, right? So, shunting starts first, then after some time ( depending on how much current is pushed by the charger and how much this current is over shunting ability of the modules ) HVC hits and charger is forced to turn off. In my mind I always had shunting and HVC to be the same voltage point, which relies on charger to shut off on its own when max pack voltage is reached while modules must shunt ALL current to prevent overcharge, this is how VB was designed. Just 2 different ways to do it. One is designed to make sure most cells have a chance to reach HVC, but requires proper match with the charger ( this is what my EV does, BTW , and it seems to work well for me ), the other is focused on safety shutoff, even if that means some cells never get to catch up all the way. After all recent discussions I definitely see merits to this approach, although it would require additional voltage sensing components to distinguish shunting voltage from HVC voltage, which adds cost.


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## JRP3 (Mar 7, 2008)

karlos said:


> My contribution to this discussion goes like this. Maybe a resolution to 'creep' in battery imbalance and need for correction is to have each cell charged by it's own charger so each time they are charged, any imbalance is corrected? They could be cheap mass produced units that are assembled on a uni-rail and located close to each battery bank location.
> Does this idea have any merit?


The only way I can see this being practical is to use individual low amp chargers for finishing charge in conjunction with a high amp pack charger. The main charger charges quickly until the first cell hits HVC, then shuts off and the individual charger finish up. You'd still need HV and LV signaling of course, though if you set the bulk charger low enough you could get away without HV signaling.


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## tomofreno (Mar 3, 2009)

> They state support for SE cells, but operating range of 2.5V - 4.1V does not match SE range


 That is why I didn't buy it. I exchanged some emails with Rod on it, but there seemed to be a disconnect, with him insisting all the LiFePO4 cells were the same as far as voltage. I bought the TBS gauge from him but decided not to go with the bms due to the 4.1V. Their website says 800mA shunt, not 600, not that it makes a lot of difference.

Tom


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## etischer (Jun 16, 2008)

I can't imagine it will shunt 800mA for long. It doesn't appear to have a heat sink or any thermal mass. Could it using the battery lugs to transfer heat into the battery? I also wonder how much noise filtering is going on, its physically a small PCB. I like how it looks though, so simple and easy to understand, they really did a nice packaging job. 



tomofreno said:


> That is why I didn't buy it. I exchanged some emails with Rod on it, but there seemed to be a disconnect, with him insisting all the LiFePO4 cells were the same as far as voltage. I bought the TBS gauge from him but decided not to go with the bms due to the 4.1V. Their website says 800mA shunt, not 600, not that it makes a lot of difference.
> 
> Tom


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## madderscience (Jun 28, 2008)

I like much of what dimitri proposes: simple and as foolproof as possible.

LVC/HVC is the easiest to do. The one thing I would add to the list of basic features is a sealed/potted design on the on-cell units. open circuit boards or even boards with some kind of silicon sealant are eventually going to have issues in an automotive environment. In my own homebuilt monitoring system, I am just using PVC pipe couplings and end caps to make nice compact little housings. THey could be filled with epoxy to make them basically indestructible. Just a thought. My voltage monitors draw less than 2mA when idle and maybe 10MA when transmitting which is less than 1% of the time, so low power consumption is easy enough to achieve.

I don't think inter-cell balancing is necessary, but I do think that basic shunting is a good idea.

Shunting can be extremely simple: (resistor + zener diode + fuse) so long as charging current is kept low. Per cell voltage monitoring (which is going to require some kind of a microcontroller unless you have a pile of analog gauges) can be kept entirely independent so that a digital system failure does not impact shunting. While charging, as long as all batteries are within safe charging voltage, the charger can shovel in the current based on whole pack voltage. as soon as one HVC signal is sent or if voltage is not being reported from one or more senders, the charger can fall back to the maximum safe charging current that can be shunted by the zener diode and resistor.

If you wanted HVC feedback (for charging) without a microcontroller on each cell, then just put a low-temp -> high resistance thermocouple on each resistor and put them all in parallel with a 2 wire circuit. The first one to get hot will cause the resistance of the whole shebang to go down, and that could be detected at the head end without any microcontrollers on the cells. Only problem here is detecting a bad / missing thermocouple might be tricky if there were too many in a circuit.

Something alone these lines is probably what I would build.


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## Bentzon (Sep 5, 2009)

I pretty temped by the single cell DC-DC chargers. Those little 20 or 40A 3.3 or 5V(can be trimmed down 50% if i remember correctly) vicor DC-DC converters looks perfect for the job. 
Couldn't this same charger be turned on once the cell reach a low voltage point by a simple LVC board to keep the cell balanced with power from the pack. 
A absolute lowest cell voltage to active regen would be needed also in case a dc-dc fails or cant transfer enough current to balance the cell. 

Might cost a little but the bulk charger can be some cheap dump charger and turned off once all the dc-dc trigger the high voltage in some way or just turned off after a certain amount of time.

Theres a guy over at endless who uses single cell vicor dc-dc to charge his headway powered vw buss.


http://endless-sphere.com/forums/viewtopic.php?f=7&t=8012&start=135


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## dimitri (May 16, 2008)

Bentzon said:


> I pretty temped by the single cell DC-DC chargers.


This seems completely impractical to me, unless I miss something. Assuming you only have one cell DC-DC and you have it on pre-determined "weakest" cell, then you might as well replace this cell and save the cost of DC-DC and wiring complexity.

IMHO, the "weakest" cell in a healthy pack is only a few AH different from all others. This title is used just to signal pack LVC and HVC, not to confuse with "poor" or "bad" cell. In this sense, there is no point in boosting "weakest" cell, but if its truly "poor" , i.e. significanlty worse than others, then just replace it.


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## Bentzon (Sep 5, 2009)

Place a dc-dc right over each cell, together with a voltage sensing board, run two wires from the bulk charger through the whole pack together with optional wires for hv/lv signal for charger and controller.

Doesn't seem that complex to me but I might be wrong once you get to wire it up


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## dimitri (May 16, 2008)

Bentzon said:


> Place the dc-dc right over each cell, together with a voltage sensing board, run two wires from the bulk charger through the whole pack together with optional wires for hv/lv signal for charger and controller.
> 
> Doesn't seem that complex to me but I might be wrong once you get to wire it up


How much does each DC-DC cost? How much will entire setup cost for a typical pack of 45 cells for example?

That's a whole lot of wiring BTW, have you assembled 30-45-60 cell pack yet? Its not as trivial as it sounds


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## Bentzon (Sep 5, 2009)

Never done car sized packs but done a 3p24s 72V30Ah motorcycle pack. I don't mind connecting and building it all as long as its not soldering 

Found the vicor dc-dc on ebay for 20-60$/piece new, not sure how much they cost from a reseller or the manufacture.

20-40$ would be acceptable for me for a 40 cell pack might be unrealistic for a 20A dc-dc.


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## dimitri (May 16, 2008)

Interesting picture is being painted by the poll results, it seems that about half of people want shunting/balancing in their BMS, I am not surprised at all. Perhaps if shunting is optional in the same product it would serve most customers, or perhaps if product can prove that shunting is implemented in a foolproof way, then more people can be convinced of its benefit, or at least lack of harm.

Again, I think EV Power product is practically perfect if only they could support growing popularity of SE cells and can be offered at a little lower price by US based resellers ( are any resellers reading this thread? ) they could really be the answer.

Does anyone have any data on how much pressure typical PCBs can handle before they crush? I saw many home made and mass produced PCBs which go under terminal bolts, but I'm not sure they are designed for this much pressure. I guess the size of the washer is the key...

Another issue with PCB design is trying to come up with PCB which can support 12-16 cells centralized, yet can be cut in pieces for those who want per cell PCBs, yet have large terminal holes,yet fit on several cell sizes. You can't have too much waste either since PCBs are priced by surface area. I guess you can't have your cake and eat it too 

Maybe PCB can have one terminal hole and the other connection by short wire similar to Volt Blocher? Or maybe just order bunch of stamped links of various sizes similar to what I have done on my modules and use small bolts to connect to PCB? Hmm... so many options...


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## karlos (Jun 30, 2008)

Dimitri it has been suggested in the past that a solution to the concern of crushing the PCB is to use a stud to first clamp the battery connections and then another nut on top to clamp the PCB of the BMS which seems to me a good solution.


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## Tesseract (Sep 27, 2008)

dimitri said:


> ...
> Does anyone have any data on how much pressure typical PCBs can handle before they crush? I saw many home made and mass produced PCBs which go under terminal bolts, but I'm not sure they are designed for this much pressure. I guess the size of the washer is the key......


Well, I'd say for the size bolts required for the TS 160 cells (and certainly the 200's) as long as you used the standard size flat washer you won't crush the board before you damage the aluminum threads (the copper threads will take quite a bit more torque before failing).

This assumes good quality FR4 that is at least 0.062" thick. Get slightly thicker for even better toughness.


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## Tesseract (Sep 27, 2008)

Bentzon said:


> ....
> Found the vicor dc-dc on ebay for 20-60$/piece new, not sure how much they cost from a reseller or the manufacture.
> 
> 20-40$ would be acceptable for me for a 40 cell pack might be unrealistic for a 20A dc-dc.


Sure... no problem. Check out this 3.3V/30A power supply made by the mostly reputable Chinese manufacturer, MeanWell:

http://www.jameco.com/webapp/wcs/st...toreId=10001&catalogId=10001&productId=693329

Only $39, which is within your budget if you buy 25 or more at a time, and you can probably modify it to output 3.6V or even 3.8V with a modest reduction in current.

Oops... take a look at the size! 8" x 4" - it's almost as big as the batteries it's meant to charge! 

Well, it's companion with half the current rating:

http://www.jameco.com/webapp/wcs/st...toreId=10001&catalogId=10001&productId=693257

is only $26 and is approximately 5" x 4". If you have to charge 40 cells, like Dimitri, then you'd be okay with the 30A model as long as you powered it from a dryer (30A) or range (50A) circuit.

Dunno about the size, though... that's a lot of extra weight and volume to add to an EV.


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## Bentzon (Sep 5, 2009)

Yeah I got a few of the 3.3V and 13.5V mean wells and that could also be a option, even a cheap one. They got a simple CC mode meant for over current protection but it can be trimmed to a output the power supply can handle for longer time.
No bulk charger needed either.

Got one of the 3.3V 10A right here and it says 100-240V input AC. I wonder if it would take DC input so It could be used to balance the cell, directly from the pack, when it hit a low voltage.

Missed it in the data sheet they do take DC input.

Just bought some mean well 13.5V 25A supplies for 35$ including shipping from china! Doesn't get any cheaper than that for a simple and power full 12v charger.


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## Dave Koller (Nov 15, 2008)

Bentzon said:


> Missed it in the data sheet they do take DC input.


\
Specification Value Number of Outputs1 
Minimum Isolation Voltage3000V 
*Input Frequency**47Hz to 63Hz* Agency ApprovalsEN|TUV|UL Minimum Operating Temperature-25°C Maximum Operating Temperature70°C Output Connector TypeScrew Terminal Block MFR Product FamilyRS-150 Product Length199mm Product Width98mm Specification Value Product Type*AC to DC Power Supply* TypeSwitching StyleEnclosed Input Voltage88V to 132V|176V to 264V Output Voltage3.3V Maximum Current Rating3A(Typ) Maximum Output Current30A Maximum Output Power99W



Well that did not copy well!!
errr you get the idea


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## dimitri (May 16, 2008)

Guys, I don't mean to sound like a moderator, but since I started this thread I will wear that hat for a moment. Can DC-DC discussion be moved to its own thread since it has very little to do with BMS, especially simple and cheap BMS 

No offense, just trying to keep this on the subject.


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## Dave Koller (Nov 15, 2008)

dimitri said:


> Guys, I don't mean to sound like a moderator, but since I started this thread I will wear that hat for a moment. Can DC-DC discussion be moved to its own thread since it has very little to do with BMS, especially simple and cheap BMS
> 
> No offense, just trying to keep this on the subject.


Yep - sorry!  You are so right...

http://www.aeva.asn.au/forums/forum_posts.asp?TID=900

cheap and simple....


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## dimitri (May 16, 2008)

After spending some time reading 17 pages long, over a year worth of posts on Australian EV forum about their ongoing BMS development, I came to following conclusions:

- Aussies are far ahead of the rest of us in using LFP cells and gaining experience 
- Same people post on most EV forums 
- Most technical people think alike and come to similar conclusions 

Seriously:

- simple analog BMS can be done at about $5 per board in parts and PCB cost, which means anything less than $10 for commercial product is just not realistic, considering assembly labor and other time consuming parts of production process

- analog BMS can be very simple and reliable, but suffers from important and often overlooked shortcoming, its LVC value is set in stone, but LFP cells voltage drop seem to vary significantly based on ambient temps and of course largely depends on discharge current. This means LVC should be temp compensated and depending on the car size and pack size people may need to choose different LVC levels. Of course latter can be accomodated by a trimpot ( adds cost and reduces reliability ), but former pretty much needs software handling, ability to adapt and dial LVC levels based on dynamic conditions.

- LVC ideally needs 2 alert levels, soft one (audible and/or visual alarm) when you get close to 20% SoC and hard one (reduce or cut throttle) when you get to say 5% SoC. Ideally it also needs to be dependent on discharge current. This is tough to do in analog BMS, it adds parts and wires.

- to get all these functions you need digital BMS, like Etischer is developing. Seems that some folks on Aussie forum have made good progress on such BMS as well, although its still in development.

- it appears that if managed and designed correctly, digital BMS can be pretty low cost in parts as well, but software development and labor can be higher, at least at first and could be reduced with mass production.

- digital BMS can be expanded via software and head end controller to integrate with other EV parts like controller, charger and most importantly instrumentation such as SoC gauges much more effective than analog one

So, if you had a choice between $10 per cell analog BMS and say $20 per cell digital BMS, which way would you go?


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## Dave Koller (Nov 15, 2008)

dimitri said:


> - digital BMS can be expanded via software and head end controller to integrate with other EV parts like controller, charger and most importantly instrumentation such as SoC gauges much more effective than analog one
> 
> So, if you had a choice between $10 per cell analog BMS and say $20 per cell digital BMS, which way would you go?


I like the idea of digital (Atmel , Arduino type processor) as I am using one for everything else - the idea of tracking changes and acting on it can be implemented with polling - price is cheap and dropping in new hex files quick and easy - They run fast enough to poll rpms, precharge, VSS and many other things - so why not new methods of BMS.. they are getting cheaper with more onboard power.. but as you say Pcbs are the bigger expense - but as far as custom for each scenario (and things going bad over time that need "tweeking algorythms") I lean strongly toward digital ..


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## tomofreno (Mar 3, 2009)

> So, if you had a choice between $10 per cell analog BMS and say $20 per cell digital BMS, which way would you go?


 Dave makes some good points. A digital bms could be very flexible and powerful. I think it might take a couple years to develop before it was ready for commercial release though, with all the added flexibility and capability, planning for different vehicles/packs, making installation and parameter setup idiot proof, alpha and beta testing in different vehicles/packs to debug...

A simple analog board like you described would be a lot better than flying blind, or bareback, and maybe keep me from destroying my cells until a reliable digital system is ready. I had said I preferred individual boards on cells, but it would be nice to have a central display where I could check status of LEDs easily. More wires though. I have cells in three boxes throughout the car so I would definitely like to cut down on wires - and connections to improve reliability. To me the LVC issues you mentioned are "I would like to have", not "I must have." If it warns me a little prematurely due to more sag during acceleration, that's ok. I'll learn how premature it is with time. I would rather have that than warn me when the cell has 5Ah left. It only took me about 1 1/2 hr to charge my 180Ah cell from 1.8 to 3V at 5A. The curve is steep there!

Tom


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## Dave Koller (Nov 15, 2008)

*Description:* Atmel's itty-bitty ATtiny85 8-Bit Processor. 8K of program space, 6 I/O lines, and 4-channel 10 bit ADC. Runs up to 20MHz with external crystal. Package can be programmed in circuit. price $2.84 for 1..

*Results*: Can run on internal oscillator, read each battery, (4 ADC's) act on those readings, and send results down a simple twisted pair serial buss to a central ATtiny to alert you via voice, lcd, leds or whatever you please.. Each would be under $10 in parts but as Dimitri says - it is the cost of PC boards that get you...


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## JRP3 (Mar 7, 2008)

Between the two choices I'd go $10 analog.


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## dimitri (May 16, 2008)

JRP3 said:


> Between the two choices I'd go $10 analog.


JRP, I think we all knew that 

Flipside of digital other than higher cost is that its likely to be less reliable, since its much more susceptible to EV's high noise environment. Also, digital makes more sense in centralized or hub design, rather than per cell modules. I know first hand how difficult it is to make Paktrakr work in EV, I could never trust it to manage my pack, but digital BMS is essentially same as Paktrakr, although proper design could make it more reliable.

Here is an idea, what if we have hybrid BMS? Simple analog cell modules with simple NC signaling loop, which is interpreted by digital head end controller along with current data ( hall effect sensor ) and temp data ( temp sensor ).

In case of LVC, head end controller will interpret LVC signal against discharge current and decide whether to trip soft or hard alert, for example LVC event at 3C-5C can be a soft alert, but same LVC event at 1C will be a hard alert. Both situations can be compensated against temp reading. All parameters can adjusted by user depending on pack size, temp limits, etc.

Its much easier to protect head end unit from EM noise and make it super reliable, while keeping cell units simple, cheap and reliable.

Any thoughts?


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## dimitri (May 16, 2008)

Expanding on hybrid BMS idea, HVC is simpler, yet must be super reliable since charging usually occurs at night, unattended. I would keep HVC head end as a simple relay tripping on first HVC and shutting off the charger, totally bypassing digital parts. LVC happens while driver is in control of the throttle, so it can be digital and fancier with 2 levels of alerts, etc.


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## JRP3 (Mar 7, 2008)

That sounds reasonable. Now how about taking a closer look at the split pack concept of monitoring? With split pack monitoring as soon as your smallest cell starts to drop you're going to see an imbalance between the halves and action can be taken. Do we really need to know which cell is low, or simply that a cell *is* low? Same thing on the charging side. As soon as one half of the pack starts to climb faster than the other half you know the smallest cell is full and you stop charging. You'd need some sort of comparative trigger instead of a fixed voltage number so it might be harder to implement. Of course if you had a cell in each half of the pack behaving in the exact same manner you wouldn't see a difference so though the odds are probably small it's not quite as safe as cell level monitoring. Ideally you'd split your pack with one half having the smallest cells to exacerbate the differences and in that case half pack voltage should be enough to use for a value.


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## etischer (Jun 16, 2008)

If you are trying to detect 0.05V rise in one cell which is part of a group of 20 cells, you are now trying to detect 0.0025V change in voltage which will be entirely masked by noise. 



JRP3 said:


> That sounds reasonable. Now how about taking a closer look at the split pack concept of monitoring? With split pack monitoring as soon as your smallest cell starts to drop you're going to see an imbalance between the halves and action can be taken. Do we really need to know which cell is low, or simply that a cell *is* low? Same thing on the charging side. As soon as one half of the pack starts to climb faster than the other half you know the smallest cell is full and you stop charging. You'd need some sort of comparative trigger instead of a fixed voltage number so it might be harder to implement. Of course if you had a cell in each half of the pack behaving in the exact same manner you wouldn't see a difference so though the odds are probably small it's not quite as safe as cell level monitoring. Ideally you'd split your pack with one half having the smallest cells to exacerbate the differences and in that case half pack voltage should be enough to use for a value.


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## dimitri (May 16, 2008)

JRP3 said:


> Now how about taking a closer look at the split pack concept of monitoring? With split pack monitoring as soon as your smallest cell starts to drop you're going to see an imbalance between the halves and action can be taken. Do we really need to know which cell is low, or simply that a cell *is* low?


If you ever use Paktrakr you know there is a screen which shows entire pack at once as a single bar graph. I find this screen most useful when driving. Under 1C it still looks like a single bar with smooth top, but under 3C it looks like a saw with very ragged edge. I am 100% positive than half pack monitoring for LVC is completely useless from what I've observed in my EV. About the only useful thing about it is to find degrading cell, which is different from others even without load, but cell level HLVC would do the same thing just as easy.


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## dimitri (May 16, 2008)

I agree that simple BMS does not care which cell is low, that is why I would prefer NC loop instead of digital serial bus. Any bad or low cell means you must stop driving anyway, so you can take time and find offending cell by looking at LEDs or using multimeter.


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## Dave Koller (Nov 15, 2008)

dimitri said:


> Here is an idea, what if we have hybrid BMS? Simple analog cell modules with simple NC signaling loop, which is interpreted by digital head end controller along with current data ( hall effect sensor ) and temp data ( temp sensor ).


This is EXACTLY what I was thinking - sort of a blend of VB and digital! As far as noise - does Paktrakr use rf noise suppression and shielding on their lines? I notice more people are learning about twisted pairs to cancel out noise... people cringe at optical fiber links (cheaper than wire in some cases) and mini processors - they seem to have a mindset that those are not trustworthy - look around you and you will see them in everyday things.. It's not that pure analog is not good cheap and reliable but tubes ( I am over 65 - lol) used to be the norm.. I think CHEAP is on the way with reliable right in there... Point is I like the idea of hybrid - but it seems to scare most when it comes to digital.. I have built and placed controls in super noise environments (old saw mills) - and yes it is a matter of great care in noise suppression when you design... but not expense - just common sense shielding..


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## JRP3 (Mar 7, 2008)

etischer said:


> If you are trying to detect 0.05V rise in one cell which is part of a group of 20 cells, you are now trying to detect 0.0025V change in voltage which will be entirely masked by noise.


Not sure how you get that. A .05 rise in one cell is a .05 rise in the total pack voltage. 20 cells at 3.45 each is 69, a .05 rise in one cell gives you 69.05 volts.


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## karlos (Jun 30, 2008)

I like the idea in principle JRP3. My question is, would the difference be too small relative to the normal differences in location of cells? Eg in front and rear of car. I say this because lets say it's an 80 cell pack, in one of the 40 of one half a cell is getting over charged by 0.5 volts near full charge. Would I be correct in understanding that would make a pack split difference of 0.0125 volts? (0.5/40=0.0125) I would say that this could easily fit into the variation you might get by splitting into a pack in two anyway, due to temperature differences, wiring lengths etc. And then you might get the case of say a battery in one side maybe say 0.25 volts over which makes the differences even smaller and harder to detect and the potential for cell failure, so much higher.
If I'm correct about the above assumptions it could be worse than riding 'bareback', it could be like riding with a saddle and the sense of security it gives but not having the girth strap done up!
So far, best of all, although I don't know much about it, I like the sounds of David85's BMS which charges the pack and at the end goes off the individually unregulated charge, checks each cell and tops them off accordingly. That way you are always starting from scratch. The only additional benefit I can see, would be to know the status of the weakest cell at say, 80% discharge.
My comments above lead me to believe that at the end of the day, when I purchase my BMS system, best of all I would like it to be very simple, very rugged, and cheap relative to the first two parameters being fulfilled.
Simply, I would like to to cut back any cell being over charged relative to the rest of the pack and show this with a diode eg; red for not charged and green for fully charged, real simple for guys like me who don't need to know everything that is going on but from time to time can visually look at the pack to see if any cells are slacking.
What qualifies it all for me is 3 things; if I sell the car, is it easy to use, idiot proof and extremely reliable eg. does not have hundreds of connections and components to fail. Another aspect is, is it easy to service should something go wrong eg., if the cell mounted BMS does not have either a green or red light showing, it is faulty!
I know very similar devices to what I have described are out there but reliability seems to be an issue. From my experience, clamping a PCB board with the pressure needed to make the cell connections, is not acceptablle even if large area washers are used. It might work for awhile but that is far from what I want, after all we are proud as EV owners of the few parts to go wrong mechanically, but where possible electrically it should be the same. As far as I'm concerned each EV made, should be able to be a rally car with no modifications, clamping PCB along with cell connection cables is asking for trouble unless a stud is used using separate nuts. For this reason I do like the design of your units Dimitri and no doubt there are others.
I hope this helps and indicates what I would spent my money on.


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## Guest (Nov 21, 2009)

I certainly like the idea. My initial thought is that I need TWO indications, one when any cell goes low, and one when any cell goes high. But why? I already know the difference between when I'm charging and when I'm driving.

Well, I might not want to use one of them for charging. I might want to use BOTH of them for LV detection. And rig one to go off at 3.0 vdc, and another at 2.8 vdc.

So, I'm still with a two wire system. 

THe MOST important thing is that it is totally isolated from the pack. That it is just open or closed analog loop pretty much takes EMI out of the game. 

I would like both upper limit and lower limit to be adjustable. There are just different voltages out there. The "sag" voltage varies by current draw. The charge voltage varies by cell type. They need to be adjustable. As above, we might want to use them for different things.


And yes, I don't want to spend $7000 to defend a $13,000 pack. I can buy 65 replacement cells for that. So cost is an issue, but it doesn't have to be free. Make em for $15 and sell em for $30 and I'll buy a bunch of them.

And that's how I would approach it. A VERY simple open or closed circuit detecting EITHER a hi or lo, with those limits being adjustable. And let the user work out HOW they are going to use it to change throttle voltage, shut off charger, or bake cookies.

What you MIGHT do to save yourself some money on the boards, is do whatever is easiest on them, and build a little relay box for the END that does the isolation and NO/NC. That's basically what EVPower has done.

That gives you a little latitude on the isolation, using optoisolators for instance, and then you build a little voltage comparator in a box that switches a relay. WE tie into the relay terminals, which are isolated from the pack. The BOX deals with the pileup of forward voltage from the isolators and makes a yes/no determination to switch a sturdy little 12vdc relay. They're $4 for 20 amps I think. Again, I'd like two relays.

That might make the board design easier.

That we have to stand there with a power supply and voltmeter and hand calibrate each cell for our pack is unfortunate. But I think being able to adjust upper and lower limits is just a must. The Sky Energy and THundersky batteries are different. My LV detection scheme may just need to know the absolute value, and yours might want to know the sag value at 300 amps. Who knows WHAT I'll be running in three years.


By taking this modular approach, and making them adjustable, you open yourself up for a lot of future success. And a world of add-ons. 

And that's where you send all the additional requirements people come up with. I think if you knew that ANY cell had reached either of TWO adjustable limits, you can add on instrumentation systems, controller interfaces, throttle voltage dividers, horn honkers, coffee makers, etc. ad infinitum.

So my hot tickets are:

1. Isolation
2. Full range adjustable voltage levels
3. TWO useful capacity relay outputs for two different voltage levels.
4. No EMI issues. 
5. Reliability.


Jack Rickard


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## Guest (Nov 21, 2009)

PS. 

And I can already see people are pulling you to do more. STAY FOCUSED on the simple thing you proposed. Almost ALL other features can be done as an additional circuit later to hook up to this. Think modular.

What will you call it?

In the precambrian tradition of EVland, the Centipede switch - a hundred legged relay creature...not very bright, but always on the march.

You could then use it with a Zilla, a Raptor, a T-Rex, or whatever.

Jack Rickard.

Oh, one more feature: It can't drain the batteries dry or burn them up. Even if it gets stepped on.


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## dimitri (May 16, 2008)

I definitely hear you on a 2 level LVC, but it means more parts and wires and that just kills me because I would love single wire NC loop. It has to be NC to be reliable, if you have NO loop you don't know if loose wire is preventing a signal to reach head end. Safety is first, which means NC loop is a must.

You seem to brush off the HVC feature, but how will you stop your charger when first cell hits HVC? Are you relying on conservative charge voltage and assume that one cell will never creep up over the limit?


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## JRP3 (Mar 7, 2008)

karlos said:


> I say this because lets say it's an 80 cell pack, in one of the 40 of one half a cell is getting over charged by 0.5 volts near full charge. Would I be correct in understanding that would make a pack split difference of 0.0125 volts? (0.5/40=0.0125)


No, as I mentioned earlier one cell going 0.05 volts higher raises that half of the pack voltage by 0.05 volts. For simple math lets say 10 cells at 3.50 volts, 35 volts total. One cell starts to go to 3.55 volts gives you 35.05 volts. Same thing happens on the discharge end. I don't know if this is enough in the real world to be useful though. If you're going to use signaling cell level is probably the way to go. Or how about monitoring cells in pairs? First pair to hit 7 volts, SE cells, shuts off the charger, first pair to hit 6 volts gives warning light. If somewhat bottom balanced that would be good enough I'd think, and half as many cell monitors.


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## JRP3 (Mar 7, 2008)

dimitri said:


> I definitely hear you on a 2 level LVC, but it means more parts and wires and that just kills me because I would love single wire NC loop.


What about my idea of a time function in the head unit? Brief LVC flashes a light, sustained LVC triggers an alarm or whatever you want.


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## Dave Koller (Nov 15, 2008)

JRP3 said:


> What about my idea of a time function in the head unit? Brief LVC flashes a light, sustained LVC triggers an alarm or whatever you want.


Forget all my "digital rantings" but I think for what you want that would be an easy way to signal on Dimitri's NC loop - fast pulses for brief LVC - slow for true LVC - off for HVC or series of flashes ... could be done...


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## Guest (Nov 22, 2009)

Dave Koller said:


> This is EXACTLY what I was thinking - sort of a blend of VB and digital! As far as noise - does Paktrakr use rf noise suppression and shielding on their lines? I notice more people are learning about twisted pairs to cancel out noise... people cringe at optical fiber links (cheaper than wire in some cases) and mini processors - they seem to have a mindset that those are not trustworthy - look around you and you will see them in everyday things.. It's not that pure analog is not good cheap and reliable but tubes ( I am over 65 - lol) used to be the norm.. I think CHEAP is on the way with reliable right in there... Point is I like the idea of hybrid - but it seems to scare most when it comes to digital.. I have built and placed controls in super noise environments (old saw mills) - and yes it is a matter of great care in noise suppression when you design... but not expense - just common sense shielding..



They're in everything, true enough. THey're in cars, true enough. The problem is, in OUR cars, we're hauling a 50kW AM radio station around with us. We call it the motor. 

There are literally DOZENS of guys who have developed digital BMS, spending months on them, thousands of dollars developing them, and I might add, some of them are absolutely FANTASTIC.

Unfortunately, after all those months, and seeing it work in truly superb fashion, they naturally take it out for a drive. THAT's when the bad news hits. Particularly a DC series motor with a commutator. Perhaps less so with an AC induction motor. Tesla has a nice display, I don't have any idea how they designed it. But they don't have a commutator. Plenty of windings of course.

I have a CAN bus on the Evision. It more or less works. So it's not impossible. But most of the inputs to the EVision are very analog, in fact part of it is MOUNTED on the current shunt, (how's that for shortening the wires) and you're basically sending numerics over the CAN bus in a race, and if it takes a hit, it picks up that data in the next cycle - I assume.

Jack Rickard


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## Dave Koller (Nov 15, 2008)

jrickard said:


> The problem is, in OUR cars, we're hauling a 50kW AM radio station around with us. We call it the motor.
> Jack Rickard


Yes and that is the rub (DC series motor with a commutator) Ham radio guy here! Know the rf side! that was why I was leaning toward the simple on\ off signal device - more or less as a single wire proposed by Demitri and added to by JRP3 - oh I haven't given up on digital BUT, on subject, it can be linear with a touch of digital pulses ... as I was saying in the other thread... That could satisfy all objectives - cheap, simple, and not so affected by EMF - AND nothing complex!


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## Guest (Nov 22, 2009)

dimitri said:


> I definitely hear you on a 2 level LVC, but it means more parts and wires and that just kills me because I would love single wire NC loop. It has to be NC to be reliable, if you have NO loop you don't know if loose wire is preventing a signal to reach head end. Safety is first, which means NC loop is a must.
> 
> You seem to brush off the HVC feature, but how will you stop your charger when first cell hits HVC? Are you relying on conservative charge voltage and assume that one cell will never creep up over the limit?


I'm not onboard with creep Dmitri. I'm not seeing creep. I understand what you're saying. It makes sense. Now why don't I see it? 

I love to type, as you can see. I read extremely well, because the nuns made me. Now all I have to do is resolve what I'm reading, with what I see on the car. I have $15,000 worth of test instruments. And a hundred grand in batteries. So it's not like I'm "unable." But I'm from Missouri. Show me.

I have charged and charged and metered and metered and it is real easy. Charge to 3.65 and quit. Life is good. Thundersky sells a charger that has no ABILITY to tie in to a BMS, has no ABILITY to even change the output, and they sell it for 24 cell 72 v systems. It charges to 87 volts and quits. And it works fine. Over time. Undertime. All the time.

So no, I have no need to detect HV. At HV, I am charging. I'm in a garage. I'm doing it at 15 amps. I don't lose cells there. And because I don't shunt I don't burn anything down there. HV detect would allow me a skosh more range because I could drive that short cell right to 4.25. But it would add a mile or two, no more.

Cell loss happens when driving. At 200 amps. Or 300 amps. Or 500 amps. So that's what I'm after.

My point was, let everyone be after what they want to be after. I like your NC loop concept that if it opens, we have a problem Houston. Agree totally that if we just break a wire, or crunch one of the circuit boards, that that would look like we have a problem Houston as well. If we have NC, then everything must be operating, and no cell can have reached the limit. 

I would LIKE to have two levels, which many might use one of them to turn off the charger. I would more likely use it for a two stage LV alarm. But that's why I urged you to stay focused on your basic idea. I think you're closed loop is going to wind up with 50 volts on it if you use optoisolators. So put a head on it and let people tie into it to do whatever they like. In fact, you can spend the next couple of years building add on devices for it as far as the imagination can go.

Ff two levels is too complex and too expensive, and agreed ONE wire is a big plus, one level would be very nice. But if it's one, I want to set it for LV, not HV. And you kind of have the competitive problem of somebody coming along and "one upping" you to two almost without fail.

Mysterious allusions to "creep" and "internal resistance variance" just don't work for me. Show me. Describe a test scenario you yourself performed to arrive at that and I'll go to the garage and perform it to see if I get the same results. That's how it's done guys. Experiment. Publish. Peer review. "I" didn't precisely invent this process by the way....

The peer review part is that others can read the description, set up an identical experiment, and they should get the same results....If they can't you have an anomaly. Something was wrong with their setup, or something was wrong with the original premise.

For my part, take a GEM, install 24 TSLFP160Ah cells, top balance to 4.000 vdc, drive until pack voltage descends to 72 vdc. Take multimeter and go check which cell is irrecoverably damaged. You CAN do this at home. In fact, you can do it at 73 vdc and incredibly, you can even do it at 74 vdc. 

My own "magic sauce" of superstition has been the Brusa charger at about $3500. I thought by being a very clever programmable charger with very good voltage measurement and CC/CV switching and letting the cells rest and having multiple stages and so forth that this was why I was getting away with it when others were just SCREAMING that you had to have a BMS or certain death would follow.

NOW that I have some clue what is really going on here, I think I can get away with a much less expensive charger, just set to a very specific voltage. I still want CC/CV. I still want to be able to set the voltage. I aint' going back to a Zivan (ever). But it probably does NOT have to be a Brusa. And no, I really have no need for HV alarm. But others probably will want it as insurance and it certainly won't hurt a thing.

The cell level LV detection on the other hand is pretty serious. If I'm ever to get to daughter mode, it pretty much has to be.... And I would have to say that's my focus at the moment. Daughter mode. I'll take your head end box, and use it to switch two itty bitty resistors in a voltage divider across the signal output of my hall effect pedal, reducing it to 25% percent of whatever it is. She'll have it floored at 20mph. I think I'll hear from her shortly after. And hopefully my car will still be good.

I DO have to figure out what that magic voltage is for MY car, which will likely be DIFFERENT for somebody else's car, or somebody else's cells of different manufacture. That's why I want it to be adjustable.

I think you'll find a chip with two optos in it isn't much more expensive than a chip with one. I think you'll find two 431s are not much more expensive than one. But the damn trim pots can be $3 to $5 apiece if you use anything that is any good and will last.

If you want to draw broad conclusions from minute "occurrences" then my sense is that cells grow "closer" in balance. Your sense is that they creep apart. We differ in this. I can't prove it. So YOU do an expensive experiment, and publish your findings and I'll snipe from the sidelines. 

But if I can reproduce it, or if it's so obvious once you've done it that I needn't bother, I'll be the first to issue a "bravo" and change my thinking, however painful that is...

If we're going to spend $10,000 each on a set of devices from China, whose entire documentation consists of a 4 inch piece of paper that says "your glad acceptance is our warmest happiness" then this is the learning curve we both severally, and as a group, embark upon. Be of good cheer. Without these devices, electric cars don't make much sense at all....

With them, we fly silently into the night... at maximum torque.

And - chics dig it....

Jack Rickard
http://evtv.me.


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## Guest (Nov 22, 2009)

JRP3 said:


> What about my idea of a time function in the head unit? Brief LVC flashes a light, sustained LVC triggers an alarm or whatever you want.


Not hard to do. LV starts a countdown timer. 10 seconds later, resample. If it's still there latch a relay.

But how do you resolve all this JRP? I can drive down the freeway at 75 mph drawing 540 amps for 15 minutes. If I get a red light, I'm pretty inspired to stop, and it was normal operation. The same thing stop and go works. 

I think there's a microswitch on my pedal. My thinking is to use it to disable the LV relay output. Then it only counts when my foots off. But that gets me back to the same question. Now if I drive 15 minutes at 75 mph, I never get an indication that anything is wrong......

The only thing I can figure is to determine what my least allowable voltage is under maximum load. If I hit that, that's the signal. If I hit it at a stoplight, unfortunately, it WAS the signal. I missed it. And now have a dead cell most probably.

This was why I wanted TWO LV signals. Resting. Loaded. Use the switch in the pedal as a simple selector between the two. It's rough. But it would more or less work. 

I'm going to do some measurements. I think on the Sky Energies I was seeing about .2 difference in 100 amp load and no load. I have some new programmable loads on the way that should let me do about 300 amps out of a single cell down to about 200 mv. Someone, I think in the Australian forum has given me a formula about three times for temperature/current/voltage to calculate this value. We'll see if it works....

Jack


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## etischer (Jun 16, 2008)

JRP3 said:


> Not sure how you get that. A .05 rise in one cell is a .05 rise in the total pack voltage. 20 cells at 3.45 each is 69, a .05 rise in one cell gives you 69.05 volts.


You aren't likely going to find a 100v analog input. The 100v will have to be scaled back down to 10v. So a 0.05v change will now be scaled down 10x making the change 0.005v. 

If you use a 100v scale DVM, you will probably not see 2 decimal places. I suspect the noise on a PWM charger will be more than the 50mV you are trying to detect. I just don't think this will work well.


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## karlos (Jun 30, 2008)

Agni Motors BMS system. 
Has anyone reviewed this BMS?
Seems like it's simple and effective.
http://agnimotors.com/home/index.php?option=com_content&task=view&id=27&Itemid=39


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## JRP3 (Mar 7, 2008)

etischer said:


> You aren't likely going to find a 100v analog input. The 100v will have to be scaled back down to 10v. So a 0.05v change will now be scaled down 10x making the change 0.005v.
> 
> If you use a 100v scale DVM, you will probably not see 2 decimal places. I suspect the noise on a PWM charger will be more than the 50mV you are trying to detect. I just don't think this will work well.


You may be correct. I was thinking with a total pack voltage of less than 200, (most people), and two half pack meters capable of 99.99V each you'd see it.


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## JRP3 (Mar 7, 2008)

jrickard said:


> But how do you resolve all this JRP? I can drive down the freeway at 75 mph drawing 540 amps for 15 minutes. If I get a red light, I'm pretty inspired to stop, and it was normal operation. The same thing stop and go works.


Yeah that's a problem. So now the head unit would have to calculate voltage sag in relation to amp draw.  Foolproof will cost more than good enough.


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## tomofreno (Mar 3, 2009)

> I'm not onboard with creep Dmitri. I'm not seeing creep. I understand what you're saying. It makes sense. Now why don't I see it?


I think this is why you don't see creep due to differences in internal resistance, ir. Due to current continuity all the cells will accumulate the same amount of charge during charging, and loose the same amount during discharge. The useful energy into the cells during charging will be different if they have different internal resistances. The charger has to do more work to move charge into the cells with higher internal resistance. A higher ir will result in higher voltage across the terminals of those cells for a given charging current. Work is the product of charge and voltage, so the charger does more work, expends more energy, charging these cells, while these cells accumulate the same amount of charge as the other lower ir cells. But the charger is doing more work because the cell is dissipating more energy during charging due to its higher ir. The cell voltage is higher precisely by the amount required for the charger to supply this extra dissipated energy. If this higher ir cell has the same capacity as other cells, its rest voltage (no current through the cell) will be the same as the others after charge (assuming they were balanced in voltage prior to charge), so it will have the same stored energy as the other cells, qV. Same charge, same cell voltage due to same capacity. The excess energy output of the charger was dissipated as heat. The energy from the charger is equal the total energy into the cell, the increase in potential energy of the charge plus energy dissipated as heat.

During discharge the higher ir cells have larger voltage sag than others for a given discharge current, so the charge in these cells falls through a smaller potential drop than other cells as it flows out of the cell. The same amount of charge flows out of each cell by current continuity, so you get less useful energy out of these cells. The rest was dissipated as heat in the cells due to higher ir. The cells have all lost the same amount of charge, so if they have the same capacity they will all have the same cell voltage (again assuming initial voltage balance prior to charging). They also have the same energy, qV'. Less useful energy was obtained from those with higher ir, but all lost the same total energy, equal to the useful energy extracted plus the energy dissipated internally in the cell. The voltage sags due to this internal energy loss. With no ir there would be no internal energy loss, all charge in all cells would fall through the same potential through a pair of cell terminals, and all cells would supply the same useful energy.


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## dimitri (May 16, 2008)

JRP3 said:


> What about my idea of a time function in the head unit? Brief LVC flashes a light, sustained LVC triggers an alarm or whatever you want.


Actually your idea of a time function on LVC is what got me thinking about hybrid BMS, where digital head controller will intepret LVC pulses based on other data such as temp and current draw. This way we can figure out some common LVC level on the cell and avoid trimpots and keep single wire loop, yet still have meaningful interpretation of LVC alerts. Applying software logic to various road conditions and vehicle conditions is where its going to be tough and time consuming, plus in order to test it all one would have to often drive the pack to LVC to see how it behaves, at a risk of killing cells in the process. But I think this approach combines the best of both analog and digital worlds and would make a nice product.

Another benefit of hybrid BMS is that a simple relay based head end controller can be offered and used while we develop the digital one and then can be swapped later without touching the battery pack.

Its interesting to learn that TS and SE show similar low end knee despite difference in their specs. Perhaps this would allow a single common LVC level for both types.

So, Jack, from your experience with both TS and SE under heavy loads etc etc, if you were to select one LVC level set in stone, assuming you had flexibility at the head end about what type of action to take, what would be your choice? Is 2.0V too low? Is 2.6V too high?

In my EV I have LVC=2.6V and it worked for me one time and it saved my pack by 50% throttle cutting, letting me crawl 1/2 mile to my garage, but since I haven't actually killed a cell I can't tell how close I got to it.

I'm not rich enough to invest $$$$ in test equipment, so all I can do is read other's experiences and draw my own conclusions.

I'm trying to find the best balance in LVC value between healthy sag under 3C load and unhealthy voltage of an almost empty cell. It seemed to me that 2.6V on TS cell was pretty good choice based on many people's reports and my own observations, but perhaps I am being too conservative?


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## Dave Koller (Nov 15, 2008)

dimitri said:


> Applying software logic to various road conditions and vehicle conditions is where its going to be tough and time consuming, plus in order to test it all one would have to often drive the pack to LVC to see how it behaves, at a risk of killing cells in the process. But I think this approach combines the best of both analog and digital worlds and would make a nice product.


Dimitri:
Software algorythms can "reset" to new, under-load, old, guessed-at, and any factory specs in the blink of an eye. Keeping in mind that NO MATTER WHAT a killer LVC is it can be delt with ( and there has to be a determination BEFOREHAND). That would be a processor for that - BUT:

As Jack suggested time delay is easy to do - I build hysterisis circuits all the time to offset noise and insure that the condition lasts for more than a pulse (flooring it!) - Jack's ten seconds sounds right.
These circuits (in hardware 15 cent cmos chip) reset each time the condition is over i.e. no latch. That same chip can pulse an opto chip from NC in a string of them to any number of warning flashes ( as JRP has asked ) - all without relays. this would be analog and digital with NO software - no processor..


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## MN Driver (Sep 29, 2009)

dimitri said:


> I'm trying to find the best balance in LVC value between healthy sag under 3C load and unhealthy voltage of an almost empty cell. It seemed to me that 2.6V on TS cell was pretty good choice based on many people's reports and my own observations, but perhaps I am being too conservative?


Winter voltage sags will move your numbers too, I don't know the TS experiences well enough(simply because there aren't many active in sub-freezing temperatures last year) to know if these can accomplish 3C at, say, 50% SOC at -20 temperatures(pick your temperature scale, you get the idea). Although I realize many living plenty further south than Minnesota don't have to ever consider temperatures possibly dropping to -40C at the absolute coldest. I know the cells are rated at -45C but 2.9 volts at 0.5C is far enough down to make me think that pulling 3C could mean pack murder. -25C looks more reasonable but at 1C or 2C, I'm not sure what I'd get.

I know JimK gave us this info from his TS manual
@-35C/-31F: 1CA (Max.Dischrg.Curr.), 1.5V (Max.Dischrg.Volt.)

Since I'm planning to get a pack big enough to be discharging less than 1C while driving at a constant speed, this might not be too big of an issue for the pack for me but the cutoff voltage might be premature. Two resistors and a microswitch to add the choice based on pack use conditions(C draw, temperatures, manufacturer of cells, etc).

EDIT: I've also got an insulated heated box plan that I'll share in a thread to bounce ideas when I get the chance. The idea is that there is no direct air heating to the battery pack so that way there will be no hot spots except for maybe slightly warmer box sides and bottom. Technical details need to be figured out.


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## dimitri (May 16, 2008)

This is exactly where I struggle, how to keep it simple and reliable, yet useful for people living in different climates, using different discharge rates, having different opinions on DoD rates, etc. It almost seems that at least one trimpot is in order. I'm against a switch since its prone to mechanical failures and dust, but a good quality multiturn trimpot seems reasonable. 

You read of many stories of DIY BMS, people go different ways, charger and controller selections play a role too, but how to combine best features in one product while keeping it simple cheap and reliable?


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## Dave Koller (Nov 15, 2008)

MN Driver said:


> Winter voltage sags will move your numbers too,
> 
> Since I'm planning to get a pack big enough to be discharging less than 1C while driving at a constant speed, this might not be too big of an issue for the pack for me but the cutoff voltage might be premature. Two resistors and a microswitch to add the choice based on pack use conditions(C draw, temperatures, manufacturer of cells, etc).


Nice point - same here in the Northwoods of Wisconsin - err one more sensor for temp


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## Dave Koller (Nov 15, 2008)

Example of pots that I have used in extreme conditions.. they are temperature stable NOT multi-turn but if I use them in series with 1% I can dial in small voltage changes - this type holds up to vibrations well also - extreme cold does not seem to change them - again this one was only a proto the finish board has held up in extreme for years.. kind of a analog with digital board Dimitri!


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## Guest (Nov 23, 2009)

dimitri said:


> Actually your idea of a time function on LVC is what got me thinking about hybrid BMS, where digital head controller will intepret LVC pulses based on other data such as temp and current draw. This way we can figure out some common LVC level on the cell and avoid trimpots and keep single wire loop, yet still have meaningful interpretation of LVC alerts. Applying software logic to various road conditions and vehicle conditions is where its going to be tough and time consuming, plus in order to test it all one would have to often drive the pack to LVC to see how it behaves, at a risk of killing cells in the process. But I think this approach combines the best of both analog and digital worlds and would make a nice product.
> 
> Another benefit of hybrid BMS is that a simple relay based head end controller can be offered and used while we develop the digital one and then can be swapped later without touching the battery pack.
> 
> ...


I don't have a good sense of it. It looks like about .15 or .2 difference between 0 and 100 amps on an SE. But my car runs at 540 amps full bore. That's why I wanted LV to be adjustable.

My "glance at the evision" while accelerating indicates a full pack voltage of 120 will drop to 108 or so under load. But I've noticed it does more when its cold. That's why I think this is a bit of a complicated situation WHAT to do with it. But if I could detect it and adjust the level, it's a start.

I'll have a constant current load that will do 300 amps probably this week. We'll find out....

Jack Rickard


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## Guest (Nov 23, 2009)

dimitri said:


> This is exactly where I struggle, how to keep it simple and reliable, yet useful for people living in different climates, using different discharge rates, having different opinions on DoD rates, etc. It almost seems that at least one trimpot is in order. I'm against a switch since its prone to mechanical failures and dust, but a good quality multiturn trimpot seems reasonable.
> 
> You read of many stories of DIY BMS, people go different ways, charger and controller selections play a role too, but how to combine best features in one product while keeping it simple cheap and reliable?


Those are the $3-$5 trimpots I mentioned.

Jack Rickard
http://evtv.me


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## Guest (Nov 23, 2009)

Dave Koller said:


> Nice point - same here in the Northwoods of Wisconsin - err one more sensor for temp


Yes, but you could put the temp sensor in the HEAD END box (one each) and be close enough. You don't need one on each cell.

Jack Rickard
http://evtv.me


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## Guest (Nov 23, 2009)

tomofreno said:


> I think this is why you don't see creep due to differences in internal resistance, ir.


It could be. I had a telephone conversation with Peter Sowensky (Designs Raptor controllers) today and he reminded me of the importance of temperature variation. In their Corbin Sparrow's, it was ALWAYS the number 4 battery that failed, because of the temperature of that particular location in the car. 

Obviously, this can vary from car to car. But yes, a factor like that can absolutely swamp "what makes sense" in a perfect world. A signficant temperature variation over time in the car location could wipe out all other factors, and indeed turn your LARGEST capacity cell into your smallest capacity cell over the course of a year of daily driving.

It gets a little complicated. I haven't noticed it being a factor. And so I just don't deal with it. I think bottom balance about once a year and charge to 3.65 vdc on TS and 3.5 or even 3.45 on SE and things will go well. But LV cell level detection would be real nice to work the "gas gage" problem and provide a little insurance. 

At that point, you're about down to having range problems. If you're not getting the range you should, you're going to have to take a look at your cells somewhat manually anyway. If one of them is destroyed, it won't tell you much. It's destroyed.

Jack Rickard
http://evtv.me


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## dimitri (May 16, 2008)

I've been looking at trimpots suitable for automotive environment and not too expensive.

Here is what I found http://www.bourns.com/data/global/PDFs/3361.pdf

These seem prefect for the car, sealed, high resistance to vibrations, etc.

$.85 when bought in quantities from Digikey. Not too bad. Opinions?

Also, I need some help from experienced EEs. I'm trying to create a single NC loop across say 100 cells and use head end controller powered by 12V aux battery. I can't use simple optocouples since voltage drop across 100 series connected units would not work under 12V. So I have been looking at photorelays which are AC/DC capable ( no need to pay attention to polarity on each cell ) designed to replace mechanical relays.

Here are specs for 2 models which are priced right for cheap BMS, less than $1 in quantities from Digikey.

http://media.digikey.com/pdf/Data Sheets/Toshiba PDFs/TLP222A.pdf

http://www.clare.com/home/pdfs.nsf/www/CPC1014N_R01.pdf/$file/CPC1014N_R01.pdf

These have 2 Ohm on-resistance, but I can't figure out if they need minimum voltage across output to conduct. For example if I string 100 of these in one loop and use it to drive a gate transistor on the head end which would drive the LVC actions, then each unit will only have 12V/100 = 0.12V across. Does anyone know if this would work with this type of unit?

Thanks

P.S. I have been prototyping an interesting approach to simple BMS, seems to work well so far, but I need to figure out how to drive NC loop with it. I am thinking about starting an open source MiniBMS thread, to get feedback from few pros out there.


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## Dave Koller (Nov 15, 2008)

dimitri said:


> These seem prefect for the car, sealed, high resistance to vibrations, etc.
> 
> Also, I need some help from experienced EEs. I'm trying to create a single NC loop across say 100 cells and use head end controller powered by 12V aux battery. I can't use simple optocouples since voltage drop across 100 series connected units would not work under 12V. So I have been looking at photorelays which are AC/DC capable ( no need to pay attention to polarity on each cell ) designed to replace mechanical relays.


That Bourns pot has good temp range and ppm/C (got some 10k's around here somewhere) - sealed well - and should work.

My question to you is:

If you set up a single wire loop terminated at the end with a current limit resistor ... Now that wire is - say + 12 volts... going back to the cheap optos - let's say you use the ground (of the 12 volt system) to put a short (via the NPN in the opto) to that signal loop - or pulse it or whatever.. wouldn't it still be a closed loop with short from any one of the BMS circuits as a signal (but with out all the voltage drop)? I thought about the number of cells and the .6 volt drop of all those NPN outputs -- yep not good ... The short would only be to the termination resistor anything wrong shows up as this Negitive ... It is a poormans NC that is alwas on and "shorts" on trouble from ANY one BMS board...


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## Tesseract (Sep 27, 2008)

dimitri said:


> ...I'm trying to create a single NC loop across say 100 cells and use head end controller powered by 12V aux battery. I can't use simple optocouples since voltage drop across 100 series connected units would not work under 12V. So I have been looking at photorelays which are AC/DC capable ( no need to pay attention to polarity on each cell ) designed to replace mechanical relays....



I looked at the TLP222, specifically, and assume the other is similar. The simple solution, and the one that is most noise resistant, is to feed the NC loop with a constant current source and measure the resulting voltage drop formed across a known resistance in series with the loop. That way it doesn't matter how many of the MOSFETs are in series, as long as the current source/sink has enough "compliance voltage". 

For example, with 12V nominal available, loop current set to 20mA, and a fixed resistance of 220 ohms, you could accommodate a loop with ~140 or so of those "photorelays" in series (subtract ~2V from the 12V supply for the current source compliance voltage then divide by the loop current for the maximum loop resistance).

So, a 20mA current source feeds the loop that connects to a 220 ohm resistor that is grounded. Monitor the voltage at the junction of resistor and loop. When the loop is present and all MOSFETs are on, the voltage at the junction will be ~4.4V (logic high, more or less). When the loop opens for any reason the resistor, being a mere 220 ohms from ground, will effectively bring that node to 0V. Tada. A noise-resistant NC loop on the cheap.

(hint: review bipolar transistor circuits for simple ways to make constant current sources).


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## Dave Koller (Nov 15, 2008)

Tesseract said:


> So, a 20mA current source feeds the loop that connects to a 220 ohm resistor that is grounded. Monitor the voltage at the junction of resistor and loop. When the loop is present and all MOSFETs are on, the voltage at the junction will be ~4.4V (logic high, more or less). When the loop opens for any reason the resistor, being a mere 220 ohms from ground, will effectively bring that node to 0V. Tada. A noise-resistant NC loop on the cheap.
> 
> (hint: review bipolar transistor circuits for simple ways to make constant current sources).


Same thing two different ways  ...


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## Dalardan (Jul 4, 2008)

Just for the fun, I've found this layout laying on my desktop...










Have fun! The Vce of 0.7V is an approximation but it should not matter in your case. Vout can be anything from 0V to VDD-0.7V, but the current will be fixed at Iout.

Dalardan


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## dimitri (May 16, 2008)

Thanks Dave, Tesseract and Dalardan, I'm looking forward for your contributions to MiniBMS open source project I just started.

Dave, I'm sorry, but I couldn't follow your idea, maybe you can post a napkin drawing?

Tesseract, your idea makes sense, but my question remains as far as ability of these relays to conduct under only 0.12V. Since these are semiconductors, I am not sure they will conduct at such low voltage. I can't see in datasheet if there is minimum voltage on the output side.

Now, does it really have to be constant current? NC loop can be between 2 Ohm ( 1 cell ) and 200 Ohm ( 100 cells ). Any one cell breaking the NC loop will cut the circuit. Perhaps I can use another comparator on the head end to sense open loop while NC voltage drop will vary based on number of cells?

Let's move this discussion to the MiniBMS thread to keep it technical.


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## Dave Koller (Nov 15, 2008)

dimitri said:


> Dave, I'm sorry, but I couldn't follow your idea, maybe you can post a napkin drawing?


Quick drawing:

The led is just to illustrate - it is lit from the 1k resistor at all times.. any opto's NPN transistor will extinguish the LED. YOU CAN HAVE AS MANY AS YOU LIKE ON THAT "BUSS" LINE...... (can be pulsed at different frequencies to represent HIGH or LOW) ( easy to delay so it has to be present for 10 secs - but that is another simple circuit) 

Hope this helps!


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## Tesseract (Sep 27, 2008)

dimitri said:


> Tesseract, your idea makes sense, but my question remains as far as ability of these relays to conduct under only 0.12V. Since these are semiconductors, I am not sure they will conduct at such low voltage. ...


Yes - N-ch MOSFETs are majority carrier devices without a PN junction so there is no "forward" voltage across the drain-source; the channel is purely resistive.


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## dimitri (May 16, 2008)

Dave Koller said:


> Quick drawing:
> 
> The led is just to illustrate - it is lit from the 1k resistor at all times.. any opto's NPN transistor will extinguish the LED. YOU CAN HAVE AS MANY AS YOU LIKE ON THAT "BUSS" LINE...... (can be pulsed at different frequencies to represent HIGH or LOW) ( easy to delay so it has to be present for 10 secs - but that is another simple circuit)
> 
> Hope this helps!


This is a NO loop and requires 2 wire bus, this is what my current BMS modules do, but it has extra wire and unable to sense loose connections. 

I will stick to true NC loop as Tesseract suggested.

Thanks


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## Dave Koller (Nov 15, 2008)

dimitri said:


> This is a NO loop and requires 2 wire bus, this is what my current BMS modules do, but it has extra wire and unable to sense loose connections.
> 
> I will stick to true NC loop as Tesseract suggested.
> 
> Thanks


Sounds good..  Each of the optos has to be on all the time then . Should work fine..


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## dimitri (May 16, 2008)

Dave Koller said:


> Sounds good..  Each of the optos has to be on all the time then . Should work fine..


Yes, having all optos on all the time is the key idea, this way ANY failure produces alert and one wire loop does both HVC and LVC.


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## Dave Koller (Nov 15, 2008)

Here is a quick draw of my BMS - high low set, takes around 10 sec to show, resets to start over (in other words wont flash opto UNLESS something is present for more than time delay) pulses at two different frequencies. The 4093 runs on 3 volts and has super noise suppression and is really cheap.. 4093 B (for buffered output) is just one chip with 4 nand gates - used in an unusual way .. Free to have or change or throw out !


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## dimitri (May 16, 2008)

jrickard said:


> I don't have a good sense of it. It looks like about .15 or .2 difference between 0 and 100 amps on an SE. But my car runs at 540 amps full bore. That's why I wanted LV to be adjustable.
> 
> My "glance at the evision" while accelerating indicates a full pack voltage of 120 will drop to 108 or so under load. But I've noticed it does more when its cold. That's why I think this is a bit of a complicated situation WHAT to do with it. But if I could detect it and adjust the level, it's a start.
> 
> ...


Jack,

if you follow my new open source MiniBMS thread, I have a plan to automatically compensate LVC for temp variations. What I need is some baseline data of voltage sag at 3C load at freezing, vs. same at room temp. Perhaps with your new toy you can help this open source project? Once I make first prototype set I can send you a sample for further testing, deal?

I'm hoping to avoid trimpots ( maybe make them optional if I can fit them on PCB ) and have 2-3 common choices for HVC and LVC that people can pick from, assuming that LVC will be temp compensated it should be doable.

Your help is appreciated. Thanks


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## tomofreno (Mar 3, 2009)

It was kind of a pain charging up my low cells (partially drained by defective VBs) to match the others. I hung a timer from my neck to remind myself to check them while charging at 5A. But you have to keep checking to ensure you charge to just the right voltage, not too little time, nor too much. It seems you have a similar problem when manually bottom balancing. In this case you drain higher V cells through some resistance to the V of the lowest cell, but have to keep checking to to discharge to the correct V. 

I thought it would be nice to have it automatically shut off at the proper V. It seems you could make a board with the "optional parts" on Dimitri's minibms to do that. The 4.7 Ohm R could be replaced by say three 3 Ohm, 5W R's in parallel to drain 3A at 3V. Swap comparator inputs 10 and 11, and pick R12 and R13 values so that the voltage at the noninverting input drops to less than 1.25V with a cell voltage at the desired low value, say 2.8V. A board with just these components can then be attached to a cell to drain it to 2.8V and stop. I don't know Q1's specs, so it may need to be replaced with something that can handle 3A, and a heat sink will be required.

You could make this part of the bms if it was connected through an ssr which had to be turned with an external signal to connect this circuit to the cell terminals, but that seems like too much added complexity/wiring to me. But I thought it might be useful as a separate board. You could make up a number of them, connect them to several of your higher V cells at a time, making bottom balancing a bit quicker and easier. I'm not a circuit designer, but it might get Dimitri and others thinking about it.

I posted this here to avoid distraction on the minibms thread. The minibms looks ready to go to me. When can I order some, sans shunting?


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## JRP3 (Mar 7, 2008)

tomofreno said:


> It was kind of a pain charging up my low cells (partially drained by defective VBs) to match the others. I hung a timer from my neck to remind myself to check them while charging at 5A. But you have to keep checking to ensure you charge to just the right voltage, not too little time, nor too much.


What time intervals did you use to check the cells to avoid them overvolting?
Can't you just set your power supply to the voltage you want and walk away?


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## dimitri (May 16, 2008)

JRP3 said:


> What time intervals did you use to check the cells to avoid them overvolting?
> Can't you just set your power supply to the voltage you want and walk away?


Ditto, just set desired voltage on your supply and walk away, I've done it several times with my cells. Current will be max the supply can handle ( may need to lower it if your supply gets too hot ), but when it gets to preset voltage it will taper to zero.


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## dimitri (May 16, 2008)

tomofreno said:


> You could make up a number of them, connect them to several of your higher V cells at a time, making bottom balancing a bit quicker and easier. I'm not a circuit designer, but it might get Dimitri and others thinking about it.


Actually, there is nothing to think about, my current BMS modules can do that today. I still have a handful PCBs left, I can simply populate HVC portion of the PCB and use a trimpot for HVC, so you can set whatever "bleeding" voltage you want. My module can "bleed" 1-2 Amps easily, but I can put a larger heatsink on the transistor and put two 10W 1 Ohm resistors in parallel and I bet it can sink 3-4 Amps without any trouble, as long as you keep it in open air for cooling.

I can make these for $20 plus shipping, PM me if interested. You can attach alligator clips to it and just clip it on one cell at a time and walk away until red LED goes off. I can calibrate it to whatever voltage you want.


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## Guest (Nov 29, 2009)

dimitri said:


> Jack,
> 
> if you follow my new open source MiniBMS thread, I have a plan to automatically compensate LVC for temp variations. What I need is some baseline data of voltage sag at 3C load at freezing, vs. same at room temp. Perhaps with your new toy you can help this open source project? Once I make first prototype set I can send you a sample for further testing, deal?
> 
> ...


Deal. I've done some preliminary testing. I can do a single cell at 90 amps now and should be able to do higher soon. I can do 150 amps on two cells.

I've been testing TS 160's at 80 amps (1/2C) The results are discouraging, but interesting. Some of the spec sheet data suddenly becomes clear. They list 2.5 vdc as the bottom end of the discharge curve. They also list 1/2C as the standard discharge rate. Well, as it turns out, if you load the cell at 80 Amps, surely enough, 2.5 vdc is where all hell breaks loose.

The problem is that under NO load, it's up around 2.8-2.9 vdc.

Worse, I think what we're going to find is that at 160 Amps, it's down between 2.2 and 2.5 vdc.

This is indeed a function of internal resistance, and as you note, I think we're going to see some wide variations based on temperature. I can easily throw one in the freezer for an hour, and then test it to get a variance.

But I don't know where all this leads. We wind up with having to pick a voltage between 2.2 and 2.9 vdc that is meaningless except for the specific conditions we state. It's too wide a world down here.

My conclusion is that all the stuff I've been reading from all these guys about LV cell monitoring BMSs demonstrably HAVE to be bullshit. They absolutely cannot be relating any real world experience - it's a feel good theory. A classic case of typing themselves smart. I can't find a voltage that is representative of any particular set of conditions.

I would like to pursue it. I will take ANY measurements you want gladly. And I'll test any circuit you send. I still think it has some merit. But increasingly I'm thinking we need actual voltages. And we need a microcontroller head end that monitors pack current and temperature to normalize the voltage indication. And just developing the functions to apply will take a lot of testing.

I would offer as a suggestion that you at least peruse the data sheets on two chips. The LTC6802-1 from LInear Technologies allows you to monitor the voltage (and temperature I think) for 12 cells.

The ISO1050 from Texas Instruments is an isolated CAN bus converter. It does isolation WITHOUT optos to 4000v. And it can translate TTL to CAN as part of the process. But all this pretty much obviates the one wire NC loop.

The question you run up against with the simple loop is "what voltage" and I not only don't have an answer, but now know with some degree of certitude that it is all over the place, depending on temperature and current. Indeed an almost meaningless range from 2.2 to certainly above 2.8 v.


Jack Rickard
http://evtv.me


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## Guest (Nov 29, 2009)

tomofreno said:


> It was kind of a pain charging up my low cells (partially drained by defective VBs) to match the others. I hung a timer from my neck to remind myself to check them while charging at 5A. But you have to keep checking to ensure you charge to just the right voltage, not too little time, nor too much. It seems you have a similar problem when manually bottom balancing. In this case you drain higher V cells through some resistance to the V of the lowest cell, but have to keep checking to to discharge to the correct V.


Welcome to the world of manually balancing. Now do you see why I look askance at Rich Rudmann's THOUSANDS OF HOURS of battery testing. It's like watching paint dry, but you almost can't look away. No going to pee. No new cup of coffee. And it's just a couple or three hours.

But you DO get to SEE it do it. It' s not a guess, or a theory. It varies very noticeably from the top, the middle and the bottom. And it does make a difference.

At either end, you CAN swing both ways. It's just as easy to take energy out of a cell. A 1/2 ohm 50 watt Dale resistor with some clips can do 5 or 6 amps to drain a cell. And I use a 3 amp 12v supply to charge. A good voltmeter, and you are partying down. But the world looks different when you're actually doing this and seeing it.

Jack Rickard
http://evtv.me


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## JRP3 (Mar 7, 2008)

jrickard said:


> The question you run up against with the simple loop is "what voltage" and I not only don't have an answer, but now know with some degree of certitude that it is all over the place, depending on temperature and current. Indeed an almost meaningless range from 2.2 to certainly above 2.8 v.


So how about a worst case scenario signal of the absolute bottom, 2V? No matter the conditions that would prevent you from taking a cell to 0 and should allow you to recover the cell if you stop driving. Or pick a slightly higher voltage and realize you're going to get some false positives. If you get a warning 20 miles into your 40 mile pack you can ignore it, if you're 35 miles into it you might want to check it out.


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## dimitri (May 16, 2008)

JRP3 said:


> So how about a worst case scenario signal of the absolute bottom, 2V? No matter the conditions that would prevent you from taking a cell to 0 and should allow you to recover the cell if you stop driving. Or pick a slightly higher voltage and realize you're going to get some false positives. If you get a warning 20 miles into your 40 mile pack you can ignore it, if you're 35 miles into it you might want to check it out.


Absolute bottom is not very useful, you may save the pack, but you get stranded in the middle of the road. 

You guys make it too complicated. Assuming typical EV rolls at 1C or less and accelerates at 3C or less, its not that hard to pick a voltage level which will produce first alerts at 3C while you are at about 80% DoD, which still lets you roll conservatively for few minutes to get home or find a place to park safely. All we need then is to compensate for temp swings.

I have been collecting data from my EV lately and will publish it soon, then I will ask Jack to confirm on the bench.

My LVC is 2.6V and I don't get false alarms, yet get enough notice at 3C to get off the road. All I need is temp compensation and I have some data I can publish shortly.


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## GizmoEV (Nov 28, 2009)

dimitri said:


> I like physical design of the unit, but it appears to be a single PCB and I am not sure that pressure of fully torqued terminal bolt will not crack and destroy fiberglass layers of PCB. Its possible they use special PCB designed to withstand pressure, don't know.


You could use a stud with a couple of nuts, one to clamp the high current wiring and one to clamp the PCB or do something like this: http://www.black-sheep.us/product_info.php?cPath=80_103&products_id=143


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## Dave Koller (Nov 15, 2008)

GizmoEV said:


> You could use a stud with a couple of nuts, one to clamp the high current wiring and one to clamp the PCB or do something like this: http://www.black-sheep.us/product_info.php?cPath=80_103&products_id=143


That"s the same website a friend told me about that has my $29 cherry sensor ( used on tach sense) for $66 ?


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## GizmoEV (Nov 28, 2009)

When you say "my $29 cherry sensor" what do you mean? I ordered my sensor from DigiKey for around $30. I'm not sure why Ron is asking such a high price.


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## Dave Koller (Nov 15, 2008)

GizmoEV said:


> When you say "my $29 cherry sensor" what do you mean? I ordered my sensor from DigiKey for around $30. I'm not sure why Ron is asking such a high price.


Not "Mine" personally  - Just I have showed it to many - errr look here so we are not off subject. http://www.diyelectriccar.com/forums/showpost.php?p=151836&postcount=37


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## Dave Koller (Nov 15, 2008)

dimitri said:


> I have been collecting data from my EV lately and will publish it soon, then I will ask Jack to confirm on the bench.
> 
> My LVC is 2.6V and I don't get false alarms, yet get enough notice at 3C to get off the road. All I need is temp compensation and I have some data I can publish shortly.


And back on subject - I am looking forward to the data, especially if Jack could do the freezer thing and test the compensation !


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## tomofreno (Mar 3, 2009)

> Can't you just set your power supply to the voltage you want and walk away?


 I tried that first of course. Three and a half hours later the current was down to around 2A and I had changed the voltage by less than 0.02V. You will reach the final voltage - asymptotically. I then set the voltage high to keep a constant current of 5A. This changed cell voltage by a bit less than 0.01A/hour (180Ah cell). I at first was checking every 10 min, then every 20, then every hr...After the first couple cells it was pretty clear what delta V/delta was, so on other cells I checked after 3 hours or so, depending on how far they had been discharged. It typically took me 7 hrs or more to charge a cell. Of course in normal operation they would not be nearly this far out of balance.


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## dimitri (May 16, 2008)

jrickard said:


> Deal. I've done some preliminary testing. I can do a single cell at 90 amps now and should be able to do higher soon. I can do 150 amps on two cells.
> 
> I would like to pursue it. I will take ANY measurements you want gladly. And I'll test any circuit you send. I still think it has some merit. But increasingly I'm thinking we need actual voltages. And we need a microcontroller head end that monitors pack current and temperature to normalize the voltage indication. And just developing the functions to apply will take a lot of testing.
> 
> ...


Jack,

here is the table I put together based on my observations and what I hear from others. Its by no means an exact science, but should be a decent approximation. Please confirm / update based on your bench testing, especially interested in cold ( 32F ) test since my data is extrapolated for those temps.

I know about those BMS chips and CAN chips, but its outside of scope for cheap and simple BMS, not really my area of expertise either. Some other folks are working on digital BMS's and there is a market for both according to the poll.

I think there is a merit for simple LVC alerts which will happen under heavy loads first, so its not really relevant what resting voltage is, you rarely see resting voltage outside of the garage anyway. You tend to accelerate often when driving, so that's when first alerts will come in, which is good, it gets you thinking how to get off road soon. Then alerts become more frequent as you press the pedal, so you know you have 3-5 minutes left to drive and you have to let off the pedal, while thinking which towing cervice to call . If buzzer alert is paralleled with throttle reduction, it works wonders, giving you extra time to think.

So, if we figure out temperature compensation range for simple BMS, it will give us pretty decent LVC in a simple analog circuit.


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## dimitri (May 16, 2008)

As suggested by Tom earlier, I am offering "bottom balancer" in Classifieds section. If you belong to "bottom balancer" crowd, get yours while supplies last 

http://www.diyelectriccar.com/forums/showthread.php/lifepo4-bottom-balancer-39025.html

This module allows you to leave the cell "bleeding" and go spend your time doing better things. It will stop bleeding when it reaches preset low voltage, like 3.0V.


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## tomofreno (Mar 3, 2009)

This was a duplicate post. I didn't think the first one went through as I couldn't seem to access the server and terminated the session.


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## tomofreno (Mar 3, 2009)

> You tend to accelerate often when driving, so that's when first alerts will come in, which is good, it gets you thinking how to get off road soon.


 Exactly. All you need is a warning. You don't need to know exactly how many Ah are left in the low cell, just that it is getting low, but you do have some charge left to go home or to a charge station at lower speed. The temperature compensation will be essential though to avoid false alarms and resulting complacency to alarms. If it alarms sometimes when the low cell is at 25% soc and at 15% other times, I think that's ok. I know its low. If it varies from 40% to 15%, I think that is a problem because you will start ignoring it.


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## tomofreno (Mar 3, 2009)

> Actually, there is nothing to think about, my current BMS modules can do that today. I still have a handful PCBs left, I can simply populate HVC portion of the PCB and use a trimpot for HVC,


 Hmmm, guess I can re-calibrate some of my VB's and do this. Set HVC to 2.8V. Run the car until the lowest cell gets to 2.9 or so. Probably have to do circles in the driveway to ease it the last 0.1V or so to 2.9. But hopefully only have to do this every 4 -6 months or so.


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## dimitri (May 16, 2008)

I have been pondering HVC levels for TS and SE cells. I think its not too crazy to have 3.6V HVC for both, since TS cell doesn't have much capacity over 3.5V anyway. I charge my cells to 3.8V because at the time I bought my Zivan NG3 charger this was common knowledge and Zivan voltage is set by the dealer. But now that I have experience with TS cells I can tell that "surface" charge disappears by the time I leave my driveway, so there really is no point to go over 3.6V.

I'm thinking of setting shunting at 3.5V ( optional for those who want it ) and HVC at 3.6V. This way BMS modules can be mass produced with same HVC and LVC for both SE and TS cells, which greatly simplifies production and reduces customer confusion when picking options.

Since BMS will interrupt the power to the charger upon first cell reaching HVC, it would be completely irrelevant what voltage the charger is set to. This also simplifies charger selection, feel free to buy $3000 charger or $300 charger, BMS could care less.

Any thoughts?


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## Tesseract (Sep 27, 2008)

I have limited experience with this so far, but so far it appears that whatever voltage you charge to, the resting voltage ends up around 3.65V at 25-30C when the TS cells are at full charge. Maybe you get an extra 1-2Ah pushing them up to 3.80V or more per cell, but you also have to wait at least 1 more hour to get it. I wouldn't say there is enough data to make this definitive, but it would appear that setting the balancing voltage to 3.65V and the HVC to 3.80-3.90V would be good.

LVC is still up in the air from my perspective, but 2.5V per cell seems a bit low. At least 2.6V and maybe 2.7V appears to be better.

We should be getting more data from a New Beetle that Rebirth Auto is converting in the next few days. It has 65 TS 160Ah cells in it to aim for around 100 miles of range. The first shakedown run with 60 of the cells delivered 80 miles, most of which were at 70mph on the interstate.


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## JRP3 (Mar 7, 2008)

Tesseract said:


> I wouldn't say there is enough data to make this definitive, but it would appear that setting the balancing voltage to 3.65V and the HVC to 3.80-3.90V would be good.


This seems too high for the SE cells. I think Dimitri has the right idea, 3.6 works for both.


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## dimitri (May 16, 2008)

Even resting voltage is irrelevant, the moment you start driving it drops to 3.4V, I am talking less than quarter mile distance here, so its not even 1 AH. I'm pretty sure 3.6V HVC will be perfect.

As for LVC, the way I have it figured out for now is 2.6V at 25C temp, 2.7V at 50C temp and 2.5V at 0C temp. This was a result of painstaking data collection from my pack and reports from many others. This assumes typical 3C draw for acceleration and 1C or less for cruising, anything higher just means you need a bigger pack. Obviously LVC is expected to produce alerts during acceleration at first, then alert frequency will increase, and once the buzzer is on all the time, you better be off the road.

Latest data I got from JimK suggests that 2.5V at 0C maybe a little too conservative for Canadian winters, but I think its better to be a little conservative when $10,000 investment is at stake  Afterall, if you are in Canada, you should be insulating and warming your pack anyway.


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## _GonZo_ (Mar 23, 2009)

I was thinking in order to add it to our BMS a feature that gives the user the posibility to choose the way that the batteryes are charged.

Let me explain:
You do not run your IC car with the tank always full so why you shoud do it with your EV...
For example I drive my car more less 10 miles per day, so I am not going to need that the battery is fully charged when I drive it out in the mornig, about half charge will be much more than enough.
About 2 or 3 times a month I need to drive around 40-50 miles so then here I may need full charged battery.

So the idea is that you have a selector (swich) on your dashboad that you can choose from low milage trips or high milage, then the BMS cuts down the charge lower or higher voltage (around 50-75% charge and 100% charge)
You will just have to choose the day before wich mode you left the car charging, depending on your next day plans...

The life of the packs is much longer if not fully charged every time, so I think this can help.

To


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## dimitri (May 16, 2008)

_GonZo_ said:


> The life of the packs is much longer if not fully charged every time, so I think this can help.
> 
> To


I don't understand the facts behind this statement  What is your definition of a "full charge" ? Don't confuse maximum charge voltage with "full charge". TS cells max voltage is 4.25V , but no one is charging them so high, most people don't go over 3.8V. So if I consider 3.8V or 3.6V to be a "full charge" then I am not hurting my cells in a long run. 

I would even go and say the opposite, not fully charging every day shortens their life, since datasheets have higher cycle life with higher SoC. Will you get more cycles if your pack is averaged at 50% SoC or 70% SoC? Why would I want another dash switch which I have to explain to my wife? I had enough fun explaining how to plug the charger and unplug it, my wife, like most wives I guess, isn't very technical


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## JRP3 (Mar 7, 2008)

Again I agree with Dimitri. If you don't want to charge your pack every day then don't, you don't need special settings for that, but by always having a fully charged pack you'll have shallower cycles which should increase pack life, not shorten it.


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## _GonZo_ (Mar 23, 2009)

> I don't understand the facts behind this statement  What is your definition of a "full charge" ? Don't confuse maximum charge voltage with "full charge". TS cells max voltage is 4.25V , but no one is charging them so high, most people don't go over 3.8V. So if I consider 3.8V or 3.6V to be a "full charge" then I am not hurting my cells in a long run.


Around 3,65V (3,6V for hot areas and 3,7 for colder areas) is full charge for Life cells.
4,2V is the voltage limit before you have to bin the cell. Over that voltage the cells components starts to change chemically talking and this changes are not reversable like when charging discharging.
Lukly the difference is quite high for this chemistry on other cells like Li-Ion or Pb the tolerance is much smaller a bit more and the cells can be close to fire or explosion.



> I would even go and say the opposite, not fully charging every day shortens their life, since datasheets have higher cycle life with higher SoC. Will you get more cycles if your pack is averaged at 50% SoC or 70% SoC? Why would I want another dash switch which I have to explain to my wife? I had enough fun explaining how to plug the charger and unplug it, my wife, like most wives I guess, isn't very technical


That is a good point, wifes rules  I like it.
All Lixx batteries have longer lifes if they are used on medium ranges (never fully charge, never fully discahrge) or dont you know how Tesla can manage to garatie so manny cicles on Lipoly cells? theyr BMS do not let the pack fully charge or fully discharge ever.

We did discover it years before they talked even about it...


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## dimitri (May 16, 2008)

JRP3 said:


> Again I agree with Dimitri. If you don't want to charge your pack every day then don't, you don't need special settings for that, but by always having a fully charged pack you'll have shallower cycles which should increase pack life, not shorten it.


Technically, with shallow cycles, it all comes down to miles driven on a pack, not cycles or calendar life. For example, if I drive 20 miles one day and then charge at night, and then drive 20 miles next day, I "used" 2 shallow cycles to drive 40 miles. But, if I drove 40 miles over 2 days on one deeper cycle, then I only "used" one cycle. Since you get more shallow cycles than deeper ones, it balances itself out and you end up with similar number of miles driven on a given battery pack. So at the end it doesn't matter what you do, as long as you keep it charged and not get to extreme ends of the curve, you will get certain number of miles out of the pack.

In my daily routine, sometimes I charge every night, sometimes I skip one, sometimes I charge twice a day. On rare occasions I whip out my badass 100 Amp charger and boost the pack between 2 long trips, making it possible to drive EV 120 miles in one day, but its a rare case for me.


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## GizmoEV (Nov 28, 2009)

Is there data to support the "don't fully charge" idea to extended calendar life? While following Tesla Motors early on I got the distinct impression that they had some data to support that idea. I seem to remember something about under charging by 10% increased calendar life and total pack miles driven significantly. It seems that there would be damage staying at the high end and damage staying at the low end but that there would be some maximizing point some where in the middle. We just need the data to find out. Is there any? Is it the same/similar for different Li chemistries?


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## MN Driver (Sep 29, 2009)

Regarding other chemistries, the lower your cutoff voltage within a certain reasonable voltage range with Lithium can add to total pack life. I read somewhere that with Lithium Cobalt ending with 4.1 volts instead of the standard 4.2 volts will give a longer cycle life, as will less agressive amp draws and not going below 3 volts at the end of charge, but the higher the better. Tesla Motors has a limit of 4.15 volts per cell with their pack which they say is 95% SOC and will be far better for the pack and has options to have it be at 3.8 volts per cell too if less range is needed.

This link has more info that you'd probably want about the topic of calendar life with Lithium Cobalt cells, I think that most of it likely carries over to Lithium Iron Phosphate as well.
http://www.teslamotors.com/blog2/?p=39

And a video of changing the settings:
http://www.youtube.com/watch?v=AQvRztMWjtQ

I don't agree with the 4+ volts for the Thunder Sky LiFePO4/LiFeYPO4's as being a safe number to charge to if I want a long pack life, I'm thinking 3.6 volts on a normal basis since pretty much every cell company says to end the charge at somewhere between 3.6 and 3.7 volts. I plan to oversize my pack so I can make a spontaneous trip to a friends house after work, then the post office, grocery store, and if I need to take a longer trip maybe I will charge to 3.7 with a pack that specifies that number or higher but on a regular basis I won't.


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## _GonZo_ (Mar 23, 2009)

GizmoEV said:


> Is there data to support the "don't fully charge" idea to extended calendar life? While following Tesla Motors early on I got the distinct impression that they had some data to support that idea. I seem to remember something about under charging by 10% increased calendar life and total pack miles driven significantly. It seems that there would be damage staying at the high end and damage staying at the low end but that there would be some maximizing point some where in the middle. We just need the data to find out. Is there any? Is it the same/similar for different Li chemistries?


I will try to post results of our experiments on Lipoly cells, I will need to dig because we done them years ago...
As well I can run tests on Life in order to get results, the only thing is that you will have to wait around a month.
Any way I think it will be very interesting...

I think that will open a tread about it.


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## dimitri (May 16, 2008)

Back to BMS topic 

For those interested in new BMS, but not following MiniBMS thread, I wanted to share a draft of the MiniBMS user manual. I should be ready to start production in 2-3 weeks and will be taking preliminary orders soon. I am working on a Web site for it...

Here is the doc, your feedback is appreciated...

View attachment MiniBMS User Manual.pdf


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## _GonZo_ (Mar 23, 2009)

I like it.
The most interesting part is the temperature diferentials sensing.


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## dimitri (May 16, 2008)

I have been getting some feedback regarding HVC feature via private messages, so I think I need to explain it in more details.

The idea of this affordable BMS is not to control the charger every time, although it can be done if your charger is not smart enough to have CC/CV profile. The idea is to protect individual cells from potential overcharge in case some cells reach HVC much sooner than others. I don't consider this a normal situation per say, but merely an additional protection layer.

In ideal world your charger will happily charge entire pack using proper CC/CV profile and shut off at the end before BMS will even trigger first HVC event. However, in less than ideal situation we want to be protected from damaging some cells even at the cost of incomplete charge cycle.

For example, if charger is programmed for Sky Energy cells at 3.6V x N ( where N is number of cells in the string ), then charger thinks that all cells are rising equally, which is true for most of the charge process. However, at the very end some cells tend to race other cells to the top, while charger is unaware of this race and pumps the current happily.

You can deal with it by intentionally reducing charger voltage if your charger even lets you change the voltage at will, or you can rely on BMS to protect the cell even at the cost of early charge termination. Its just a matter of picking lesser evil.

I might have made a mistake assuming that 3.6V which is good for Sky Energy is also acceptable for Thundersky cells. It is arguable that Thundersky cells at 3.6V may have few percent of capacity left unused at this HVC level.

I am willing to offer BMS with 2 selectable HVC levels at the time of order, say 3.6V for Sky Energy and 3.8V for Thundersky.

Same goes for LVC, it is not meant to be used every day as SoC gauge, its meant to protect the pack from early grave. In day to day EV use you should never encounter LVC events and you should have other means of gauging SoC level, be it a simple voltmeter or a fancy SoC display.

I hope this all makes sense, I will update the draft document accordingly.


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## dimitri (May 16, 2008)

Here is the updated user manual for MiniBMS. I added lots of new comments and corrected some data. Cell module design has been finalized and parts have been ordered for a first production batch. I am working on the Web site, planning to do a YouTube video showing MiniBMS operation in my own EV and will be taking orders probably right after New Year.

Your feedback is appreciated 

View attachment MiniBMS User Manual.pdf


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## Dave Koller (Nov 15, 2008)

dimitri said:


> Here is the updated user manual for MiniBMS. I added lots of new comments and corrected some data. Cell module design has been finalized and parts have been ordered for a first production batch. I am working on the Web site, planning to do a YouTube video showing MiniBMS operation in my own EV and will be taking orders probably right after New Year.
> 
> Your feedback is appreciated
> 
> View attachment 5244


You have supplied great drawings and adequate warning's I think you have a winner Dimitri ! You have put a monumental effort into every aspect of this BMS.. Good Job!


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## JRP3 (Mar 7, 2008)

More pictures. I like pictures.


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## dimitri (May 16, 2008)

JRP3 said:


> More pictures. I like pictures.


More pictires of what specifically? I will be adding pics of BMS installed in my car, but those pics will likely go to my Web site and/or forum, not sure they belong in a user doc. What other pics do you think belong in a general user manual document?

Thanks for comments.


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## JRP3 (Mar 7, 2008)

I guess the diagrams cover it pretty well but I was thinking of the control board wiring in the vehicle.


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## dimitri (May 16, 2008)

Got the Web Site setup for MiniBMS, here is the link, its also in my signature.

http://www.cleanpowerauto.com/MiniBMS.html

If you are planning to get MiniBMS soon, feel free to submit the form, I will not send Paypal invoice until I have production modules ready to ship.

Got my head end control board installed and working in my EV, but still waiting for some parts for cell modules, Holiday shipping schedule is a mess...


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## GizmoEV (Nov 28, 2009)

The BMS looks great! I like the simple 1 wire signal line. The BMS I'm using takes a 3-wire set.

I was wondering, if someone wanted to test, on the road, the LVC with throttle cut back with out actually running a cell to the LVC point could an extra BMS module be connected to a variable output power supply and adjust the output voltage to simulate a LVC?


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## dimitri (May 16, 2008)

GizmoEV said:


> The BMS looks great! I like the simple 1 wire signal line. The BMS I'm using takes a 3-wire set.
> 
> I was wondering, if someone wanted to test, on the road, the LVC with throttle cut back with out actually running a cell to the LVC point could an extra BMS module be connected to a variable output power supply and adjust the output voltage to simulate a LVC?


No need for such complexity, since I have NC signaling loop, you just need to bring the signaling wire with a NC switch to the cabin and flip the switch to simulate LVC event. I have covered this in the MiniBMS manual.


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## GizmoEV (Nov 28, 2009)

Of course! When you said that I realized that should have been obvious from the design. I missed that obvious point in the manual.

Maybe the name needs to be "Ultra Simple miniBMS."


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## tomofreno (Mar 3, 2009)

I didn't see the price of the shunt option, may have missed it.


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## dimitri (May 16, 2008)

tomofreno said:


> I didn't see the price of the shunt option, may have missed it.


You didn't miss it, there is no separate price for shunt option. There is very little cost difference to add shunting parts, so no sense to make it complicated, one price for all.


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## tomofreno (Mar 3, 2009)

What a deal! Almost feel I should order it with the shunt option now.
Looks very good Dimitri.


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## dimitri (May 16, 2008)

Here are 2 new diagrams for charger interface with those chargers which have BMS input.

This is based on documentation, not first hand experience, but should work as described.

Please note that apparently not all Elcon ( aka Chinoz ) PFC chargers come with red/black BMS input wires, so if yours doesn't have it, then use the original diagram with AC relay. You should know if your charger has the BMS input since you wouldn't be able to use the charger until you apply voltage on red/black wires 

I will add these to the MiniBMS user manual as well.


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## Travdude (May 11, 2009)

Will your head end board work with Voltblochers?


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## dimitri (May 16, 2008)

Travdude said:


> Will your head end board work with Voltblochers?


Unfortunately no, since VB uses NO ( normally open ) signaling bus and MiniBMS uses NC ( normally closed ) signaling bus. You could add one transistor in between VBs and my head end board to invert the signal, then it could work. Diagram of my head end board is published in MiniBMS thread, if you are familiar with electronic circuits you can figure it out.

Hope this helps.


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## cycleguy (Oct 7, 2009)

Dimitri,

Great work, nice job! I am interested in purchasing a system for my Headway 30s10p pack. I think the 3.6V HVC is fine, but I'm worried the short 2-4 second LVC delay is not long enough. I will be pushing my pack to 10C regularly, these cells will sag to 2.5V- 2.6V at 10C at a full charge. I'm wondering how difficult it would be to increase the LVC delay to 8-10 seconds. This will prevent LVC warning under load and be a more useful indicator of "at rest" conditions.


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## dimitri (May 16, 2008)

cycleguy said:


> Dimitri,
> 
> Great work, nice job! I am interested in purchasing a system for my Headway 30s10p pack. I think the 3.6V HVC is fine, but I'm worried the short 2-4 second LVC delay is not long enough. I will be pushing my pack to 10C regularly, these cells will sag to 2.5V- 2.6V at 10C at a full charge. I'm wondering how difficult it would be to increase the LVC delay to 8-10 seconds. This will prevent LVC warning under load and be a more useful indicator of "at rest" conditions.


Very simple, just swap C1 capacitor on the head end board with double or triple capacity and make fine adjustments with a trimpot.

Since you are not using prismatic cells, are you planning on centralized BMS location? My boards come in 4x4 matrix for 16 modules per board, so you'd need 2 boards and will have 2 spare modules unused.

When you order MiniBMS, just put in comments that you want longer LVC delay and I will make it happen.


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## cycleguy (Oct 7, 2009)

dimitri said:


> Very simple, just swap C1 capacitor on the head end board with double or triple capacity and make fine adjustments with a trimpot.
> 
> Since you are not using prismatic cells, are you planning on centralized BMS location? My boards come in 4x4 matrix for 16 modules per board, so you'd need 2 boards and will have 2 spare modules unused.
> 
> When you order MiniBMS, just put in comments that you want longer LVC delay and I will make it happen.



Yes, I am planning on a centralized BMS. This sounds great, I'll be placing my order right now.

Thanks


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## dimitri (May 16, 2008)

cycleguy said:


> Yes, I am planning on a centralized BMS. This sounds great, I'll be placing my order right now.
> 
> Thanks


As I mentioned before, I will not be ready to ship the product until after the New Year and I will not bill you until I am ready to ship, but you can submit the order to get in line anytime 

BTW, 4x4 boards are designed such that all 16 modules already connected to each other and don't require signaling connections in between, only 2 signaling connections per 4x4 board. You will need to connect your cells to BMS boards via QD tabs, but only N+1 wires are needed, since modules are already connected in series on the PCB. Hope this makes sense, I will provide better documentation for this option in a few days, once I snap some pics.

Each 4x4 board is 196mm x 130mm, I hope its not too large for your bike. They can be cut in halves or quarters, but then it would require more wiring between the boards. You have many choices. I need to know final config up front, so I can put correct amount of QD tabs on them. Its easy to add tabs later and I can throw some extra tabs in just in case you decide to cut boards later.

Thanks


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## cycleguy (Oct 7, 2009)

dimitri said:


> As I mentioned before, I will not be ready to ship the product until after the New Year and I will not bill you until I am ready to ship, but you can submit the order to get in line anytime
> 
> BTW, 4x4 boards are designed such that all 16 modules already connected to each other and don't require signaling connections in between, only 2 signaling connections per 4x4 board. You will need to connect your cells to BMS boards via QD tabs, but only N+1 wires are needed, since modules are already connected in series on the PCB. Hope this makes sense, I will provide better documentation for this option in a few days, once I snap some pics.
> 
> ...


The delivery time is fine, I won't be ready for this until some time in January anyway. I'm not sure about the configuration yet, so 2 complete boards with extra tabs would be fine, I'll have to configure these myself once I get to that point.

I'm not too sure what you mean by the N+1 wires. I'm assuming each cell board will need one + and - wire connected from each paralleled cell pack.
This would be 60 wires total for a 30s pack? I understand that the cell boards are would already be wired to each other.


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## dimitri (May 16, 2008)

cycleguy said:


> I'm not too sure what you mean by the N+1 wires. I'm assuming each cell board will need one + and - wire connected from each paralleled cell pack.
> This would be 60 wires total for a 30s pack? I understand that the cell boards are would already be wired to each other.


Since cell modules on 4x4 board are already connected to each other, you just need to connect most negative cell to (-) on most negative BMS module, then only connect (+) from each paralleled cell packs to BMS modules, there is no need to connect adjacent (-) since they are connected on the board already. So, you need 31 wires from your pack to BMS, which is N+1 . Don't worry, it will be clear once I have pics posted and more info.


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## racunniff (Jan 14, 2009)

dimitri said:


> As I mentioned before, I will not be ready to ship the product until after the New Year and I will not bill you until I am ready to ship, but you can submit the order to get in line anytime


I put my order in to get in line  Looking forward to a successful completion of your design.


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## EVComponents (Apr 20, 2009)

_GonZo_ said:


> I was thinking in order to add it to our BMS a feature that gives the user the posibility to choose the way that the batteryes are charged.
> 
> Let me explain:
> You do not run your IC car with the tank always full so why you shoud do it with your EV...
> ...


That is exactly how Tesla handles it. 
There is a standard mode that charges the cells to about 80%. On my Tesla that equals a range of about 190 miles.

Then there is a range mode that allows you to charge to 90%. That is about 244 miles on the screen.

Roughly here is what the two modes provide from the battery pack. low end to high end
Standard mode = 20% to 80% of the battery pack
Range mode = 10% to 90% of the battery pack

I have used range mode about twice since last July.

Tesla is giving the owner the ability to have different levels of high voltage cutoff and low voltage cutoff.

Using range mode I have an extra 25 miles of range on the top end and 25 miles of range on the bottem end of the battery pack. It warns you many times on the screen that this is not recommended for long term battery life. Every time you open the charge port it defaults back into standard mode.

Of course, the Tesla battery pack and BMS has 9 processors and costs about $30,000. So you get what you pay for. I don't see many people around here willing to pay for that.


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## _GonZo_ (Mar 23, 2009)

On Li-Ion cells the spam life of the battery is longer if used with partial cycles, but with LiFe batteries still not well defined or demostrated.
I am running a tests on LiFe cells in order to see how they perform on full cycles or partial cycles, and if it may worth do the same system on LiFe cells.
You can see about the test here:
http://www.diyelectriccar.com/forums/showthread.php/lifepo4-life-tests-39439.html
Only around 200 cycles have been elapsed (100 on each cell) so still no noticiable results.


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## tomofreno (Mar 3, 2009)

> That is exactly how Tesla handles it.


 Well, no it is not. He wanted half charge, or a variety of choices, not a choice between 80 and 90%. You don't need 9 processors to do this. Just adjust the voltage limit on your charger.


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## JRP3 (Mar 7, 2008)

_GonZo_ said:


> On Li-Ion cells the spam life of the battery is longer if used with partial cycles, but with LiFe batteries still not well defined or demostrated.


All of the information from the battery manufacturers clearly shows longer cycle life with shallower cycles. TS shows 3000 cycles at 80% and 4000 cycles at 70%. Other cells show similar trends.


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## EVComponents (Apr 20, 2009)

tomofreno said:


> Well, no it is not. He wanted half charge, or a variety of choices, not a choice between 80 and 90%. You don't need 9 processors to do this. Just adjust the voltage limit on your charger.


Tesla also has a mode that stops charging at 50%. It is called "storage" mode. That mode is designed if you are going away for a few weeks and don't want to leave your battery pack at 80% standard mode. You just leave the Tesla plugged in, click storage mode on the screen, then it keeps the battery pack temp at the optimal level and draws enough juice to keep it right at 50%.

Another cool feature is that when you are in range mode, it limits your acceleration so that the car is more efficient for a long range drive. I have only used this when driving from Seattle to Portland for the Wayland Invitational last July. 200 miles and the temp was 95 degrees. I was trying to be conservative because it was a long drive. 60 mph on cruise control. But it was no problem. I had 30 miles of range left over after the trip.


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## MN Driver (Sep 29, 2009)

JRP3 said:


> All of the information from the battery manufacturers clearly shows longer cycle life with shallower cycles. TS shows 3000 cycles at 80% and 4000 cycles at 70%. Other cells show similar trends.


For the TS spec: When they changed to the 3000 at 80% DOD, the 70% DOD spec became 5000. It's the same with Lithium Cobalt, there is a certain point that might be a sweet spot. It seems the USABC tests with A123 LiFePO4 seems to point pretty much directly in the middle of SOC.

My own idea was to get a pack that would be at least 100 miles at 90% DOD and that is a rare occurrence for me but allows me to cover everything but a road trip with an electric car in my own use. This way I could drive my 30 mile work commute and charge every other day and be at less than 60% DOD for my daily charging. With a 70% DOD spec of 5000, if the specs live up to themselves I'd get over 25 years. I think there would be calendar life limitations and I've got my own doubts to the provided specs at the discharge rates we use but if I got 15 years from a pack and the replacement pack costs me less than the first one I buy, I think I'd be thrilled. If I designed my pack for 50 miles at 80% and charged every day after my 30-40 miles of daily use and got 10 years, I'd be thrilled in that case too. The only issue I have is that only 2 blocks out of my entire 15 mile each way commute aren't at slow speeds so a smaller pack, say for 30 miles at 80% DOD means a high draw situation and likely reduced cycle life.



EVComponents said:


> Tesla also has a mode that stops charging at 50%. It is called "storage" mode. That mode is designed if you are going away for a few weeks and don't want to leave your battery pack at 80% standard mode. You just leave the Tesla plugged in, click storage mode on the screen, then it keeps the battery pack temp at the optimal level and draws enough juice to keep it right at 50%.
> 
> Another cool feature is that when you are in range mode, it limits your acceleration so that the car is more efficient for a long range drive. I have only used this when driving from Seattle to Portland for the Wayland Invitational last July. 200 miles and the temp was 95 degrees. I was trying to be conservative because it was a long drive. 60 mph on cruise control. But it was no problem. I had 30 miles of range left over after the trip.


200 miles at 60 mph is excellent. I'm glad Tesla gives control over the end of charge in order to maximize life of the pack. If the Tesla pack was at 60% state of charge and it goes into storage mode, does it discharge the pack to 50% or does it simply not charge it and just maintain ideal temperature? Ideal temperature for Lithium Cobalt long term cell storage is around refrigerator temperature.


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## EVComponents (Apr 20, 2009)

MN Driver said:


> If the Tesla pack was at 60% state of charge and it goes into storage mode, does it discharge the pack to 50% or does it simply not charge it and just maintain ideal temperature? Ideal temperature for Lithium Cobalt long term cell storage is around refrigerator temperature.


It will lose about 1% every two days until it gets to 50%. 
Then it will draw electricity from the wall to maintain 50%.

I am actually out of town right now. I left my Tesla at about 70% charge, plugged in and programmed for storage mode. It will likely lose about 2% or 3% while I am gone.

The Tesla Roadster is never "off". It is always on as some basic level to maintain the battery pack within it's limits. It can heat the battery if the temp is too low. It can cool the battery if too hot.

If the outside temp is below freezing, then you plug in the Tesla, it won't start charging right away. It will warm up the battery pack slowly, then once it reaches the minimum temp for charging, it will start the charge cycle.

(My Tesla at Wayland Invitational)
http://www.youtube.com/watch?v=_b7P8pjHpgM


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## _GonZo_ (Mar 23, 2009)

EVComponents said:


> It will lose about 1% every two days until it gets to 50%.
> Then it will draw electricity from the wall to maintain 50%.
> 
> I am actually out of town right now. I left my Tesla at about 70% charge, plugged in and programmed for storage mode. It will likely lose about 2% or 3% while I am gone.
> ...


All that working around battery is because they are using Li-Ion cells that are very high capacity density but they are quite sensible to extreme temperatures and have short life in comparation to LiFePO4 cells.
The have to manage very carefully the pack in order to keep it in good shape for at least 3 years (garantie time )
As well it is sport car so lightest batteries was a must 
Another point is that when the Tesla design started no LiFePO4 cells were available in the market. I am sure now they are looking to them...


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## EVComponents (Apr 20, 2009)

_GonZo_ said:


> Another point is that when the Tesla design started no LiFePO4 cells were available in the market. I am sure now they are looking to them...


Tesla has said that they plan on using the same commodity cells in the Model S sedan. If I recall, another major auto manufacturer recently said they are doing the same. There are advantages to using cells that are common commodities and have multiple suppliers.

All of Tesla's IP is based around the battery pack design and BMS.


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## _GonZo_ (Mar 23, 2009)

EVComponents said:


> Tesla has said that they plan on using the same commodity cells in the Model S sedan. If I recall, another major auto manufacturer recently said they are doing the same. There are advantages to using cells that are common commodities and have multiple suppliers.
> 
> All of Tesla's IP is based around the battery pack design and BMS.


I agree on what you say, and obviously Tesla have to cover cost of the development of the battery system, that works and works well.
But in my point of wiew LiFePO4 cells are now better suited for EVs but it is just my opinion.


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## jorhyne (Aug 20, 2008)

_GonZo_ said:


> I agree on what you say, and obviously Tesla have to cover cost of the development of the battery system, that works and works well.
> But in my point of wiew LiFePO4 cells are now better suited for EVs but it is just my opinion.


For a high end, high performance sport car I would make the argument that LiFePO4 batteries are not more suitable. Because Tesla makes no pretenses of being a low-cost provider (maybe they'll get there someday) they can use the absolute best batteries money can buy. As much as you want to argue the merits of LiFePO4, they simply have lower discharge rates and take up more space than a quality Lithium Cobalt battery. Sure LiCo's might be a little more finicky, but when your target market can afford just about whatever they want you have the blessing of being able to use some pretty advanced and expensive technology to make a nice product.

Written by somebody whose next conversion will be using thunderskys!


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## _GonZo_ (Mar 23, 2009)

As I said on a post before I agree with you Horhyne it is a fancy sport car so light weight, hi-power is a must 

I think we should move this discussion about Tesla car batery to other thread, if we keep on going away from the subjet...

This thread is about BMS and the discussion about Tesla started talking about its BMS, les try to keep it in the line...
The over all conclusion is that Tesla has a great battery management system with a lot of features.
I propose, why we do not make a list with all of those features that we know that Tesla BMS does and others BMS does and check what may be most/least important and then make a list of minimum/maximun requirements for optimun and afordable BMS?


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## Jan (Oct 5, 2009)

Hi Dimitri,

I saw jack's Fridayshow last night about charging. Well my last night, your timezone could say something completely different at that time. But anyway, did you see it? He makes some remarkable statements about arbitrary high voltage cut offs in BMS during charging.


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## JRP3 (Mar 7, 2008)

I just watched it myself. I think his points are more applicable to high amp charging. If your charger is only putting out 15-20 amps you won't see the same voltage rise as you would at higher amperage, and when the cell hits 3.5 or so volts it's going to be closer to full than if you're charging at a higher amp rate. Ideally the HVC would trigger the charger to go into constant voltage, if it hadn't done so on it's own already. If it's a low amp charger and doesn't have a constant voltage capability and just shuts off on HVC I'm not sure it will be as much of a problem as Jack implies.


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## dimitri (May 16, 2008)

Jan said:


> Hi Dimitri,
> 
> I saw jack's Fridayshow last night about charging. Well my last night, your timezone could say something completely different at that time. But anyway, did you see it? He makes some remarkable statements about arbitrary high voltage cut offs in BMS during charging.


Can't answer it better than JRP3 just did . Yes, Jack makes lots of remarkable statements, that's how TV personalities lure you to the screen. Then its up to the audience to interpret those statements and separate facts from bullshit.

As I stated in MiniBMS manual, the idea of BMS HVC function is not to interfere with normal charging process, but serve as a safe guard in case some cells reach the top sooner than others, even at the cost of potentially incomplete charge. In real life, not on the bench with 100 Amp supply, your charger will be 95%-99% done with its charging process when first cell hits HVC, at that time charging current should be less than 5 Amps, in most chargers less than 2 Amps, so if BMS cuts off last 10-15 minutes of the process, you may have lost 0.1 Ah of your capacity, but may have protected the cell from overcharge.

Sure, when you dump 100 Amps into a cell on the bench trying to learn its behavior, then you don't need BMS to cut off your experiments.

Draw your own conclusions....


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## Jan (Oct 5, 2009)

dimitri said:


> Draw your own conclusions....


I'll try. But it is complicated stuff. What happens with the voltage above the 3,4?

I started another thread: 
http://www.diyelectriccar.com/forums/showthread.php/charging-high-voltage-39965.html


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## tomofreno (Mar 3, 2009)

When charging my pack at 15A it took less than 2 minutes for my highest voltage SE180Ah cell to go from 3.5V to 3.6V (its max), significantly longer at 5A. I would expect it to be very fast at 100A. Apparently internal resistance climbs fast when a cell gets close to its max voltage, as evidenced in the change in slope in the supplier's charge/discharge curves there, and it clearly is a function of charge current. It takes a long time for the same cell to go from 3.4 to 3.5V when charging the pack at 15A, like around an hour. I have set up the voltage limit on the charger so this cell doesn't get above 3.45V. The timer is triggered at around 119V pack voltage, when this cell is at about 3.41V. When the charger times out and stops the cell is typically at about 3.43V dynamic voltage (voltage with charge current into it), others are around 3.41V. It, and all the others typically "relax" to about 3.345V, all within +/-1 to 2mV range. Not a full charge, but there is very little energy storage beyond this point - in the entire pack - as I can only charge to the max voltage of this cell. I guesstimate maybe 600Wh from my measurements. If I used shunts and charged the other cells up to 3.5V I could store more energy than this, but I wouldn't be able to discharge it all because I would be limited in how low I drive my lowest capacity cell. 

I think the cells will last longer if they are not charged right to their max voltage each time as is done with most shunt balancing circuits - but have no data to support this. I plan to use HVC simply as a safety back up, in case something changes and another cell reaches its high voltage before the one that presently does. Ordinarily, the HVC will not be triggered. If it is, it will simply shut down the charger, my pack voltage will be a bit lower than normal, and I'll know I have to monitor cells with my dvm while charging to see which one is now higher, and adjust the charger voltage limit appropriately. If the change is small I likely won't notice it and won't care. If the charged pack voltage is unacceptably low, then I'll need to replace the cell.

Right now I have no bms (order is in for the "mini" because I may not always be able to monitor cell voltages with my dvm, and know my wife won't). I know how much Ah charge was removed from the pack from the reading on my Coulomb counting gauge (TBS), so I know how long I have to charge at xA to recharge the pack. Around 20 minutes or so before this time I saunter out with my dvm and measure that high voltage cell to ensure it is around 3.4 -3.42V, and spot check some others to ensure they are at their normal difference to that one. So far, it always is, and they always are. To me LiFePO4 cells are as simple to use as lead acid batteries - AS LONG AS YOU STAY AWAY FROM THE STEEP SLOPE PARTS OF THE CHARGE/DISCHARGE CURVE. Just size your pack to do this. So say charge until your highest voltage cell reaches 96% soc or less, and discharge to only about 30% soc, use HLVC as backup, and life is very simple.


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## dimitri (May 16, 2008)

Tom,

I think you have a typo in voltages when you say 4.43V and 4.41V...


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## tomofreno (Mar 3, 2009)

> I think you have a typo in voltages when you say 4.43V and 4.41V...


 Whoops, should be 3.43, 3.41 of course. Thanks.


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## Guest (Dec 24, 2009)

dimitri said:


> Can't answer it better than JRP3 just did . Yes, Jack makes lots of remarkable statements, that's how TV personalities lure you to the screen. Then its up to the audience to interpret those statements and separate facts from bullshit.
> 
> As I stated in MiniBMS manual, the idea of BMS HVC function is not to interfere with normal charging process, but serve as a safe guard in case some cells reach the top sooner than others, even at the cost of potentially incomplete charge. In real life, not on the bench with 100 Amp supply, your charger will be 95%-99% done with its charging process when first cell hits HVC, at that time charging current should be less than 5 Amps, in most chargers less than 2 Amps, so if BMS cuts off last 10-15 minutes of the process, you may have lost 0.1 Ah of your capacity, but may have protected the cell from overcharge.
> 
> ...


Talk about BULLSHIT. You have a goofy shunt system that has damaged numerous cars. It finally gets debunked and you IMMEDIATELY go into designing another in your quest for cash from an innocent community, and in TWO WEEKS have another BULLSHIT design on the market, and spout ENDLESSLY across the entire DIY web site one bit of misinformation after another. IF YOU CUT IT OFF at 3.6 v, there IS no constant voltage phase. And the current DOESN'T decrease. You just bulk charge to 3.6 v and quit. A MORON can see this.

Your explanation is that if it is done at a HIGHER current to demonstrate this, then the laws of physics are altered in YOUR time zone and it is all different? 

Yes, I'm trying to COUNTER the bullshit. But you can type faster than I can reply or video tape.

Jack Rickard


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## Guest (Dec 24, 2009)

tomofreno said:


> To me LiFePO4 cells are as simple to use as lead acid batteries - AS LONG AS YOU STAY AWAY FROM THE STEEP SLOPE PARTS OF THE CHARGE/DISCHARGE CURVE. Just size your pack to do this. So say charge until your highest voltage cell reaches 96% soc or less, and discharge to only about 30% soc, use HLVC as backup, and life is very simple.


Precisely so. The HLVC accomplishes very little.

Jack Rickard
http://evtv.me


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## dimitri (May 16, 2008)

jrickard said:


> Talk about BULLSHIT. You have a goofy shunt system that has damaged numerous cars. It finally gets debunked and you IMMEDIATELY go into designing another in your quest for cash from an innocent community, and in TWO WEEKS have another BULLSHIT design on the market, and spout ENDLESSLY across the entire DIY web site one bit of misinformation after another. IF YOU CUT IT OFF at 3.6 v, there IS no constant voltage phase. And the current DOESN'T decrease. You just bulk charge to 3.6 v and quit. A MORON can see this.
> 
> Your explanation is that if it is done at a HIGHER current to demonstrate this, then the laws of physics are altered in YOUR time zone and it is all different?
> 
> ...


You are truly delusional, Jack. Seek professional help. Every one of your statements is false and every response from you is predictable, take bits of info out of context and turn it around to show how superior you are. You really think people can't read between the lines?

I have been planning new BMS for many months after I saw some shortcomings of previous versions, I only started seriously investing my time and money into it recently and made it public. So far I invested more time and money in this project than I ever hope to get back, I am only driven by passion to help DIY EV community. The price of BMS modules doesn't even begin to cover the time it takes to make and test every one of them. Unlike you, I believe that BMS is essential in EV, but should not be shrouded in mystery or cost more than the battery itself, so I am on the quest to make it simple and affordable. That is why I started the poll, to see what people think on the subject. As it turns out, against your delusional remark in your last blog entry, everyone did not follow you in 8 seconds, and people were using LVC before you claimed to have invented it.

Time will tell who is who....


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## dimitri (May 16, 2008)

jrickard said:


> Precisely so. The HLVC accomplishes very little.
> 
> Jack Rickard
> http://evtv.me


HLVC accomplishes precisely that, it makes you stay away from steep slopes on both ends of the curve. True, you can do it without BMS if you oversize your pack and have discipline in driving your EV for next 10 years, but some people prefer the simple system to help them and allow less technical family members to enjoy EV as well.

Its amazing how you manage to turn your own postulates around and make them look either good or bad, whichever makes you look more superior at a time.


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## tomofreno (Mar 3, 2009)

Jack, 

There are a couple things I would like to address here:



> You have a goofy shunt system that has damaged numerous cars.


 Not to my knowledge. The Voltblochers caused discharge of some cells. In my case it was due to a faulty MCP131 voltage supervisor chip which was directly connected between the cell terminals and had high leakage current. There was no resistor in series with this chip to limit current in event of such a failure, so several of my cells were discharged. Dimitri's version of this circuit did have a resistor in series with this chip, which would have prevented this. His new design is completely different and has numerous features to protect against such failures.



> IF YOU CUT IT OFF at 3.6 v, there IS no constant voltage phase.


 He stated that 95% to 99% of the charging is done, and the current is down to less than 5A before HVC is triggered. That is the CV phase. He has said before that his system on his car is set up so that the charger is in CV phase and at low current before shunts come on. So he is in agreement with you on how to charge. 

I am of the same mind as you, that shunts are probably not required. I am also not sure what they are really doing since the voltage of cells during charging is a function of both soc and internal resistance, as your latest video nicely demonstrated. So it is not clear to me what kind of "balancing" is really going on, or that it makes much difference in cell operation at all. But I do want HLVC to warn me if something changes significantly, so I don't start chronically overcharging or discharging a cell. I prefer to intercede manually at this point to determine what has changed and decide what to do about it. I am not confident enough to make a definitive statement that balancing is not required, since I am not sure what its end result is, nor whether it has some benefit for much older cells that have significantly degraded.

I think you, Dimitri, and me are all in agreement that you should just set the voltage limit on a CC/CV charger so it can do its job charging the cells without interruption from an HLVC circuit normally. I like the clarification in your latest video on 3.4V rest voltage being full charge, charging at 0.05C to 3.6V being one way to get there, and demonstration that there is little energy storage remaining above 3.5V. We need good data and clear explanations like this to demystify LiFePO4 cells.

Tom


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## dimitri (May 16, 2008)

tomofreno said:


> We need good data and clear explanations like this to demystify LiFePO4 cells.


That is exactly my point, I have nothing against the data shown in the videos, afterall its demonstrating simple laws of physics. However, I have a problem when explanation of the data is twisted to serve one's point, depending on what point he is trying to sell in a particular video, which tends to turn 180 degrees every week. When people before Jack say that 3.6V is a good end of CV phase for Sky Energy and 3.8V is good for ThunderSky, he calls them MORONS, but when he clearly demonstrates the same exact thing on video, he is some kind of Messiah. What a hypocracy.

So, the disagreement is not really about technical terms, but their interpretation and most importantly the attitude towards fellow EVers.


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## tomofreno (Mar 3, 2009)

Give it a rest Dimitri.


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## cycleguy (Oct 7, 2009)

A Merry Christmas to everyone. Dimitri, what would it take to now create a dash display add-on battery monitor system that interfaces with the existing cell boards, or create an interface to use a Paktrakr?
I hate duplicating cell boards for lvc/hvc and now battery monitoring too.
The ultimate would be a programmable LCD display that incorporates speedo, indicator lights, odometer, and battery monitoring all in one unit


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## dimitri (May 16, 2008)

cycleguy said:


> A Merry Christmas to everyone. Dimitri, what would it take to now create a dash display add-on battery monitor system that interfaces with the existing cell boards, or create an interface to use a Paktrakr?
> I hate duplicating cell boards for lvc/hvc and now battery monitoring too.
> The ultimate would be a programmable LCD display that incorporates speedo, indicator lights, odometer, and battery monitoring all in one unit


If you want SoC display and digital battery monitoring then you would want a full scale digital BMS, which was outside of scope for inexpensive analog BMS. There are digital BMSs out there if that's what you want, for example Hardy BMS and Elithion. They have more functionality, but cost more and more difficult to install and configure. Also keep in mind heavy EMI inside EV, which tends to mess with digital signals. For example I can no longer recommend the PakTrakr to anyone after all the troubles I went thru to make mine work somewhat stable.


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## MN Driver (Sep 29, 2009)

cycleguy said:


> A Merry Christmas to everyone. Dimitri, what would it take to now create a dash display add-on battery monitor system that interfaces with the existing cell boards, or create an interface to use a Paktrakr?
> I hate duplicating cell boards for LVC/HVC and now battery monitoring too.
> The ultimate would be a programmable LCD display that incorporates speedo, indicator lights, odometer, and battery monitoring all in one unit


This exists, but you have to build it and learn a little bit about PICaxe programming while you are at it. Peter on the BVS site did this:

http://www.youtube.com/watch?v=gAaSTQ2cWrk
http://www.youtube.com/watch?v=Wgyg41GIGxU

A link to his thread is in the video. He built it because he wanted more than what was being offered by others in terms of monitoring and also wanted something that catered best to his needs. It can do pretty much everything that it seems most people would want. It displays temperature, voltage range of the cells with highest and lowest voltages, alarms for high voltage and low voltage with a trigger to shut the load or charger off, the PIC can be reprogrammed and now you can suddenly change your high or low voltage safety cutoff point if you'd like. He used to work with a van as an EV that he drove around in back in 2002(maybe earlier), and then ended up putting Lithium Ion(cobalt) cells in and used those for a number of years until he eventually sold the van. His new project involves 50 40Ah HiPower LiFePO4 cells but the BMS is flexible and can be programmed for whatever someone wants.

The disadvantage, if you consider it a disadvantage, is that you need to make sure the programming is set to suite your needs and you would need to buy all the loose parts and assemble it yourself.

In short, it's not a package solution that can be bought up by the masses and since it is digital some care needs to be taken to filter electrical interference but he has it running digital now I think he solved that issue.

If you know electronics and are willing to put the time into it, you could find the parts and solder a whole BMS together. It's the BMS that I'm considering right now because I like how it monitors everything I need and for the level of complexity, it IMO does the best job at bringing important information such as Ah used/regenerated, temperature, high/low cell voltage range all together. Is it for everyone, no. Is it overkill to do the monitoring, I don't think so, but many here might not think its worth the effort. I come from the Lithium Cobalt background (polymer form factor) where if you go over recommended charge rates or too far over voltage at any point in charging, the cells violently destruct and if you drop too low in voltage, the next time you charge them you could find either low capacity or a very unstable cell that will swell and fail. Just because a cell doesn't necessarily get hot or swell doesn't mean it isn't damaged, sometimes it takes a few dozen cycles to until you know damage was done. Same goes with charging below freezing, although I think cold charging was something that may have been solved with Lithium Iron Phosphate, with a Lithium Cobalt cell, you'll have all of your capacity but the internal resistance will rise sky high and you will have terrible voltage drop that gets exponentially worse until a cell that normally handles 2C now handles 1C for 10 minutes but 0.1C for 80% of its original capacity.

On a side note, although getting a little off topic: It appears that the mechanisms for aging of LiFePO4 based on manufacturers graphs indicate that these will likely get old and eventually be useless as a function of increased internal resistance where the voltage sag will get worse and worse at a specific load. If you don't know what I'm talking about, check out the Thunder Sky spec sheets for their LiFeYPO4 cells and compare the high rate 5C discharge chart with the 8,000 cycle chart. It gets 70% capacity at 8,000 cycles, but only if you can live with 0.5C. Not many people can live with that much sag, but then again, not many people are planning for 8,000 cycles either, it's an extreme situation, but if 5,000 cycles can still pull 3C, it will likely give you more peukert-style results with a lower voltage while its at it. ...This is why I plan to oversize my pack as I generally want to fight my future electricity thief, which I'm sure will be increased internal resistance down the line than anything else. Time will tell for sure, but manufacturers seem to follow the standard LiIon issue of reduced amp-rate capabilities. I only brought this up because it seems premature cell failures come up but not the topic of pack aging. The only Lithium solution that doesn't seem to lose its internal resistance is LiMn but the batteries that we can afford are rated at 500-1000 cycles and still cost more than LiFePO4 in an electric car sized package.

Back on topic, In short: For a complex solution, if Peter's digital do-it-yourself solution isn't the answer for you, it's probably going to be Hardy BMS or Elithion for off the shelf solutions, but I really don't know anything about those two solutions other than you generally pay the price for what an off the shelf solution provides. There are other options, many of them, in fact here is a list of over 20 with many not even on the list: http://liionbms.com/php/bms_options.php

I think Dimitri has the best bang-for-the-buck solution for something that you can just order and attach right now for battery monitoring and damage-preventing cutoffs with shunting as an option if its desired. If you know your general capacity, if you want extra monitoring to use as some sort of fuel gauge, you could go with an Ah counting device, if I'm not mistaken, the EVision does this among other things, but does it at the pack level, not cell level. Another one that someone mentioned here I think it was the TPS or TBS or something like that which was monitoring different things amongst the pack in addition to what their Curtis AC controller/HPGP(HPEVS) AC50 system's Spyglass was watching, but I don't remember what those details were off hand and I don't currently have the time to look.


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## dimitri (May 16, 2008)

I have been struggling with a wiring diagram for centralized installation, where each PCB has a matrix of 4x4 cell modules. Here is what I came up with, I don't like how busy it is with all the red lines, but its the best I came up with, trying to show the sequence of cell modules as its related to a string ( or substring ) of 16 cells. This design has minimum number of wires ( N+1 ) for centralized BMS, plus 2 signaling loop wires.

If you need more than one board, then signaling wires connect in series from one board to the next one and then meet at the head end.

Does this make sense? Any ideas on making it more readable? Or maybe this is good enough? Once finalized, I will add this diagram to the main user guide.

I am ramping up MiniBMS production process and starting to ship first orders this week, I apologize for the delay, Holidays messed up my work schedule.


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## tomofreno (Mar 3, 2009)

Makes sense. I suppose there are labels on the boards for things like the cell loop connections?


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## etischer (Jun 16, 2008)

I know I'm not Dimitri, but I am working on an all digital BMS which will be programmatically configurable and have a 2 line LCD keypad for displaying data. It will have many other features which we will reveal when the product is released. 






cycleguy said:


> Dimitri, what would it take to now create a dash display add-on battery monitor system that interfaces with the existing cell boards, or create an interface to use a Paktrakr?
> I hate duplicating cell boards for lvc/hvc and now battery monitoring too.
> The ultimate would be a programmable LCD display that incorporates speedo, indicator lights, odometer, and battery monitoring all in one unit


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## dimitri (May 16, 2008)

I updated MiniBMS User Guide with centralized wiring diagram. Updated doc is on MiniBMS Web site and also attached here.

View attachment MiniBMS User Manual.pdf


I caught up with my workload and shipped all paid orders up to date. At this point lead time is only couple of days. If you are interested, place your order on MiniBMS Web site, the link is in my signature.

Here are pics of MiniBMS installed in my EV and what 4x4 board looks like for centralized install.


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## Jan (Oct 5, 2009)

Dimitri,

Are you asking more than Elition per cel? Or am I something missing?

http://liionbms.com/php/1PR0xxxX.php#1PR0106P


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## dimitri (May 16, 2008)

Jan said:


> Dimitri,
> 
> Are you asking more than Elition per cel? Or am I something missing?
> 
> http://liionbms.com/php/1PR0xxxX.php#1PR0106P


You should compare complete cost of the system out of the door rather than individual parts. Numbers on above Web site are misleading since Elithion has significant surcharge for direct orders ( call them and ask for a quote ), which is designed to do business with resellers instead of individuals. When you buy from reseller, like EV Components, then Elithion comes to about $27 per cell, which is twice the cost of MiniBMS.

Of course Elithion has much more functionality and much more complicated install process, so its not fair to compare the two systems just by cost. Pick what works best for you.

Hope this makes sense.


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## tomofreno (Mar 3, 2009)

I don't see it stated anywhere in the manual (may be overlooking it), so am I correct in assuming the tab on the cell level boards attaches to the cell ground terminal. I like the connectors. Heavy duty! Not going to come loose. If a cell hits HVC during charging and shuts off the charger does its green LED turn off and stay off, or does it come back on when the voltage drops? Wondering if I can tell if a cell hit hvc or the charger timed out normally.


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## JRP3 (Mar 7, 2008)

From the picture it looks as if there is a + next to the red lead so I'd assume you are correct
http://www.cleanpowerauto.com/MiniBMS.html


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## roger (Jan 24, 2010)

I´ve done tests with lab supply on dimitris boards, the green LED comes back when the voltage drops. I don´t installed the system yet because batteries from evcomponents are still missing, they should arrive every day. When installation completed I will report here.


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## dimitri (May 16, 2008)

tomofreno said:


> I don't see it stated anywhere in the manual (may be overlooking it), so am I correct in assuming the tab on the cell level boards attaches to the cell ground terminal. I like the connectors. Heavy duty! Not going to come loose. If a cell hits HVC during charging and shuts off the charger does its green LED turn off and stay off, or does it come back on when the voltage drops? Wondering if I can tell if a cell hit hvc or the charger timed out normally.


Yes, tab is negative and red wire is positive, also marked on PCBs 

Green LED bounces back quickly after HVC event, so you can't tell by looking at the cells afterwards, but I use solid state AC relay to control the charger and it has a little red LED on it, so I can tell if relay is off, then HVC shut the charger, otherwise charger stopped on its own. In your case, Tom, with Manzanita Regbus input, I don't know if its possible to tell how charger was stopped. Does it really matter?


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## tomofreno (Mar 3, 2009)

> Does it really matter?


 I want to know if a cell has changed. If for some reason the capacity of a cell has changed it could be limiting the pack capacity. But now that I think about it, I would see that on the TBS gauge. It would show some negative Ah (partially discharged) rather than Full after charge. Still, I like to know what is going on. Did the charger time out as expected, or did HVC stop it?

The Manzanitas are a bit funny in that they continue to climb in voltage in the "constant voltage" phase. So you have to play around and set the voltage limit and timer so that the timer is triggered at the appropriate voltage with appropriate time for the charger to shut off when your high voltage cell is about full. If you charge at a different current setting, the above settings won't work and it may well overcharge your high voltage cell. Also, if you set the timer too long, like 2 hrs, and then charge after a discharge that requires less than 2 hours charging time (< 60Ah in my case), the timer won't shut off the charger soon enough to avoid overcharging. I do this for example after doing some shorter local trips, before going on a longer trip that requires a full charge. In this case the HVC function will likely kick in. It may also kick in if I "opportunity charge" since that will be at a different current than I use at home with 240V, so the charger may well not time out soon enough. Currently I avoid overcharging in these cases by using a HBMS - human bms - me and a dvm. I divide the TBS Ah discharged by the charge current to estimate the charge time and start checking cell voltages about 15 minutes before this time. My wife won't do this though, hence the need for an HVC function. This works very well by the way. My weakest cell always hits full within minutes of this estimated time if I have the timer set at about 35 minutes or less. If the timer is set much longer the current is throttled back more, so a bit more charging time is required.


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## JRP3 (Mar 7, 2008)

Hmmm, I never considered the different timeout needs with the Manzanita.


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## dimitri (May 16, 2008)

You should be able to rig an LED somewhere between HVC control and Regbus input so it will indicate when HVC event occurs. For example, if there is voltage between Regbus pins 1 and 2, you could put LED with appropriate resistor across those pins. When HVC relay trips, it shorts pins 1 and 2 and LED would go off.

This is just a theory, I don't know if Regbus pins can drive LED without any negative effect on charger's functions. There are many other ways to do this. You can even solder LED with resistor across the coil of HVC relay on MiniBMS board, it would work very well. Make sure LED current is limited to less than 5 mA, so it won't overload comparator's output.


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## dimitri (May 16, 2008)

I have promised a YouTube video for MiniBMS, I had troubles with my video camera, so I ended up buying a new one.

This is my first YouTube upload, I hope its not too bad, I am not very good at making videos 

http://www.youtube.com/watch?v=lOuCH8hc6V0

EDIT: I tried embedding, but can't figure out how, so I just have the link.


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## tomofreno (Mar 3, 2009)

> You should be able to rig an LED somewhere between HVC control and Regbus input so it will indicate when HVC event occurs. For example, if there is voltage between Regbus pins 1 and 2, you could put LED with appropriate resistor across those pins. When HVC relay trips, it shorts pins 1 and 2 and LED would go off.


 Yes, I thought of maybe doing something like this.

Other than that it is installed. Breaking a connection between two cell boards triggers the buzzer. Breaking a connection while charging shuts off the charger and sounds the buzzer, and breaking a connection while driving cuts back the throttle and sounds the buzzer. I wasn't expecting the buzzer for HVC, but I guess HVC and LVC are the same to the main board, its just an open condition in the loop.

All systems go! 

My controller is set up for full throttle at 3.5V. I assumed that is 3500 Ohm from the potbox (5kOhm being 5V), so paralleled a 2.5kOhm R with the potbox. It very noticeably slows the car, but I can continue driving slower. The buzzer is quite very audible.

Easy to install with just one wire loop, but man those spade clips fit tight! Very difficult to remove one. They sure won't come loose. Nice tiny surface mount LEDs. Nice small boards too. The design seems pretty bullet proof to me. I really like that the buzzer goes off if there is a loose connection - not much chance of that with those spade clips though.

Photo of 8 cells with boards:


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## tomofreno (Mar 3, 2009)

> Hmmm, I never considered the different timeout needs with the Manzanita.


 Yeah, its a pain. I wish it would just switch to constant voltage mode and hold that limit voltage until the timer times out. But it doesn't. It very slowly throttles back current, but not fast enough to keep the voltage from continuing to rise. How much more it rises depends on the timer setting and the initial charging current level.


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## Dave Koller (Nov 15, 2008)

Dimitri!

Nice video !!

It took me 3 hrs to download (my dialup) - converted it to MPG and put it on a DVD - plays nice on the TV!!

It was a good run-through of the BMS!


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