# Balancing: Who Needs It?



## mark1030 (Jul 28, 2008)

I think in theory if you don't exceed the current beyond which the cells become imbalanced, everything should stay balanced. I would think, though, that over enough time the cells would still become a little imbalanced. But maybe not enough to cause a problem.


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## JRP3 (Mar 7, 2008)

There are different levels of "BMS", some do more than others. You definitely need balancing if you are seeing out of balance conditions developing. Let's say you have one cell that's slightly lower than the others. What could happen over time is that as the pack is charged to the finish voltage the other cells get charged a little higher to make up for the weak one which remains undercharged. When driving the weak cell gets discharged more than the others, further weakening it. Eventually it will be damaged. Or maybe one of the overcharged cells will get damaged by overcharging. You need a way to keep a cell from over discharging during driving and overcharging during charging.


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## JRP3 (Mar 7, 2008)

Looking at your BMS it appears to be a Battery Monitoring System as opposed to a Battery Management System.


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## david85 (Nov 12, 2007)

Why doesn't your BMS work? Is this something that perhaps demands a refund?


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## JRP3 (Mar 7, 2008)

david85 said:


> Why doesn't your BMS work? Is this something that perhaps demands a refund?


I think his "BMS" is working fine, it's just a monitoring system not a management system.


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## Zemmo (Sep 13, 2007)

Yeah that website is misleading. They do call it a battery management system but everything they describe is just a battery monitoring system. If it was a management system they would brag about the power it can shuttle around, burn off, bypassing, or shutoff but they only talk about the monitoring features. You do need to have a battery management system of some sort on your batteries. From tests in our EAA club shows that it is the charging that needs to be regulated. I hope that helps.


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## david85 (Nov 12, 2007)

JRP3 said:


> I think his "BMS" is working fine, it's just a monitoring system not a management system.


Stuff like this really pisses me off. VERY misleading on their part.

So what are you supposed to do with that info while driving and find a cell out of ballance? pull over and send for a new battery?


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## madderscience (Jun 28, 2008)

If you already have a battery MONITORING system, then perhaps all you need to add are individual bypass regulators on each of the cells. These are fairly simple (at their simplest, a fuse, zener diode and a big resistor) such that the cells cannot exceed a certain voltage during charging. These don't help any with undervolt protection, but the existing BMS and your own discipline could prevent that, and bypass regs will allow your battery pack to balance out during charging.

Look into getting some bypass regs for your batteries.

Good Luck.


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## sailfish11 (Sep 15, 2007)

I should add that I've ordered (and paid for) the BMS, but Elite Power Systems has not yet shipped it. They've agreed to hold the order for me while I investigate and provide me with a full refund if I decide to go with another system. They also have said they will update their website to make things more clear.

When I spoke with Elite they said that their system DOES provide under and over voltage protection -- but it doesn't do balancing. Is balancing (as described in a previous post) easy/cheap to add? If so, then I may keep this system and just add the balancers.

Is there something else a BMS should do?


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## david85 (Nov 12, 2007)

Does the under and over voltage protection work on each cell or only on the full pack voltage?


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## mark1030 (Jul 28, 2008)

It monitors each cell, but the way it protects is that it is linked to the charger and won't allow the charger to damage the cells.


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## sailfish11 (Sep 15, 2007)

mark1030 said:


> It monitors each cell, but the way it protects is that it is linked to the charger and won't allow the charger to damage the cells.


How do you know this? Have you talked to the guys at Elite?


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## david85 (Nov 12, 2007)

mark1030 said:


> It monitors each cell, but the way it protects is that it is linked to the charger and won't allow the charger to damage the cells.


So basically its a charger that has a cell ballancer built in. If so, its what I have on mine, not ideal, but it does work.


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## JRP3 (Mar 7, 2008)

sailfish11 said:


> How do you know this? Have you talked to the guys at Elite?


On their charger page:


> The charging process is monitored and controlled by BMS systems (not included). It provides protection against over-voltage, over-current, over-heat, short circuit and input voltage too low.


http://elitepowersolutions.com/prod...id=67&osCsid=a8bcef8873621b73740d10cbcfc99405


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## sailfish11 (Sep 15, 2007)

david85 said:


> So basically its a charger that has a cell ballancer built in. If so, its what I have on mine, not ideal, but it does work.


I don't understand. Does this over/under voltage protection somehow perform the same function as a balancer?

Also, I saw a comment on another forum that stated that LiFePo4 batteries don't need balancing -- or least it's not as important as it is for other lithium battery chemistries. The posted didn't explain why. Can anyone comment?


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## sailfish11 (Sep 15, 2007)

JRP3 said:


> On their charger page:
> http://elitepowersolutions.com/prod...id=67&osCsid=a8bcef8873621b73740d10cbcfc99405


Right, but it doesn't specify whether it monitors each cell, or just the whole pack voltage. Or maybe that's implied due to something about the nature of chargers that I don't understand.


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## david85 (Nov 12, 2007)

sailfish11 said:


> I don't understand. Does this over/under voltage protection somehow perform the same function as a balancer?
> 
> Also, I saw a comment on another forum that stated that LiFePo4 batteries don't need balancing -- or least it's not as important as it is for other lithium battery chemistries. The posted didn't explain why. Can anyone comment?


LiFePO4 is more forgiving than older lithium chemistries, but it should still have BMS. I heard that too. Most of that info seems to be comming from LionEV and friends. I don't know if its true, but I would advise against running something that can't ballance the cells.

What I have is a charger that works normally untill the battery voltage is near max, then the charge current drops off and the charger starts to ballance each of the cells individually. It does this with a set of wires that connects the charger to the individial cells in addition to the normal + and - connections.

This means that I have overvoltage and ballance protection, but I can still over discharge the battery if I wanted, which is why I say its not ideal. In effect the BMS only works druring the charge cycle not discharge cycle, so I still need a seperate low voltage cutoff.


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## madderscience (Jun 28, 2008)

Balancing ensures all cells are at as close as possible to the same state of charge. This can be done:

during the charging phase by:

1) circuits that shunt charge current around the full batteries and allowing the single large charger to keep running and charging up the weaker ones
2) an individual, independent charger for each cell.

or:

all the time via a mechanism that shuffles charge around on the fly by using the strongest cell(s) to charge the weakest one(s) until they are all even.

Overvoltage and undervoltage protection applied per cell or per pack is intended to ensure that no cell (or the entire pack) goes out of its "happy place". Excessive charge or discharge will damage lithium cells. It is best if this is monitored per cell (perhaps as part of the balancing operation) but it can be applied to the pack as a whole, so long as you've got some kind of balancing operation going on too. 

Any smart charger does the overvoltage protection by turning itself down and off as the battery voltage comes up, however the profile/voltage will differ depending on your battery chemsitry. Said charger may or may not take feedback from individual cell battery monitoring to adjust its output on the fly depending on the complexity of the system.

Undervoltage is usually only managed by a minimum battery voltage at which controllers start cutting back to protect themselves and the battery. For example, with a 144V curtis I think it won't let the battery voltage drop below 96V under load. Of course, that is probably WAY too low to be really very effective as an under voltage safeguard.

Yes, unfortunately its rather confusiming and complimicated. (and expensimive) for the best systems.

OK its way to late for me. Off to bed.


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## sailfish11 (Sep 15, 2007)

david85 said:


> This means that I have overvoltage and ballance protection, but I can still over discharge the battery if I wanted, which is why I say its not ideal. In effect the BMS only works druring the charge cycle not discharge cycle, so I still need a seperate low voltage cutoff.


What do you use for your low voltage cutoff?


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## JRP3 (Mar 7, 2008)

sailfish11 said:


> Right, but it doesn't specify whether it monitors each cell, or just the whole pack voltage. Or maybe that's implied due to something about the nature of chargers that I don't understand.


I assume it's per cell since a whole pack voltage cutoff would not need a BMS. Any automatic charger should stop charging the pack when full voltage is reached, but without a BMS it can't tell what is happening to individual cells.


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## sailfish11 (Sep 15, 2007)

JRP3 said:


> I assume it's per cell since a whole pack voltage cutoff would not need a BMS. Any automatic charger should stop charging the pack when full voltage is reached, but without a BMS it can't tell what is happening to individual cells.


I see your logic. Maybe it's sensing 'module' voltage -- it says it comes with 10 voltage sampling modules (each monitoring up to 10 cells). I wonder if it can achieve adequate protection then, since it seems it doesn't have visibility of each cell -- just 10-cell modules.

By the way, since I've been 're-researching' chargers, does anyone have experience with the Zivan NG3 charger? I see some retailers selling for use with LiFePo4 cells, but I can't find anywhere where it says if the NG3 has this over-charging protection (and pseudo balancing). I like the characteristics of the Zivan better than the one at Elite, and it weighs 1/4 as much. Plus it's a brand with a proven track record.


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## david85 (Nov 12, 2007)

sailfish11 said:


> What do you use for your low voltage cutoff?


The car's volt meter will be my fuel gauge, so I can be the low voltage cutoff. I may also fit an alarm to the vehicle, but the real solution would be to have the motor controller cut power after a certain point. I'm in discussions with a BLDC motor builder right now, but I am not sure if his controllers can be configured for this or not. I'll have to ask next time we talk.


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## JRP3 (Mar 7, 2008)

sailfish11 said:


> I see your logic. Maybe it's sensing 'module' voltage -- it says it comes with 10 voltage sampling modules (each monitoring up to 10 cells). I wonder if it can achieve adequate protection then, since it seems it doesn't have visibility of each cell -- just 10-cell modules.


Well they do say "Voltage of each battery cell", so each monitor must have wires to each cell.


> By the way, since I've been 're-researching' chargers, does anyone have experience with the Zivan NG3 charger? I see some retailers selling for use with LiFePo4 cells, but I can't find anywhere where it says if the NG3 has this over-charging protection (and pseudo balancing). I like the characteristics of the Zivan better than the one at Elite, and it weighs 1/4 as much. Plus it's a brand with a proven track record.


I doubt it would work with the BMS however.


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## mark1030 (Jul 28, 2008)

I think questions directed at me have been answered. I have discussed the capabilities of the ElitePowerSolutions BMS with them. Hopefully they won't mind if I post a couple excerpts from our conversations:


> 4) BMS will monitor voltage of each cell and current. It will control and stop in case of over-charge and under-discharge. It will not limit current draw, an normally you need to set up motor controller for current limit. It has capability of displaying all the parameters and setting up warning limits. Our BMS does not do cell balancing.


And:


> The charging process has 2 steps: constant current (current recommendation is 0.3CA) and cnstant voltage (current will drop gradually to zero). During charging process, if an over-voltage condition is detected for an individual cell, BMS will send signal to the charger. Charging current and voltage will be automatically reduced to prevent over-charging. When under-voltage condition is detected by BMS, the signal will be fed to charger. User needs to confirm under-voltage condition and push an "recharge" button on the charger, the charger will start will low current charging until under-voltage condition disappears and resume to normal charging current.
> 
> During discharge process, the BMS will provide warning according to voltage and current limits set up. User will need to react based on BMS feedback. Currently it is not set up to shut off current when individual cell voltage is low. We are working on to provide this option. If this is what you like, we can send you this upgrade later (free of charge) when it becomes available. This also depends on whether your motor controller is able to interface with our BMS.


 
A bigger question is: Does anybody know where to get or how to make regulators or balancers for 160AH LiFePO4 cells? That's what I'm planning on using, but even the ones at hotjuiceelectric.com only go up to 100AH.
When I emailed Smartspark in August about BattEQ, I was informed they have no products that work with Lithium technologies.


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## Zemmo (Sep 13, 2007)

One of the guys in our NTEAA club is currently building a BMS which works similar to the one in question. Each of the modules can monitor N number of cells and sends that data back to a main controller through a CAN bus. Evie-Systems is going to add on more to each module to do more and more. Here is his website : http://evie-systems.com/

He isn't in full production yet, but you can watch some of his YouTube videos. They are learning a lot about the behavior of Lithiums.


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## sailfish11 (Sep 15, 2007)

Thank you to everyone for the great discussion. I feel more confident now, and I think I'll keep the BMS and charger from Elite. My controller is the Azure DMOC445, which says it provides over and under voltage limits for the batteries -- so the only remaining question is whether it will 'talk' to the BMS.

In any case, I think I've got the essentials covered.


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## sailfish11 (Sep 15, 2007)

mark1030 said:


> A bigger question is: Does anybody know where to get or how to make regulators or balancers for 160AH LiFePO4 cells? That's what I'm planning on using, but even the ones at hotjuiceelectric.com only go up to 100AH.
> When I emailed Smartspark in August about BattEQ, I was informed they have no products that work with Lithium technologies.


Check out EV Power in Australia. They make balancers that may work for you. They're only about $16 each -- although that doesn't include shipping.... Still, I've heard good things about their products.


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## JRP3 (Mar 7, 2008)

sailfish11 said:


> My controller is the Azure DMOC445, which says it provides over and under voltage limits for the batteries -- so the only remaining question is whether it will 'talk' to the BMS.


Probably not. It most likely only does over and under voltage for the whole pack voltage.


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## jbrecher (Jul 26, 2007)

I'm the person with the lithiums in NTEAA. I have them in my Fiero. There are 32 cells for a 96 volt pack. I currently have regulators on each cell which were made buy Hot Juice Electric in Austin Tx. They let each cell get to 3.65 volts and then shuttles and drops current to maintain the voltage no higher during the charge cycle. 
MOST LITHIUMS ARE DESTROYED IN THE CHARGE CYCLE! 
The cells must be balanced. It only took 6 or 8 charge cycles to get balance. Now when I charge they all come up the 3.65 volts within a few minutes of eachother.
Now we are prototyping a monitor/regulating/charge controlling system. One step at a time. We are currently doing a monitoring system as I have regulation from the Hot juice units. We will follow with the regulating.
We are not trying to misguide anyone. We are trying to be careful with my moocho dollar lithium pack and make sure of a good reliable product in the end.
Bear with us while we perfect this thing. Please ask me direct questions so I can answer things rather than folks saying we are being mis-leading. There is too much of that going on in our EV world. 
Stay in Touch.
John Brecher
President NTEAA
214-448-2911
[email protected]


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## JRP3 (Mar 7, 2008)

jbrecher said:


> They let each cell get to 3.65 volts and then shuttles and drops current to maintain the voltage no higher during the charge cycle.
> MOST LITHIUMS ARE DESTROYED IN THE CHARGE CYCLE!
> The cells must be balanced. It only took 6 or 8 charge cycles to get balance. Now when I charge they all come up the 3.65 volts within a few minutes of eachother.


I thought most lithiums went to 4.25 volts? Aren't you undercharging them? Or is it better to go lower with lithium?


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## david85 (Nov 12, 2007)

jbrecher said:


> I'm the person with the lithiums in NTEAA. I have them in my Fiero. There are 32 cells for a 96 volt pack. I currently have regulators on each cell which were made buy Hot Juice Electric in Austin Tx. They let each cell get to 3.65 volts and then shuttles and drops current to maintain the voltage no higher during the charge cycle.
> MOST LITHIUMS ARE DESTROYED IN THE CHARGE CYCLE!
> The cells must be balanced. It only took 6 or 8 charge cycles to get balance. Now when I charge they all come up the 3.65 volts within a few minutes of eachother.
> Now we are prototyping a monitor/regulating/charge controlling system. One step at a time. We are currently doing a monitoring system as I have regulation from the Hot juice units. We will follow with the regulating.
> ...


Glad to have you on the forum, John. I think I may have heard of your lithium fiero before. Did you work with TexomaEV on some projects?



JRP3 said:


> I thought most lithiums went to 4.25 volts? Aren't you undercharging them? Or is it better to go lower with lithium?


3.65V is for lifepo4 because of the lower nominal voltage than LiCoO2, which is the one that goes all the way to 4.25V. The Ballance charger that came with my chinese sample battery regulates each cell to 3.65V and I can say that it does work in much the same way as John describes. Ballance time is less with each cycle as they get closer and closer in ballance. LiCoO2 has nominal voltage between 3.6-3.7V usually and LiFePO4 is typically only 3.2V.


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## JRP3 (Mar 7, 2008)

Thundersky and some other LiFePo4 cells list 4.25 as the upper range 
http://www.thunder-sky.com/products_en.asp?fid=66&fid2=70
But I guess keeping them slightly undercharged is better for them, unlike lead acid.


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## david85 (Nov 12, 2007)

Interesting.....and those are LFP series as well, hmmm.

The documentation that I have for my batteries states that you can actually push the voltage of each cell up to 10.5V before it fries. This does not mean that you can charge up to say, 9V and expect good performance and cycle life, but maybe different manufacturers have different versions of "safe masimum" voltage. I am more comfortable with my charger maxing at 3.65V. As it is, the batteries had plenty of reserve at that charge level (slightly more than spec) and a nearly flat discharge curve.


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## dimitri (May 16, 2008)

sailfish11 said:


> By the way, since I've been 're-researching' chargers, does anyone have experience with the Zivan NG3 charger? I see some retailers selling for use with LiFePo4 cells, but I can't find anywhere where it says if the NG3 has this over-charging protection (and pseudo balancing). I like the characteristics of the Zivan better than the one at Elite, and it weighs 1/4 as much. Plus it's a brand with a proven track record.


Zivan NG3 can be set for Lithium charge profile, but it cannot monitor individual cells, only whole pack. However, it has a connector for external BMS ( simple NO/NC contacts ) which can shutoff charging when BMS detects some problem or bad condition. I believe that EV Power in Australia that was mentioned here developed BMS which balances and monitors each cell, and feeds into Zivan NG3. IMHO, this is how I want to setup my LiFePO4 pack. I haven't run numbers yet, but I think this is most cost effective and I trust Zivan and the guy at EV Power more than I trust Chinese charger and BMS which have very little technical specs published and there is so much confusion of what they do and how they work. As for discharge BMS, I'd rather buy PakTrakr because its much cheaper and has reliable record and its all local to US. They now offer Lithium support, although I think its limited to 40 sells, so that is a bummer.
I don't mean to bash Elite Power, if I was to buy LiFePo4 cells today, I would buy from them, but why can't they post detailed specs on their BMS? The info on the Web site is not cohesive at all, which doesn't help them get new customers.
I hope someone will buy their BMS and posts the results, maybe its a good product, just not well known yet...


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## dimitri (May 16, 2008)

Check this out, just found a US based BMS company with excellent Web site, tons of info, still reading thru all the pages, but looks like there are some really nice options and decent prices.

http://liionbms.com/php/index.php


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## jbrecher (Jul 26, 2007)

You folks need to watch this site for soon to come regulators and monitoring systems.
http://evie-systems.com/


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## PhantomPholly (Aug 20, 2008)

dimitri said:


> Check this out, just found a US based BMS company with excellent Web site, tons of info, still reading thru all the pages, but looks like there are some really nice options and decent prices.
> 
> http://liionbms.com/php/index.php


Awesome - love their pack calculator. Too bad prices are still so high.


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## Technologic (Jul 20, 2008)

david85 said:


> Stuff like this really pisses me off. VERY misleading on their part.
> 
> So what are you supposed to do with that info while driving and find a cell out of ballance? pull over and send for a new battery?


This is why a real over and under voltage BMS is so important Without it you can drain a battery down to 0 cutting it's life cycle, or overcharge some while undercharging others.

Anyone that claims at least a small analog circuit isn't necessary for each cell is probably trying to scam you. Last thing we need is lithium packs dying after 2000 miles 

Obviously a digital monitoring system with a central monitor for each cell is better, but not practical in DIY EVs. Over and under voltage circuits probably wouldn't cost over $5 per cell in parts for an analog circuit.... maybe $6 if you want it to cut charging of the entire pack at a certain voltage 

not sure why you didn't just use 1 or 2 larger prismatic cells though.... C rating isn't worth THAT much hassle IMO.


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## sailfish11 (Sep 15, 2007)

dimitri said:


> I don't mean to bash Elite Power, if I was to buy LiFePo4 cells today, I would buy from them, but why can't they post detailed specs on their BMS? The info on the Web site is not cohesive at all, which doesn't help them get new customers.


I've spoken with them about just that, and they agreed their website needed improvement -- they say they're going to update their website to add more info.



dimitri said:


> I hope someone will buy their BMS and posts the results, maybe its a good product, just not well known yet...


Well, that would be me -- I just ordered their BMS and charger. When paired together, I think they provide good over-voltage protection and balancing. I'll have to see about adding some sort of under voltage cutoff -- other than just watching my gauges and stopping when voltage gets too low.


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## dimitri (May 16, 2008)

Sailfish,

I can't wait to see your feedback on their BMS, I hope you will find it worth the money. Based on info and pictures I am afraid this BMS doesn't balance, only monitors cells. I bet that first cell to reach full charge will shut off the charger. To balance you must have current bypass modules on each cell or charger must have individual charging wire for each cell. According to their picture of BMS, you can see 10 cell monitor modules, which connect in series to a display unit, which goes back to the charger. It doesn't look like charger has a circuit for each cell, which means no balancing, IMHO.

I would love to be wrong if someone can tell me how this BMS can do balancing. It looks just like PakTrakr with fancy LCD screen. 

Anyway, this BMS can be a great product when combined with cell modules for balancing.


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## Technologic (Jul 20, 2008)

sailfish11 said:


> I've spoken with them about just that, and they agreed their website needed improvement -- they say they're going to update their website to add more info.
> 
> 
> 
> Well, that would be me -- I just ordered their BMS and charger. When paired together, I think they provide good over-voltage protection and balancing. I'll have to see about adding some sort of under voltage cutoff -- other than just watching my gauges and stopping when voltage gets too low.


 
Undervoltage can be more risky than overvoltages towards battery lifespan...

Undervoltage circuits can be done with just a low wattage MOSFET though.


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## dimitri (May 16, 2008)

If pack is balanced during charge then undervoltage of one cell pretty much means you ran whole pack to zero. This is no different than running out of gas in ICE car, normal people watch their gas meter and don't let it go to zero. In EV, knowing your range to avoid getting stuck in the middle of the road is pretty much given, for any battery type or chemistry, every EV has some sort of SOC meter or voltmeter. If your undervoltage protection kicks in the middle of the traffic, your damages may be worse then a couple of cells, hmm..... I'd rather watch my SOC and not let it go too low then rely on such "protection". Isn't that just common sense?


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## Zemmo (Sep 13, 2007)

I would agree with Dimitri, it would be very inconvenient to have your pack turn off while driving. An alarm would be a much better option but you would normally be watching your SOC meter anyways. If you balance the cells during the charge, that is a controlled and convenient way to balance them.


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## JRP3 (Mar 7, 2008)

Someone pointed out to me that good cells should stay fairly close to each other after being balanced, and that a good BMS cannot make up for bad cells. Makes sense to me. If they aren't balanced after charging the BMS probably can't save them.


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## david85 (Nov 12, 2007)

A 3 fold safety system would be ideal IMO. Say, a warning light at 70% DOD, alarm at 80% and automatic shut off at 90%. That should give plenty of time to get out if traffic.

This is how the setup I have works. There is one wire that goes to each connection of the battery, and when the battery is nearly charged the ballancer kicks in. There is a seperate quick connect for the ballancer function.


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## mark1030 (Jul 28, 2008)

Sailfish, it looks like you're going with the same setup I'm planning. The one question I haven't broached with Elite is the regen braking. How are you going to handle that? I figured I'd just start out easy to get feedback from the BMS to make sure I don't put the batteries at risk while I adjust the parameters on the DMOC. Maybe you have a better plan.


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## Technologic (Jul 20, 2008)

dimitri said:


> If pack is balanced during charge then undervoltage of one cell pretty much means you ran whole pack to zero.


Not necessarily... if voltage of 1 cell drops before other's (say the wires get slightly hotter than the others... say because it's longer... this will increase electrical resistance and draw more from that one cell) this could destabalize the ENTIRE pack without an undervoltage style cut off for each cell and proper designing of the wiring itself.

I can just visualize the voltage cascade if this happens without a undervoltage cut off for each cell. Voltage drops lower in 1 cell... motor draws more current, causing the voltage to continually drop, wires to heat up etc.

Now if that cell was cut off until it's cell voltage matched the other cells voltages, that would solve the issue permanently


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## david85 (Nov 12, 2007)

Sooner or later we need to come up with a list of terms to describe different levels of BMS instead of calling them all BMS.

I'm not sure if what I have should even be called a BMS. Its really just a automatic balance charger.

What Ttechnologic is describing sounds more like a real time system that works to monitor and adjust cells on the fly and not just at the minimum and maximum cell charge limits. More sophisticated, but if done right, the best way to do it.


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## Technologic (Jul 20, 2008)

david85 said:


> What Ttechnologic is describing sounds more like a real time system that works to monitor and adjust cells on the fly and not just at the minimum and maximum cell charge limits. More sophisticated, but if done right, the best way to do it.


 
Well this could be accomplished with a "dummy" analog circuit... The problem with SO many cells (the 96 the OP has) is that it's nearly guaranteed, in my opinion, that some cells will fall in and out of balance. I think an active digital BMS is the ONLY real way to stabilized such a large amount of cells for a long 2000 cycle life span 

With a few prismatic cells I don't think you need an active balancing system (though one is certainly possible with an analog system). I still really like the "cool factor" of having a fully digital system that relays all of the voltage data from every cell to a central program that displays it on an LCD. But I think that's a rather poor use of time for the DIY community. Such systems on any scale less than the 100,000 volume would be extremely cost prohibitive. I really do not think a BMS for any group of lithium packs should cost over $500 *sighs*. 

I THINK a system with say 12 larger prismatic cells could get away with imbalance in the cells themselves... obviously this would hurt range when the undervoltage cut off switched one cell out. I'd imagine this would happen very rarely though, and it could easily be wired to reset all of the undervoltage switches once charging is engaged (automatically balancing the cells again).

Anyone see any reason that kind of circuit would unbalance regularly?

At any rate that's the kind of design I intend to work on soon for the BMS of my 8kwh lithium pack I'll be starting soon. I worry a lot about the BMS/charger ruining the Lifepos' cycle life (and therefore making them a waste of money IMO). Maybe I have a tinfoil hat about this, not sure. But there seems to be a lot of "tendacies" for imbalance in multiple batts in series.


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## sailfish11 (Sep 15, 2007)

dimitri said:


> Sailfish,
> 
> I can't wait to see your feedback on their BMS, I hope you will find it worth the money. Based on info and pictures I am afraid this BMS doesn't balance, only monitors cells. I bet that first cell to reach full charge will shut off the charger. To balance you must have current bypass modules on each cell or charger must have individual charging wire for each cell. According to their picture of BMS, you can see 10 cell monitor modules, which connect in series to a display unit, which goes back to the charger. It doesn't look like charger has a circuit for each cell, which means no balancing, IMHO.
> 
> ...


I've been emailing Jennifer Wenren at Elite and she assures me that the BMS monitors the voltage for each cell (not just 10 cell modules). I don't think the BMS does balancing by itself, it accomplishes it during charging in coordination with the charger.

You (and others) raise a good point though -- it's not clear if the charger will stop charging as soon as the first cell reaches the cutoff voltage, or if it will continue to charge the other cells until all are fully charged (thereby balancing the pack). I've sent another email to Jennifer to clarify and I'll post her response.



mark1030 said:


> Sailfish, it looks like you're going with the same setup I'm planning. The one question I haven't broached with Elite is the regen braking. How are you going to handle that? I figured I'd just start out easy to get feedback from the BMS to make sure I don't put the batteries at risk while I adjust the parameters on the DMOC. Maybe you have a better plan.


For this one I have to plead ignorance. I'm paying someone else to do the conversion for me (Joe Strubhar, of Gremco, Inc.) and to some extent I'm relying on his expertise.

I've been assuming that the controller handles the regen and that it was a totally separate process from the BMS/charger. I hadn't thought about the possibility of the regen overcharging the batteries. I'll run this question by Joe and see what he thinks.

Whew, who would have thought building an EV would be so complicated!


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## sailfish11 (Sep 15, 2007)

I just got the reply from Jennifer and here's her reply:


> If one cell reachs cut off voltage, over-voltage signal will cut off the output power of the charger. The charger output is for the whole pack, and there is no individual output to each cell.


So it sounds like it will NOT do a very good job of balancing.
The monitoring system will display the voltage of each cell though, so as long as I'm paying attention, I would know when the cells are out of balance. I wonder if there's some way to re-balance the pack in that case? The system ships this Monday, so I have until then to decide if this is really the system I want (without incurring shipping charges)...

The challenger in this case is the BMS from EV-Power in Australia: http://www.ev-power.com.au/-Thundersky-Battery-Balancing-System-.html

They each cost roughly the same. EV-Power's seems to be a better system, but Elite's would have the advantage of pairing better with their charger -- assuming I still use Elite's charger.

I like Elite's charger because it is dual voltage (the website says it isn't, but Jennifer assures me it is) and seems like a solid unit. I also like Zivan's NG-3, but that is not dual voltage and isn't designed for LiFePo4 (although they claim it can be programmed to work).

Choices, choices! I'm definitely open for suggestions!


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## favguy (May 2, 2008)

I can't see how there is any decision to be made!, the elite system doesn't balance the cells, so if you buy this, you're going to have to find another way to deal with individual cells as they go out of balance! I can't imagine why anyone would want such a system, it's impotent, all it can do is watch, not act!

The Australian system will do the job properly, as a simple, effective analogue system, allowing current to bypass and "burn off" as each cell reaches fully charged voltage at the end of the charge cycle until they are all up and matched after which it will turn off charging. It also has a low voltage warning output that you can use as you please. Unless anything else comes up in the meantime that's as cost effective, it's the system I'm getting when the time comes.

There are other very sophisticated, programmable can-bus systems out there also for much more money if you're a techy!

Paul


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## dimitri (May 16, 2008)

I bet Elite charger has simple NO/NC trigger input, similar to Zivan. If you confirm it, you can hook up EV Power BMS to Elite charger.

Although they cost the same, one system does the balancing, but lacks fancy LCD, the other shows tons of data, but doesn't do anything with it, so its not a fair comparison. You basically need both, or you at least need some cheap and simple clampers on each cell for current bypass, but in your case of 96 cells, even the cheapest clampers will turn into a grand or two 

Also, to clarify my comment about 10 cell module, each module monitors 10 cells using individual wire per cell, it collects analogue voltage data from each cell, converts to digital stream and sends via serial bus to the head unit. So , yes, each cell is monitored, but less spagetty wiring is going to head unit. Its same design as in PakTrakr.


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## favguy (May 2, 2008)

Well you only "need" tons of data if thats your thing, but I would argue we all need a reliable EV and the knowledge that the very expensive cells are bieng treated OK, even if that isn't shown in glorious LCD colour, so I know what gets my vote! (IMHO of course...)


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## sailfish11 (Sep 15, 2007)

Ok, ok, I get it!  Joe says the same thing -- get the one that actually does the job, not just watches.



dimitri said:


> I bet Elite charger has simple NO/NC trigger input, similar to Zivan. If you confirm it, you can hook up EV Power BMS to Elite charger.


Dimitri, could you explain this? What does 'NO/NC' mean? I'll ask Jennifer about it.


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## favguy (May 2, 2008)

Relay switch outputs NC = Normally closed (ie, it opens a circuit when activated) NO = Normally open (ie, it closes a circuit when activated)


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## dimitri (May 16, 2008)

Normally Open/Normally Closed , basically a couple of contacts, like a switch or relay. When NO is closed, or NC is open, it triggers an action. In this case the action is to stop charging.

http://www.zivanusa.com/pdf/NG3.pdf

See page 6, AUX2 input is what BMS will drive. 

Hope this makes sense.


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## sailfish11 (Sep 15, 2007)

Thanks Guys! Maybe I'll splurge for a Manzanita Micro charger -- they accept almost any input voltage and seem pretty versatile. Does anyone have any experience/feedback on how well they work with LiFePo4 (or other lithium)?


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## Technologic (Jul 20, 2008)

dimitri said:


> each module monitors 10 cells using individual wire per cell, it collects analogue voltage data from each cell, converts to digital stream and sends via serial bus to the head unit. So , yes, each cell is monitored, but less spagetty wiring is going to head unit. Its same design as in PakTrakr.


Wouldn't it be more simple to take analog signals from each cell and route them to an analog voltage system that then cuts each one off according to it's voltage (up or down)?

PakTrakr is nice... but at $1800 it better be. You're talking about throwing in a full fledged quad-core comp system for HALF that price .... Any programmer could write code in C to display voltages... It's not worth $1800 IMO, especially when the goal here is hands off, so easy a monkey/average human (being synonyms) could run this without concern over messing things up. 

Damn things aren't even including chargers either (and chargers tend to be the most expensive to actually build into the system, as transformers and large AC/DC recitifiers aren't so cheap on the parts scale).


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## judebert (Apr 16, 2008)

I'm a programmer. While I could make a GUI that displays measured voltages, the problem is obtaining the measurements in the first place. That requires hardware that the computer doesn't necessarily have. 

I can't design hardware. I don't know what a quad-core box with touchscreen and all the voltage measuring hardware would cost. Experience shows it would probably be close to the TrackPakr cost (for subjective definitions of "close", including things like enclosure, safety features, &c).


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## jbrecher (Jul 26, 2007)

Your BMS should have a monitor on each cell like you say but the undercharge is more of a watch the gauges deal. The management part of the system that I'm using on my lithiums can see each cell's voltage which would show an undervoltage condition. If you were on a different screen on the display, it would show a voltage alarm and you could see what cell is nearing the low setting.


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## madderscience (Jun 28, 2008)

...And for what its worth, you don't need a lot of CPU horsepower for a monitoring/management system unless you are doing something totally over the top for a UI, or the box is multitasking with something else.

The homemade monitoring system I built uses a Pentium-II class 300mhz single-board computer (actually a re-purposed compaq windows terminal) that is hosting a modern linux distro for the data logging and display. The UI that my system presents provides individual battery voltage and a 24-hour pack voltage graph but doesn't include too many interactive features. It is meant to be glanced at while driving, not played with. Individual batteries each have a microcontroller driven voltage monitoring circuit that reports battery voltage back to the main box via a combination of wireless and USB stuff. The big problems you will face (hardware design aside) in measuring all the voltages of individual batteries or cells is the voltage isolation issue and finding a way to coordinate data coming from all the individual senders.

As far as choosing computer hardware, Things like number of available I/O options, ability to boot from a solid state disk of some sort, compact size, fanless design and low power consumption are far more useful for a car computer system in a car than raw CPU horsepower is.


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## Zemmo (Sep 13, 2007)

Thats in impresive DIY BMoS (Battery Monitoring System)! I looked on your website to get a general idea on what you ended up spending on your BMoS, but I didn't find a price. Could you give us an idea on what you spent on your BMoS?


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## madderscience (Jun 28, 2008)

In terms of dollars I probably spent under $500. Most of that was the retail prices of components for the individual battery senders which came out to around $20 each. I spent a LOT of time on the software and on hacking the Compaq windows terminal to make it run linux. In retrospect I could have gone with an off-the-shelf "carputer" and been done much faster but I wanted to incorporate some castoff junk into the project, as is usually the case with me.

thanks for the compliment.


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