# Cell Voltage Increase Abnormally When Pack Chargin



## iruraz (Sep 4, 2012)

Hi All,

I have 82 cells (in series) LiFePO4 Sinopoly 90Ah. I charge them first time but one cell 's (called "cellX") voltage increase rapidly to 3.6V, BMS (Elithion Pro) step in and closes to charger (Eltek 3kW) to balance cells. BMS try to decrease cellX voltage from 3.6V to 3.4V and this process proceeds about 1.5-2 hours. After cellX 's voltage is under 3.4V, BMS opens to charger and battery pack start to draw current but cellX is up to 3.6V very fast (it is about 3-4 minutes). And BMS closes to charger to balance again. This repeats over and over again .

I tested cellX when battery pack is under load. CellX 's voltage drop seems very normal and voltage value is near to average value during battery pack is under load (load draws up to 350A).

What could be the reason of cellX 's voltage fast increasing when it charges?

Note:
Vcell-max: 3.6V Vcell-high: 3.4V according to BMS configuration

Regards


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## Tesseract (Sep 27, 2008)

iruraz said:


> ...
> I tested cellX when battery pack is under load. CellX 's voltage drop seems very normal and voltage value is near to average value during battery pack is under load (load draws up to 350A).
> 
> What could be the reason of cellX 's voltage fast increasing when it charges?


The obvious answer: "cellX" is already fully charged, or nearly so.

Less obvious answer: despite the above statement to the contrary, "cellX" has a higher internal resistance. Carefully measure terminal voltage at two different currents (e.g. - 1C and 2C) then compare with several other cells in the pack. They should all match within a few percent.


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## iruraz (Sep 4, 2012)

@Tesseract, thanks for your reply.



Tesseract said:


> The obvious answer: "cellX" is already fully charged, or nearly so.


How can it be possible? I measured all cells voltage before installation and all of them were 3.3V . 



Tesseract said:


> Less obvious answer: despite the above statement to the contrary, "cellX" has a higher internal resistance. Carefully measure terminal voltage at two different currents (e.g. - 1C and 2C) then compare with several other cells in the pack. They should all match within a few percent.


82 cells are in the box and it is difficult to measure voltage value of cellX. As I mentioned before, when battery pack under the load BMS keeped log and I analyzed them there was no abnormal voltage drop neither cellX nor other cells. If cellX has internal resistance, there is abnormal voltage drop at cellX according to others, right?

Regards.


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## onegreenev (May 18, 2012)

3.3 volts resting is pretty much a full cell. Since you use a BMS you should top balance your cells so you know they are all at the top before you begin. Did your cells arrive sitting at 3.3 volts? Did you buy these from a private party? Usually cells don't come fully charged. My guess is you purchased used from someone who charged them then let them sit. If thats the case then ask them what they did. But you should top balance them first. Then you will be sure your system will work much better out of the gate. If you can change your end voltages to 3.55. No real need to go above that level.


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## Tesseract (Sep 27, 2008)

iruraz said:


> How can it be possible? I measured all cells voltage before installation and all of them were 3.3V .


A meaningless statement... A mere 10mV of difference in resting voltage is equivalent to a much larger difference in state of charge, so if you can't confidently report that all cells were within 10mV of each other then you probably shouldn't even bother measuring the voltage. Note that a meter might have terrible absolute accuracy but as long as it is repeatable (ie - has high relative accuracy) the data will be still be usable. For example, if you measure ten other cells and they are all within 5mV of each other but cellX is 20mV higher then you can confidently conclude that cellX is at a higher state of charge.


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## iruraz (Sep 4, 2012)

onegreenev said:


> 3.3 volts resting is pretty much a full cell. Since you use a BMS you should top balance your cells so you know they are all at the top before you begin. Did your cells arrive sitting at 3.3 volts? Did you buy these from a private party? Usually cells don't come fully charged. My guess is you purchased used from someone who charged them then let them sit. If thats the case then ask them what they did. But you should top balance them first. Then you will be sure your system will work much better out of the gate. If you can change your end voltages to 3.55. No real need to go above that level.


@onegreenev, they arrived sitting at 3.3V and as far as I know they are not private part. How can I top balance them? I mean, which voltage level should cells sit at the end of top balance? Cut-off voltage is 3.8V according to its document? 

If I discharge the pack until cells average is 2.8V and then I charge the pack until 3.6V, does any problem occur?



Tesseract said:


> A meaningless statement... A mere 10mV of difference in resting voltage is equivalent to a much larger difference in state of charge, so if you can't confidently report that all cells were within 10mV of each other then you probably shouldn't even bother measuring the voltage. Note that a meter might have terrible absolute accuracy but as long as it is repeatable (ie - has high relative accuracy) the data will be still be usable. For example, if you measure ten other cells and they are all within 5mV of each other but cellX is 20mV higher then you can confidently conclude that cellX is at a higher state of charge.


@Tesseract, you comment "cellX is already fully charged" your previous message and in your last message above you comment "higher state of charge" for cells voltage differences. And they are different status. Yes there may be voltage differences between cells within 5mV, 10mV or so on.. 

For initial charge I try to balance them with "proper way". Which way or technique should I apply for initial charging?


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## EVfun (Mar 14, 2010)

You can't really bottom balance because you have a BMS that tries to establish a top balance by design. So pulling them all down to 2.8 volts doesn't really work for you.

What I would do is use a load resistor to pull about 5 amp hours out of the high cell and them resume charging. I use a 0.2 ohm 100 watt resistor or a 0.5 ohm 50 watt resistor, both purchased from surplus places. You can also use a car headlight, though it is harder to figure out how fast it is discharging unless you have a 50 amp shunt handy. The idea is to pull some out, then go back to charging until either that cell goes up in voltage again, or the rest of the cells go up in voltage. If that cell goes up repeat the few amp hours of discharge on it and try again. When the rest of the cells go up that cell will likely stay behind. At that point you can finish charge that one cell. I find a dumb transformer and timer 6 volt car charger works well with a variac to control the input voltage. There are also inexpensive single cell LiFePO4 chargers that would work better. In a pinch you could use a car battery with a light bulb to limit current, but you gotta stay and watch it because it will overcharge if left on too long.


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## frodus (Apr 12, 2008)

Just let it cycle. The bms is doing its job. Your pack is out of balance ....that cell is at a higher soc than the others. Let it keep cycling. As long as it can shut off the charger you're fine. 

So basically if you start with an unknown pack you would need to balance first if you want to have a balanced pack.....you didn't do this....so If you just leave the bms going and charge and discharge normally, the cells will come into balance after a handful of charge/discharge cycles.


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## iruraz (Sep 4, 2012)

EVfun said:


> You can't really bottom balance because you have a BMS that tries to establish a top balance by design. So pulling them all down to 2.8 volts doesn't really work for you.
> 
> The idea is to pull some out, then go back to charging until either that cell goes up in voltage again, or the rest of the cells go up in voltage. If that cell goes up repeat the few amp hours of discharge on it and try again. When the rest of the cells go up that cell will likely stay behind. At that point you can finish charge that one cell


@EVfun thanks. I have 82 cells and they are unbalanced. I should have balanced them before discharge. If I try your methods on cellX I am sure that one of other cells have same problem like cellX because they are unbalanced.



frodus said:


> Just let it cycle. The bms is doing its job. Your pack is out of balance ....that cell is at a higher soc than the others. Let it keep cycling. As long as it can shut off the charger you're fine.
> 
> So basically if you start with an unknown pack you would need to balance first if you want to have a balanced pack.....you didn't do this....so If you just leave the bms going and charge and discharge normally, the cells will come into balance after a handful of charge/discharge cycles.


@frodus, thanks for your recommendation. You are right, I did not balance my cells because I did not know that batteries need this before discharge. If batteries cycle (charge/discharge) a few time, they will be balanced according to you message and I will try this method.

I am confused about charge level of LiFePO4, it's nominal value is 3.3V, balance value is 3.4V (BMS's default value) and maximum value is 3.6V (BMS's default value). Which value should I see after fully charge of cells, 3.3V, 3.4V or 3.6V? And does "3.4V balance value" indicate top balance? 

What should I do before discharge new battery pack if they come full of charge (3.3V)?


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## dougingraham (Jul 26, 2011)

iruraz said:


> I am confused about charge level of LiFePO4, it's nominal value is 3.3V, balance value is 3.4V (BMS's default value) and maximum value is 3.6V (BMS's default value). Which value should I see after fully charge of cells, 3.3V, 3.4V or 3.6V? And does "3.4V balance value" indicate top balance?


All BMS systems with shunt balancers are top balance systems.

If you compare two LiFePO4 cells who have been resting for a half day or more where one has a voltage of 3.3000 volts and one with a voltage of 3.3004 volts they will be out of balance by about 1%. This means that with cells of 100 AH they would be different in state of charge by 1 AH. In a car that takes 2AH per mile this would be about half a mile of driving. The resting voltage of fully charged cells is around 3.4 volts. Above this level a difference in voltage of a few thousandth doesn't mean much which is one reason why you can balance here. At voltages below 3.0 a few thousandths again doesn't have much significance. It is easy to bottom balance as well. The farther you get from 3.3 volts the more value the resting voltage has in telling you something about State Of Charge.

In your case you can just let the BMS do its job and after a few days to weeks of daily charging you will eventually have a top balanced pack. You can speed this process up by buying or borrowing a RC hobby charger that knows about LiFe type cells (often called A123 in the chargers literature) and individually charging every cell in the pack. If you do this the pack will be top balanced and the BMS won't actually need to do much of anything. If you choose to let the BMS handle it you need to listen to it when it tells you to stop driving because the cell with the lowest state of charge will be ruined if you continue driving. As the pack comes into balance your range will go up unless you have some truly weak/bad cells in the pack.




iruraz said:


> What should I do before discharge new battery pack if they come full of charge (3.3V)?


3.3V is not full. It is about half charged. And you should choose to either bottom balance which excludes use of the shunt balance portion of a BMS or top balance the cells before assembling into a pack and hooking up the BMS.


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## iruraz (Sep 4, 2012)

dougingraham said:


> All BMS systems with shunt balancers are top balance systems.
> 
> If you compare two LiFePO4 cells who have been resting for a half day or more where one has a voltage of 3.3000 volts and one with a voltage of 3.3004 volts they will be out of balance by about 1%. This means that with cells of 100 AH they would be different in state of charge by 1 AH. In a car that takes 2AH per mile this would be about half a mile of driving. The resting voltage of fully charged cells is around 3.4 volts. Above this level a difference in voltage of a few thousandth doesn't mean much which is one reason why you can balance here. At voltages below 3.0 a few thousandths again doesn't have much significance. It is easy to bottom balance as well. The farther you get from 3.3 volts the more value the resting voltage has in telling you something about State Of Charge.
> BMS.


@dougingraham, thanks for you detailed explanation.



dougingraham said:


> In your case you can just let the BMS do its job and after a few days to weeks of daily charging you will eventually have a top balanced pack. You can speed this process up by buying or borrowing a RC hobby charger that knows about LiFe type cells (often called A123 in the chargers literature) and individually charging every cell in the pack. If you do this the pack will be top balanced and the BMS won't actually need to do much of anything. If you choose to let the BMS handle it you need to listen to it when it tells you to stop driving because the cell with the lowest state of charge will be ruined if you continue driving. As the pack comes into balance your range will go up unless you have some truly weak/bad cells in the pack.


I will let the BMS do its job for balancing and I will listen BMS as you said. If there is a warning state for instance a cell's voltage drop under 2,8V or 2,7V when battery pack discharges what should I do? Should I charge battery pack and wait for BMS balancing job?

Regards.


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## iruraz (Sep 4, 2012)

dougingraham said:


> All BMS systems with shunt balancers are top balance systems.
> 
> If you compare two LiFePO4 cells who have been resting for a half day or more where one has a voltage of 3.3000 volts and one with a voltage of 3.3004 volts they will be out of balance by about 1%. This means that with cells of 100 AH they would be different in state of charge by 1 AH. In a car that takes 2AH per mile this would be about half a mile of driving. The resting voltage of fully charged cells is around 3.4 volts. Above this level a difference in voltage of a few thousandth doesn't mean much which is one reason why you can balance here. At voltages below 3.0 a few thousandths again doesn't have much significance. It is easy to bottom balance as well. The farther you get from 3.3 volts the more value the resting voltage has in telling you something about State Of Charge.
> 
> ...


@dougingraham, thanks. I let the BMS do its job. I will inform you about the result. But I am still confused about full charge voltage. You said that 3.3V is not full but according to @Tesseract it is fully charged:



Tesseract said:


> The obvious answer: "cellX" is already fully charged, or nearly so.


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## Tesseract (Sep 27, 2008)

iruraz said:


> @dougingraham, thanks. I let the BMS do its job. I will inform you about the result. But I am still confused about full charge voltage. You said that 3.3V is not full but according to @Tesseract it is fully charged:


*groan*

No, I did not say that 3.3V is fully charged. I said that you did not report enough significant digits of your cell voltage readings to reliably infer anything about state of charge. Period. A cell that reads 3.255V is much lower in state of charge than one that reads 3.344V, but both readings can be rounded off to 3.3V. Cell voltage is a poor proxy for state of charge (for LFP, anyway) because the discharge and charge curves are so flat. Ie - very little change in voltage for a very large change in state of charge.

Dougingraham made the same mistake as you when he said that 3.3V is half charged - you simply can't tell the state of charge with so coarse a voltage reading. You need to use a meter that can repeatedly and accurately measure out to 4 significant digits (e.g. - 3.305V) if you want to use voltage as a proxy for SOC.

Furthermore, I said that if cellX is tripping the BMS' high voltage cutoff (HVC) earlier during charging, but does not exhibit significantly higher internal resistance than the other cells, then the most obvious and simplest reason is that it began at a higher state of charge out of the box. I really don't see what is so difficult to comprehend or believe about this. These cells are made in China, after all, and thus exhibit all of the usual traits of such products: wide variations in technical specs and likely even a few duds in every box.


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## dougingraham (Jul 26, 2011)

Tesseract said:


> Dougingraham made the same mistake as you when he said that 3.3V is half charged - you simply can't tell the state of charge with so coarse a voltage reading. You need to use a meter that can repeatedly and accurately measure out to 4 significant digits (e.g. - 3.305V) if you want to use voltage as a proxy for SOC.


I know what I meant and I didn't mean that 3.3V is half charged. 3.3079 volts is about half charged is what I meant. I did some testing a couple of years ago and found that a cell at 82 degrees F with a resting voltage of 3.3076 had a state of charge of 48.94%. A cell that had a resting voltage of 3.3089 at a temperature of 93 degrees F had a state of charge of 53.02%. Temperature makes a difference to the readings and I wish I had been doing this in a temperature controlled environment. My Fluke meter displays 5 significant digits but at the time I did my testing it had been too many years since its last calibration to trust absolute accuracy.

In order to use voltage as a SOC indicator you need a meter that can display at least 5 significant digits and you need a temperature compensation formula, and you need to wait at least 12 to 24 hours to allow the cell voltage to settle. If you do all of that you can probably get a better than 1% estimate of SOC. However, with a single meter that can reliably measure 4 or more significant digits you can compare the state of charge of two cells that are at the same temperature in the same pack. A BMS does not typically have the capability of measuring to this kind of resolution because multiple A/D converters utilizing different reference voltages are present. People see a number in these things with several digits displayed and for some reason they trust them when they shouldn't.

From your described behavior I am still guessing that the one cell was delivered at a higher SOC than the others. If you have a meter that can read 4 or more significant digits you can disconnect everything from the battery (yes, the BMS as well), wait a couple of hours for everything to settle down and then measure all the voltages of all the cells with your accurate meter and then do it again to see how much drift there is. If you let it sit overnight before taking any measurements the drift will be minimal. If you just came back from a drive or took it off the charger then there can be some voltage change between the readings. If you are near half charge the numbers will be very close together and still there will be significant differences in the state of charge with tiny changes in measured voltage. In your case you want to be near full SOC when you make your measurement.

Having said all of that your issue might be the BMS or it might be you have a bad cell. Manually topping up each cell individually with a single RC hobby type charger set for the correct chemistry type will quickly get you into an initial state of balance. Waiting for the BMS, assuming it is working correctly, to do it could take days or even weeks, or if there is something wrong with it might never happen. You need to measure and understand what you are measuring.


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## Tesseract (Sep 27, 2008)

dougingraham said:


> I know what I meant and I didn't mean that 3.3V is half charged.....


Nobody likes a nitpicker, but you did write this:



dougingraham said:


> 3.3V is not full. It is about half charged.


And that seems to be all the OP got from your otherwise excellent post explaining why you need to measure cell voltage with a lot more precision than 2 significant digits if you want to assess its state of charge that way.


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## dougingraham (Jul 26, 2011)

Tesseract said:


> Nobody likes a nitpicker, but you did write this:
> 
> 
> 
> And that seems to be all the OP got from your otherwise excellent post explaining why you need to measure cell voltage with a lot more precision than 2 significant digits if you want to assess its state of charge that way.


Thanks. And all I had to do was append 3 zeros to the number to prevent the nitpick. Maybe define what the word about means here.

"It all depends on what the definition of the word is is." President Bill Clinton.


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## dougingraham (Jul 26, 2011)

iruraz said:


> I will let the BMS do its job for balancing and I will listen BMS as you said. If there is a warning state for instance a cell's voltage drop under 2,8V or 2,7V when battery pack discharges what should I do? Should I charge battery pack and wait for BMS balancing job?


I just went back and re-read a bunch of this thread. I missed answering the above.

If the BMS gives you a low cell warning indication while you are driving then stop driving the car. If you keep going you will destroy that cell. And it will happen fast. And you probably won't be happy about it.

I recommend against driving the car until you get this balance issue resolved. You have a few options.

1) You can let the BMS bring the state of charge into balance. This might take a while. Maybe weeks depending on the cell capacity, imbalance, charge current, and shunting current of the devices involved. Some cells will be held high for extended duration while the rest are brought up to similar state of charge. This is probably not the best for the cells but it is what shunt balancing BMS's do on every charge cycle although hopefully not for very long normally. If you are going to just let the BMS handle it I suggest you reduce the charge current to a level that is less than or equal to the shunting current. This will reduce the chance of damage due to long term overcharging.

2) Help the shunt balancers along by discharging manually the cell(s) that are fullest. I don't much like this suggestion since you have to manually watch what is going on and could spend a considerable amount of time fiddling around which could be spent in more productive pursuits. There is no clear path to what you need to do and it would be easy to go too far.

3) As I have said before, buy or borrow one of the RC smart chargers and charge the cells one at a time with the charger. Turn off the BMS shunt balancers while doing this as it can interfere with the charger. This is preferred because you can turn it on and work on something else until it beeps at you. Switch to the next cell and hit the start button. This will get you well top balanced with the least amount of time with not much fooling around. And you should only have to do this once unless you replace or add cells to the pack.

The Power Lab series of chargers PL6 or PL8 and the iCharger line of chargers (something like the iCharger 3010b which is what I have) can all do this if you are looking for a recommendation.

Best Wishes!


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## green_EV 2000 (May 2, 2014)

This is 90% of the normal capacity of the cells about 3.4V, 3.4V-3.6V maximum battery capacity is 10%, you can refer to our discharge curve:http://www.liyuanbattery.com.cn/En/...D=788672&CorpProductClass1_ID=81491&id=926731


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## pm_dawn (Sep 14, 2009)

@iruraz:
From looking at the picture in the first post I would urge you to check your connections and cables. The BMS reports a really high Internal Resistance for cell #80, and that is not the same cells as is reported for high voltage.
So you may have a long cable run or a loose connection on the #80 cell.
It would have been interesting to see the per cell report on Internal resistance, since it will give you an indea of where you might have bad connections or bad cells. Regarding the #76 cell being hihg in voltage I join the rest of the thread in saying that it probably is an unbalanced cell.

@Jack_winston:
The charge curve that you referr to are at 0.5C and 1C, which is far from where this application is at, iruraz stated that a 3 kw Elcon is used to charge the 82s 90ah pack giving that a max charge current to about 11A which is about 0.12C charge rate. The charging curve will look a lot flatter att that rate and the cells will be more like 95-98% when 3.400v is reached on Constant Current mode.


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## iruraz (Sep 4, 2012)

pm_dawn said:


> @iruraz:
> From looking at the picture in the first post I would urge you to check your connections and cables. The BMS reports a really high Internal Resistance for cell #80, and that is not the same cells as is reported for high voltage.
> So you may have a long cable run or a loose connection on the #80 cell.
> It would have been interesting to see the per cell report on Internal resistance, since it will give you an indea of where you might have bad connections or bad cells. Regarding the #76 cell being hihg in voltage I join the rest of the thread in saying that it probably is an unbalanced cell.


@pm_dawn, thanks. There is a warning about in BMS GUI that cell resistance values are not stable when BMS works with VS supply and cells are charged. When BMS works with VL supply I observe cell resistance and nothing seem abnormal (resistances are in the optimal range)



dougingraham said:


> 3) As I have said before, buy or borrow one of the RC smart chargers and charge the cells one at a time with the charger. Turn off the BMS shunt balancers while doing this as it can interfere with the charger. This is preferred because you can turn it on and work on something else until it beeps at you. Switch to the next cell and hit the start button. This will get you well top balanced with the least amount of time with not much fooling around. And you should only have to do this once unless you replace or add cells to the pack.
> 
> The Power Lab series of chargers PL6 or PL8 and the iCharger line of chargers (something like the iCharger 3010b which is what I have) can all do this if you are looking for a recommendation.
> 
> Best Wishes!


@dougingraham, thanks for your help and recommendations. 3rd one is the sanest for me.

@Tesseract, thanks for your explanations. I convienced about SoC and voltage level 

Regards.


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