# Lithium BMS and Charging



## Sunking (Aug 10, 2009)

Ok I am an electrical engineer with a lot of lead acid battery plant design, so I have a good grasp of that technology and charging algorithms. I also understand Lithium charging and chemistry.

What I would like to know more about is Lithium Battery Management Systems, in paticular when and where to use them.

Right off the top of my head is they are suited to be used when 2 or more parallel strings are used to keep the strings balanced. Or are they used to monitor every cell in a string, and used to charge each cell individually for maximum performance. 

THX

SK


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## dimitri (May 16, 2008)

Most basic BMS is used on each cell in series connected string to make sure charging stops when first cell reaches HVC and discharge stops when first cell reaches LVC, which may or may not be the same cell every time.

Balancing is additional function and its been controversial topic lately since there is no concrete historical data on LFP chemistry behavior over long time usage. Some prefer top balancing, some prefer bottom balancing, which is basically a method of lining up all cells either at the top voltage or bottom voltage, to get a State Of Charge reference point. Without refence point you have no idea of SoC since voltage curve in LFP is so damn flat, cell at 3.2V can be 30% SoC or 70% SoC, you have no idea.

I'm currently designing MiniBMS with goals of simplest, reliable and cost effective LFP management. See my MiniBMS thread in this section...


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## Sunking (Aug 10, 2009)

dimitri thank you for the reply. That is kind of what I was thinking of. IMO that is basically Nucking Futs. If I understand you correctly you are telling me we need a separate charger and monitor circuit for each individual cell?


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## Tesseract (Sep 27, 2008)

Sunking said:


> ...If I understand you correctly you are telling me we need a separate charger and monitor circuit for each individual cell?


Nah - just the monitoring circuit (for low & high voltage cutoff). You could use individual chargers, but that's, well... Nucking Futs!


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## JRP3 (Mar 7, 2008)

You should check out Jack's balancing video on EVTV.me. He's been running his Speedster without any balancing for about 5K miles.


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## ahambone (Jan 13, 2009)

Tesseract, it may be a little nucking futs but I don't think it's as bad as you are making it out to be.

Almost every BMS I've seen for any Li* chemistry only does LVC/HVC or balance on the parallel groups of cells. That means for a (1p) x 4s LiFePo4 there are 5 wires to the BMS and for a (5p) x 4s battery there are still only 5 wires. Parallel connected cells self balance and it's the different voltages of the cells connected in series that need to be monitored externally.

The attached diagram shows where the BMS would attach in a (4p) x 4s configuration of Li* cells.









In my car I have a (5p) x 24s pack, which gives me a 76.8v LiFePO4 battery pack. I have 25 separate wires going from my pack over to my BMS boards to monitor for low-voltage on any cell and to make sure no cell goes over high-voltage when the charger is attached. 25 wires is a lot of wires but it's entirely manageable as long as you are patient when hooking things up.

Here's a photo of my BMS board in use, where charging is almost completed. (Photo taken from before moving the BMS into the car). There's a shot of the battery pack it is managing as well. On the end of the pack are a bunch of molex connectors where the BMS connections wire up to the pack.

















It may be a lot of wires but I'm certain it's the right thing to have if you want to get the full life span out of a any Li* chemistry. 

Here's a link to the assembly instructions I used for my kit-based BMS. On page 2 or 3 you can see the circuit diagram - it's not too terrible and easily fits on one page (granted with one group that repeats per channel). No MCU or processor, everything is analog. (Same as Dimitri's and the volt blochers if I recall correctly.)

http://www.tppacks.com/documents/4-24 - Cell LiFePO4 BMS-v2.6c Kit Assembly-Test Instructions.pdf

Cheers,
--Adam


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## MN Driver (Sep 29, 2009)

Sunking said:


> ...If I understand you correctly you are telling me we need a separate charger and monitor circuit for each individual cell?


To add to what everyone else has said.

The 'top balancing' is done with a circuit that bleeds off the extra voltage through a resistive shunt and basically burns off the power through a resistor. This occurs when the charger has already ramped down in current and is nearly at the point of shutting off so the amount of wasted energy is minimal. This usually has a circuit on each cell but there are other designs where multiple cells are handled with the same circuit board with wires going to the terminals of the cells.

For a monitoring circuit, it could be done in a similar fashion with one board and wires going to each board, as long as the wires are short as possible then voltage loss over the wiring shouldn't be too much of an issue. Whatever the system is that someone is using there are concerns over wiring runs affected by electrical interference however so a digital system is more prone to problems. An individual who made his own BMS with serial communication with a master bus and slave boards would get false triggers that would shut down the connection to the pack in digital mode until the noise problem was taken care of.


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## Tesseract (Sep 27, 2008)

ahambone said:


> Tesseract, it may be a little nucking futs but I don't think it's as bad as you are making it out to be.


I agreed with Sunking that using individual chargers for each cell was Nucking Futs; individual monitoring/balancing of each cell is perfectly reasonable. Cumbersome, sure, but still reasonable. 

An interesting philosophical question is whether it better to have a centralized BMS with lots of wires radiating out to each, and that have to be hooked up in the proper sequence, or individual boards that may cost more overall but are more or less "foolproof"?


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## dimitri (May 16, 2008)

> Tesseract, it may be a little nucking futs but I don't think it's as bad as you are making it out to be.


Adam,

he was only speaking against multiple chargers, not against cell level monitoring , so it was not contradicting what you have done 

Even in LA world one charger per battery was pretty extreme, but in LFP world with 3.2V cells it would be insane.

As for BMS wires, your choice of remote BMS is well, your choice  , which is perfectly suitable for a bike with smaller pack, but in a car with large prismatic cells, there will only be 1-4 wires running across the pack, depending on which BMS you pick.

The one I am working on now will only have 1 wire, super easy to install and super reliable since any broken or loose connection will result in alarm.


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## _GonZo_ (Mar 23, 2009)

BMS is a battery management system, it can be very simple or extremely complicated...

There is different tipes of circuits for managing batery packs and different ways to do it.

They can be inside the battery pack like in the phone or computer batteries, or outside of the pack like in RC that the funtions are taken by the charger/balancer and the motor controler.

It can be as simple as a volt watcher that just burns extra current when voltage reases certain level so protects te battery from over charge like in a normal IC car battery charging system years ago.

Others that control each cell voltage and when any of the cells reaches maximun voltage cuts down the charge, as well they monitor the voltge on discharge and if any of the cell goes under the limit, shuts down the out put.
This are usually called PCM (protection circuit module) and are used on phones, computers etc.

The next level is that as comented before can be very complicate, one important point is now added that is balancing of the cells. (quite important on large packs)
Any way just to let you know there is two ways the BMS control the balancing of the cells.
One tipe is the one you describe that burns the extra power (Like the volt wathchers) by compare cells voltages. That are ususlly called resistive balancing.
And another new tipe that I am working on is the ones that redirects current from the cells that are higher on voltage to the ones that are lower. you can see more info here: http://www.diyelectriccar.com/forums/showthread.php/new-bms-technology-39249.html

As well many other features are or can be on this systems like, short cut protection, maximun current protection, temperature control, charge gauge, cycle recording, error recording etc...


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## _GonZo_ (Mar 23, 2009)

> I agreed with Sunking that using individual chargers for each cell was Nucking Futs;


We do individual charger for each cell in some systems  yes we are nuts LOL


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## ahambone (Jan 13, 2009)

dimitri said:


> he was only speaking against multiple chargers, not against cell level monitoring , so it was not contradicting what you have done


Oh I agree - for some reason I read the original question through the lens of "wires sensing every cell's voltage is nutz"; for some reason the charger per cell part of it slipped through my mind. 

A charger per cell is nutz, but I know of at least one EV car that has done it in a fashion. Not entire chargers per-se but DC-DC converters, one per cell, for a 30s pack of LiFePO4 cells. Each of the DC-DC's is tuned to charge up each cell to the same 3.7x volts and they are supplied by a common high-current 48VDC supply. That's not a fully qualified charger per cell, but serves the same purpose.

Cheers,
--Adam


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## _GonZo_ (Mar 23, 2009)

As commented we use paralel charging (that is how we call it) in some battery sistems but they areonly 4S sistems and it is done because it is the easyest and lighter way in that cases.
Actually are systems that feed especial army equipment that already include a big power suplie, and the easy way to control the charge was to do it by individual chargers so the cells were always balanced even if one of them was smaller or had some damage.
The discharge protection was currently taken by the computer transmitor so no more circutis needed to be added in order to control the battery pack.

As well I have a short of 12V transportable power suplie that has 4 small chargers inside, very practic when you do not have mains to conect.

In this cases , lets say from 2S to 6S on small packs a charger per cells is a posibility you can think, but as you say it will be nuts for big packs like yours...


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## MN Driver (Sep 29, 2009)

Here is a link of some nucking futs idea that was posted in a different thread after someone asked if they could charge all cells fully without using shunts. People responded and suggested using a number of these plugged into power strips.

http://www.all-battery.com/TenergyLiFePO4BatteryCharger01300.aspx

At 2 amps, a 100Ah pack that was exhausted would take about 50 hours to charge, but there could be additional charge inefficiencies at such a slow rate too. That idea might work for an ebike with a 10Ah pack and less cells than a car conversion but even then there are better solutions.


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## JRP3 (Mar 7, 2008)

Actually I proposed using those in conjunction with a higher amp pack charger. The high amp charger is set to an average of 3.4 V per cell or so then the individual chargers do the finish charging on the individual cells and allows top balancing. I don't think it's necessary but it should work and be more efficient and safer than shunt balancing, and fairly cheap.


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## tomofreno (Mar 3, 2009)

The night before last I charged my pack of 35 series connected SE180Ah cells. The static or rest pack voltage before charging was 113.5V, and cell voltages varied from 3.236 to 3.251V, a range of 0.015V. I charged the pack to 120.8V dynamic voltage (voltage with 30A charging current into the pack) or about 3.45V/cell. The next morning I measured the static or rest pack voltage at 116.9V, and the cell voltages varied from 3.345 to 3.352V, a range of 7mV. I think this makes sense, since the slope of the V versus Ah curve is smaller at 3.35V than at 3.2V, so a given difference in soc of cells will result in much smaller difference in voltages. 

I did not see the larger variation in "top" voltages Jack reported when he "bottom balanced". I think this is because the cell voltages he reported were dynamic, ie, the cell voltages with 99A charging current into them, and the differences were mainly due to differences in internal resistance, not capacity. I think that differences in internal resistance cannot unbalance a pack, they only affect the amount of useful energy, work, you get out of each cell. You get the same amount of charge out of each cell during discharge, just at lower energy from the cells with higher internal resistance due to more energy dissipation in those cells, and as a result, more voltage sag, so the electrons fall through a lower potential in those cells during discharge. 

So do we really care about differences in dynamic voltage of cells? Are the max charge and min discharge voltages specified by cell manufacturers static or rest voltages, or dynamic voltages? If the latter then at what charge current and what discharge current? The voltage increase during charge, and decrease ("sag") during discharge, depend on this. Or does it matter, ie, are the high and low voltage specs magic numbers that cannot be exceeded no matter what the current magnitude? What is the physical basis for these limits? Some of you have said you understand the LiFePO4 chemistry, so please tell me the physical basis for the spec. One said there are irreversible changes. What changes, and why? 

Thanks,
Tom


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## Overlander23 (Jun 15, 2009)

These little per-cell chargers may not be as crazy as everyone thinks (except for JRP3). Microchip Inc make/sell a LiFePO4 charger chip for about a buck (less in volume). 

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en543954

Multi-stage charging, etc... Wouldn't have enough juice to do the whole charge, but would work for end of charge.

Brings up the issue of top/bottom balancing, but it does seem more elegant than shunting.




JRP3 said:


> Actually I proposed using those in conjunction with a higher amp pack charger. The high amp charger is set to an average of 3.4 V per cell or so then the individual chargers do the finish charging on the individual cells and allows top balancing. I don't think it's necessary but it should work and be more efficient and safer than shunt balancing, and fairly cheap.


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## JRP3 (Mar 7, 2008)

tomofreno said:


> I charged the pack to 120.8V dynamic voltage (voltage with 30A charging current into the pack) or about 3.45V/cell. The next morning I measured the static or rest pack voltage at 116.9V, and the cell voltages varied from 3.345 to 3.352V, a range of 7mV. I think this makes sense, since the slope of the V versus Ah curve is smaller at 3.35V than at 3.2V, so a given difference in soc of cells will result in much smaller difference in voltages.
> 
> I did not see the larger variation in "top" voltages Jack reported when he "bottom balanced".


I can't remember but he may have charged to a higher per cell voltage which might take individual cells higher on the knee of the curve and show greater variation. It also might have been interesting to see your individual cell voltages just before charging stopped.


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## _GonZo_ (Mar 23, 2009)

> These little per-cell chargers may not be as crazy as everyone thinks (except for JRP3). Microchip Inc make/sell a LiFePO4 charger chip for about a buck (less in volume).
> 
> http://www.microchip.com/wwwproducts...cName=en543954


Similar to that chip is the one we use for paralel charging and wors great.
We only have one charger fail once about a couple of years ago. We were not able to determine if the chip was faulty or a condensor on the circuit.
The rest are all working perfectly.


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## MN Driver (Sep 29, 2009)

tomofreno said:


> So do we really care about differences in dynamic voltage of cells? Are the max charge and min discharge voltages specified by cell manufacturers static or rest voltages, or dynamic voltages? If the latter then at what charge current and what discharge current? The voltage increase during charge, and decrease ("sag") during discharge, depend on this. Or does it matter, ie, are the high and low voltage specs magic numbers that cannot be exceeded no matter what the current magnitude? What is the physical basis for these limits? Some of you have said you understand the LiFePO4 chemistry, so please tell me the physical basis for the spec. One said there are irreversible changes. What changes, and why?
> 
> Thanks,
> Tom


The charging shouldn't exceed the maximum voltage specified by the manufacturer of the cell, if you look at their charge charts they show that you reach the maximum charge voltage and then taper down your charge amperage from there until down to a small fraction of your original amperage, numbers like C/20 and C/24 come to mind. In a series pack this may vary a bit, especially getting beyond 3.6 volts, higher depending on the exact cell, especially with what we have seen with Sky Energy cells.

Discharging seems to be the one that most people are debating, with Lithium Cobalt, going below 2.8 volts has always been a big no-no, usually people wouldn't go below 3 volts to avoid slipping down to 2.8. Since that chemistry would produce fire with some pressure behind it, people are more careful. ...but with Lithium Iron Phosphate we have different numbers than Lithium Cobalt and whether or not the principals remain the same, I am not sure. It seems that the characteristics of the cell in most respects are the same as Lithium Cobalt with a flatter charge and discharge curve but this seems to be from a smaller difference due to a lower nominal voltage and better charge/discharge rate characteristics.

So I'm with you on that question regarding the discharge voltages, personally if a manufacturer specs 2.5 volts as their minimum I'll do what I can to not drop below it because I haven't seen anything official showing me that this 'misconception' as many have put it on this forum is truly a misconception. ...I may be a bit too careful though, but 'minimum discharge voltage' sounds me to like the lowest voltage it should reach during discharge or static. I don't see manufacturer spec sheets go below their minimums in their test charts if that says anything. ...many are doing it without reporting dead cells but who it might impact cycle life, but we don't know by how much if any at all. I'm also planning to oversize my pack so by the time I get to 2.5 volts at 3C I'm probably pretty close to the end as it is, and will likely not be pulling 3C when I know I'm running low unless I have a real reason to do so. The same way I would operate with a gasoline car, I'm not about to do a 'stop light drag race' when I'm 10 miles from a gas station and my gas gauge is past the E line.


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## Sunking (Aug 10, 2009)

Hey this is the OP, remember me?

Ok I have been reading along, and investigating Lithium Ion battery charger algorithm. After eading all this it leaves my wondering what the heck. 

The charging algorithm is very easy and simple. Apply a float voltage (4.2) charger current limited to .4 to .7C and apply it until the charge current drops to 3% of C rate. You are done at that point, and anything else it just asking for trouble.

So what the heck is all this BMS stuff for. Just from POV it just seems to adding a whole lot of complexity with lots of eggs in the basket. Is this some sort of plot to generate the repair industry when EV become mainstream?


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## JRP3 (Mar 7, 2008)

Float charging takes forever with an EV sized pack. 4.2 volts is too high and there aren't many AH's above 3.8 volts anyway. If you charge a pack to an average of 4.2 without a BMS how do you prevent some cells from going higher? The cells won't all reach the same voltage at the exact same time.
Now if you stop charging your pack at an average of 3.7 per cell it won't be a problem if some drift higher to 4 volts or so. This assumes TS cells. SE cells have a lower high voltage so you need to stop charging around 3.45 cell average or so if you aren't using a BMS. That's what I'll be doing.


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## AndyH (Jun 15, 2008)

I'll take a step or two back and comment on the view from here. 

I consider a 'battery management system' the overall plan for keeping the pack where it wants to be. This doesn't have to be a 'device' and probably should not be completely a 'device'.

The cell wants to be kept within a high and low voltge limit. It wants to be kept between a max charge and discharge limit. It wants to be kept within a comfortable temperature range. The current and voltge limits change with temperature. A complete system should take all this info account.

The old lead-acid paradigm that most all of us have stuck in our minds is completely charge the battery and keep it full. Periodically overcharge it to try to balance the cells. For longest life, only use the top 50% capacity. For batteries designed for this use, only use the top 80%. Recharge after every use. If in doubt, recharge. Is it charged yet? 

LiFePo4 is different. Most of the energy is in the 'middle' voltage range. There is very little energy at either end of the charge/discharge curve. I just ran a three-voltage comparison on a Thunder Sky cell. A full charge was considered to be 4.2V. I used 2.5V for low voltage. Next cycle was between 3.7 and 2.8. Last cycle was between 3.4 and 2.7. Filling to 3.7 gave us 98% capacity and filling to 3.4 gave us 97%.

Since we generally want to stay inside 80% capacity for longest life, I think the above shows us that it not inly isn't necessary to top balance, but it's not desired for long life. Same for 'bottom balancing' - we don't need to go there either.

Starting at the beginning, I think we should start with our performance parameters - voltage, current needs. Look at our climate and pack heat production. Do we need to heat or cool the pack?

Size the pack for the voltage and current range we need. Leve a bit of voltage room above and below - 10% on either side? That means our charger should not routinely fill the pack, and our controller's pack-level LVC should not be down at the bottom of the voltage range.

Use shunts or other cell-level balancing devices - but set the limits ABOVE the normal charging voltage. In normal use, the shunts should not activate. They're only there to protect the cell from being overcharged if the pack is out of balance. Same for cell level LVC. Make it within the safety zone for the cell, but at or below the controller's pack-level LVC. Same as the shunts - it should normally not be used, but it's there to keep us from doing something expensive. The LVC signal - like running out of gas in an ICE vehicle - should leave us on the side of the road. It's there to protect the pack from us. To get to this point, we've either run to far and ignored the volt meter and fuel gauge, or we have a problem with the pack and if we keep going we're going to destroy the pack.

I'm going to propose connecting all cells in parallel for as much time as necessary to balance the entire pack somehwere in the middle. Maybe it will require discharging all the cells to a set LVC point of maybe 3.0V. When the pack is installed, it will be somewhat 'middle balanced' - capacity will be 'ragged' top and bottom. If we've done our planning correctly, we'll get a lot of cycles without activating either the cell level LVC or the shunts.

Taking it a step further, temperature compenstated charging and discharge limits would be nice. We can either computer control everything and stick thermisters all over the place, or fall back to our climate controlled box and know that we can leave the charger on 'max current' because the cell is within it's tropical comfort zone.

I hope that's useful,
Andy


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## JRP3 (Mar 7, 2008)

AndyH said:


> I'm going to propose connecting all cells in parallel for as much time as necessary to balance the entire pack somehwere in the middle. Maybe it will require discharging all the cells to a set LVC point of maybe 3.0V. When the pack is installed, it will be somewhat 'middle balanced' - capacity will be 'ragged' top and bottom.


Actually you've just described bottom balancing. You can't really balance in the middle since as you've already pointed out the middle voltages are not indicative of the state of charge. You only get that near the ends. By the time you hit 3 volts you're starting down the steep part of the curve. Bottom balancing does make sense since it prevents you from discharging any single cell too far and damaging it. This, combined with limiting the amount of charge into and out of a cell should keep everything in a happy place. Doing no balancing at all is closer to the middle balancing you propose, and as long as you keep your charge and discharge limited should also work. This is exactly what Jack has done in his speedster with good results.


> I hope that's useful,
> Andy


Not so much actually, this has all been covered before


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## tomofreno (Mar 3, 2009)

> I can't remember but he may have charged to a higher per cell voltage which might take individual cells higher on the knee of the curve and show greater variation.


 I looked again at Jack's data for SE cells and he charged at 75A to 3.760V/cell. After 15 minutes they had relaxed to 3.3668V/cell. He then discharged at 99A. My cells relaxed to an average of around 3.348V/cell after charging to 3.45/cell (measured the following day, so likely would have been a bit higher 15 minutes after charging), so is there much to be gained by charging to higher voltage? Looks to me as if much of the added energy is being dissipated as heat in the cells due to increase in internal resistance as shown by the steep rise in voltage above about 3.5V/cell.

This data makes me think there is not a set high cell voltage to stop at. What you want to do is stop charging before that part of the curve where the slope starts increasing. I say that because I am guessing the voltage versus time or Ah slope starts increasing since all the easy to access sites in the cathode have been taken, and now lithium ions have to be forced deeper into the material, straining bonds to get to additional sites, which degrades the material over time. More work must be done, more energy expended per added unit of charge to do this, so it shows up externally as an increase in internal resistance. I expect the voltage value where this "knee" in the curve occurs will depend on the charge current magnitude. It will be higher at 75A than at 30A charging current. You will gain a bit more energy stored, but at the cost of decreased cell life.

I also looked at Jack's blog where he presented the graphs for TS cells showing "ragged bottom" after starting at 4.00V on each cell and discharging, and ragged top after starting with all cells at 2.8V and charging. Nowhere does he say these are based on actual data. It appears he assumed a distribution of cell capacities, different Ah, then calculated the time to fully discharge each at 100A discharge current. So as far as I can see there is no data to show this is what actually happens. I did not see a "ragged top" in rest voltages after charging. I may well have seen a "ragged top" if I had measured voltages while charging, but is that indicative in differences in capacity or internal resistance? If the former, why aren't the rest voltages different? We are implicitly modeling the cells as capacitors with some series resistance, but I don't think that completely captures their behavior. It is more complex. Capacitors don't "relax" in voltage.


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## AndyH (Jun 15, 2008)

JRP3 said:


> Actually you've just described bottom balancing. You can't really balance in the middle since as you've already pointed out the middle voltages are not indicative of the state of charge. You only get that near the ends. By the time you hit 3 volts you're starting down the steep part of the curve. Bottom balancing does make sense since it prevents you from discharging any single cell too far and damaging it. This, combined with limiting the amount of charge into and out of a cell should keep everything in a happy place. Doing no balancing at all is closer to the middle balancing you propose, and as long as you keep your charge and discharge limited should also work. This is exactly what Jack has done in his speedster with good results.
> Not so much actually, this has all been covered before


Brutal!  

My preferred method is to cycle the cell on one of my test units until I have the capacity, then leave it at 50% SOC. I hope you'll agree this is fairly close to the middle.  I used 3.0V in my thumbnail sketch as I figured most folks could hit that with a lamp an a VOM. Sorry - lazy.

This 'no balancing' 'ragged top and bottom' is also close to what the owners of Chinese electric scooters filled with Thunder Sky cells have been doing for a number of years. No BMS is installed, cells are installed in widely varying states of charge and the Thundersky charger is set to undercharge the pack. Even with varying states of charge and no cell-level LVC the failure rate is very low. Adding only cell-level LVC would have saved more than 95% of the failures with which I'm aware.


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## AndyH (Jun 15, 2008)

tomofreno said:


> I looked again at Jack's data for SE cells and he charged at 75A to 3.760V/cell. After 15 minutes they had relaxed to 3.3668V/cell. He then discharged at 99A. My cells relaxed to an average of around 3.348V/cell after charging to 3.45/cell (measured the following day, so likely would have been a bit higher 15 minutes after charging), so is there much to be gained by charging to higher voltage? Looks to me as if much of the added energy is being dissipated as heat in the cells due to increase in internal resistance as shown by the steep rise in voltage above about 3.5V/cell.
> 
> This data makes me think there is not a set high cell voltage to stop at. What you want to do is stop charging before that part of the curve where the slope starts increasing. I say that because I am guessing the voltage versus time or Ah slope starts increasing since all the easy to access sites in the cathode have been taken, and now lithium ions have to be forced deeper into the material, straining bonds to get to additional sites, which degrades the material over time. More work must be done, more energy expended per added unit of charge to do this, so it shows up externally as an increase in internal resistance. I expect the voltage value where this "knee" in the curve occurs will depend on the charge current magnitude. It will be higher at 75A than at 30A charging current. You will gain a bit more energy stored, but at the cost of decreased cell life.
> 
> I also looked at Jack's blog where he presented the graphs for TS cells showing "ragged bottom" after starting at 4.00V on each cell and discharging, and ragged top after starting with all cells at 2.8V and charging. Nowhere does he say these are based on actual data. It appears he assumed a distribution of cell capacities, different Ah, then calculated the time to fully discharge each at 100A discharge current. So as far as I can see there is no data to show this is what actually happens. I did not see a "ragged top" in rest voltages after charging. I may well have seen a "ragged top" if I had measured voltages while charging, but is that indicative in differences in capacity or internal resistance? If the former, why aren't the rest voltages different? We are implicitly modeling the cells as capacitors with some series resistance, but I don't think that completely captures their behavior. It is more complex. Capacitors don't "relax" in voltage.


I know that Jack uses a computerized test device to measure at least capacity of the cells he uses. I think he's using a West Mountain CBA and at least one 500W amplifier.

Yes - TS and SE cell voltage will sag after removing them from the charger. A123 cells are the only ones that I've seen that will hold a 'surface charge' until initially loaded - then they'll join the pack at somewhere around 3.3-3.4V.


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## Sunking (Aug 10, 2009)

JRP3 said:


> Float charging takes forever with an EV sized pack. 4.2 volts is too high and there aren't many AH's above 3.8 volts anyway.


Ok this is just a friendly debate for educational purposes. I disagree with that statement. We can argue about C rates rather it be .7C or 10C, or what float or finish voltage of 3.6 to 4.2 as it depends on the actual Li chemistry. But let's say I use a float type charger that fist goes in constant current of say 10C, then tapers back to .03C at some finish voltage say 3.6, 4.0, whatever. Every thing from multiple vendors say this method is the best practice which is known as Float with Current limit.

What am I missing?


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## AndyH (Jun 15, 2008)

Sunking said:


> Ok this is just a friendly debate for educational purposes. I disagree with that statement. We can argue about C rates rather it be .7C or 10C, or what float or finish voltage of 3.6 to 4.2 as it depends on the actual Li chemistry. But let's say I use a float type charger that fist goes in constant current of say 10C, then tapers back to .03C at some finish voltage say 3.6, 4.0, whatever. Every thing from multiple vendors say this method is the best practice which is known as Float with Current limit.
> 
> What am I missing?


I don't think you're missing a thing - sounds like CC/CV to me.


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## JRP3 (Mar 7, 2008)

Yeah I think it just a terminology confusion. To me a "float" charger is simply a low power charger that is left on the battery to keep it at a specfic voltage. The charge method you describe is appropriate, as long as you have a way to prevent individual cells from getting overcharged. The easiest and cheapest way to do that is to slightly undercharge the pack. People are putting a lot of time and money into some complex BMS's that probably are not necessary, in my opinion.


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## MN Driver (Sep 29, 2009)

Sunking said:


> Every thing from multiple vendors say this method is the best practice which is known as Float with Current limit.
> 
> What am I missing?


I think the confusing part about this is the term 'float'. I can't think of any vendors who use that term.
With lead acid batteries float charging takes forever because the batteries don't absorb charge as quickly when they are at 2.3 volts per cell versus 2.4 volts per cell. Float charging is normally the lower voltage trickle charge after a lead acid battery is done charging so it doesn't spend all of its time gassing and corroding the grid.

With Lithium I prefer the terms CC/CV. Which is constant (limited) current(or in most electric car cases with Lithium, can be whatever your charging infrastructure can pump into the cell), followed by constant (limited) voltage of where you don't want to go higher as to prevent damage to the cells.

Constant current, constant voltage is the preferred term. Float makes it sounds like the final cycle on a lead acid charger where it never stops trickle charging, which is bad because Lithium isn't designed to be overcharged, it is supposed to stop when it is done.


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## tomofreno (Mar 3, 2009)

Ok, now I have some data showing a "ragged top" like Jack's TS graph. I increased the time on the Manzanita timer to 55 seconds, left the volt trim (max charge voltage) where I had set it before the last charge, and turned the charger back on. The cells had been sitting since that last charge on Tuesday. Initial pack voltage was 116.5V according to the TBS, 117.0 according to my DVM at the pack terminals. Initial cell voltages in the front box were: 3.348, 43, 46, 42, 44, 42, 42, 44 (44 means 3.344). I turned on the charger, turned the current up to 30A and started measuring cell voltages, but they were steadily increasing. I measured 3.425, and 3.422 on the first two but they were increasing at a bit more than 1mV/sec, so I quit. Pack voltage read 118.6. Within less than 2 minutes (by the time I measured those first two cells) from starting the charger, the timer came on, and the charger started throttling back current. Pack voltage was 120.3V when the timer came on. Current was at 20A after 1 min, 14.9A after 5 min, 12.6A after 10 min, 11A, after 15 min. After 16 minutes pack voltage was at 121.8V, or an average of 3.48V/cell. I checked cell voltages in the front box: 3.67, 3.88, 3.76, 3.57, 3.63, 3.465, 3.547, 3.554! I shut off the charger. 

So there is Jack's ragged edge at the top. Evidently most of the cells in the other boxes are at considerably lower voltage to get a pack voltage of 121.8V. Immediately after I shut off the charger, the pack voltage was 121.0, and decreasing steadily. The cell that had been at 3.67V just before I shut off the charger, read 3.54V but was falling quickly. I am pretty confident when I measure them tomorrow they will all read less than about 3.36V. The cell that went to 3.88V had been drained by a VB to low voltage, and then over-discharged while driving to 0.753V. This cell evidently has significantly lower capacity than the others. No swelling or heating observed.


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## Sunking (Aug 10, 2009)

JRP3 said:


> Yeah I think it just a terminology confusion. To me a "float" charger is simply a low power charger that is left on the battery to keep it at a specfic voltage.


Well that is a trickle charger. 

I work with Telephone company battery plants at 24 and 48 volts up to 10,000 amps. We use float chargers. A 10Ka plant consist of many strings of 2000 Ah batteries (14) in parallel, to provide 4 hour reserve time, with 16 800 amp rectifiers @ 110% current limit, operating in float mode at 54.2 volts.


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## JRP3 (Mar 7, 2008)

Sunking said:


> Well that is a trickle charger.


 This wiki differentiates float and trickle by noting the float charger is automatic and can remain connected, so I guess any charger that automatically regulates itself is technically a float charger, regardless of output.
http://en.wikipedia.org/wiki/Trickle_charging


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## Sunking (Aug 10, 2009)

JRP3 said:


> This wiki differentiates float and trickle by noting the float charger is automatic and can remain connected, so I guess any charger that automatically regulates itself is technically a float charger, regardless of output.
> http://en.wikipedia.org/wiki/Trickle_charging


Will WIKI is not my bible. . A float charger by industry standard is a constant precision voltage power supply that is current limited capable of fully charging a battery from 0% to 100% in 24 hours or less. Some come with an Equalization feature with a time limit of 24 hours. These types are used in emergency DC plants like a UPS or Telecom battery plant. Not something you buy at Wally World or Auto World. 

A trickle charger is a toy you buy at Wally World, that supplies a C rate of about .01C or less, aka battery maintainer to over come self discharge and keep a battery topped of but incapable of charging a battery in any meaningful time frame.


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## RagnarLodbrok (Jun 20, 2021)

you can check for modular and plug'n play system






Battery Management Systems







volrad.com.tr


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