Issues with balancing on new battery pack

pomah

10 W
Joined
Jan 2, 2016
Messages
83
Location
Gothenburg, Sweden
Hi guys, I recently built my first battery but it seems I have run into some balancing issues.

I have built using N.E.S.E modules so it is possible to reassemble it if needed, but now to my issue.

LG INR21700-M50LT cells

13s3p pack

Cells bought in december, pack assembled back then but it has been waiting for the bike until now (May)

After first charge up the final voltage was 52,12V, battery seems to function for my purpose, 1kW ebike.

After inspection I found two of the series groups to be at 4,15V and the rest to be at around 3,98V, so the pack is not balanced, or?

The bms is connected and the pack is charged over the bms, but I guess it for some reason can not balance the whole pack? But it does cut of the charge so it is alive.

So at this stage I am a bit confused and would love for some hands on tips. Thank you for your time.
 
As evidenced by the gage of the balancing wires it is my understanding that the balancing current is very small, so it may take some time.
Your 4.15 volt cells are not overcharged (4.2 max) and you are less than 2/10ths of a volt apart. There are others here that are far more knowledgeable than I, but my suggestion is to give it a few cycles to balance out.
When I am building a battery (do not have many under my belt) I put the cells into parallel for a while, days or weeks, so that they are dead nuts balanced before the build.
 
Danger? Not if the BMS is correctly functioning; it will shut off when the lowest group reaches LVC...but the controller's LVC should shut off operation before that happens. (the BMS LVC is a safety shutoff and shouldn't be depended on as the regular shutdown LVC...the controller is designed to do this job; a "48v" controller should have a 41-42v LVC, typically)

pomah said:
After inspection I found two of the series groups to be at 4,15V and the rest to be at around 3,98V, so the pack is not balanced, or?

How are the cells being measured, under what conditions?

Which groups are high?


The bms is connected and the pack is charged over the bms, but I guess it for some reason can not balance the whole pack? But it does cut of the charge so it is alive.

Is it a balancing type of BMS? They are not uncommon. If so, it won't ever balance the pack; it doesnt' have the hardware.

If it is a balancing type, how much balancing current can it shunt away from a cell group? This is usually on the order of 50mA (or less).

If the capacity difference between 3.98v and 4.15v, for your cell model per the manufacturer's spec sheet, is say, 100mAh, then for a 3p group that's 300mAh.

300mAh / 50mA = 6 hours, minimum, to balance the cells. (it will actually take longer as the charging turns on and off and low cells slowly top up and high cells are drained).
 
charge the pack at a very low rate, like 1 amp. If you fast charge at high amps the bms will always shutoff the charge as soon as the high cells reach 4.20 volts. By slow charging it gives the pack the ability bleed off the high cells and give the low cells time to catch up.

If you still have balancing issues, its usually a cell that has high IR. Just one cell being high IR will cause balancing problems. I encounter it many times when building packs before I got an IR tester.
 
jonyjoe303 said:
charge the pack at a very low rate, like 1 amp. If you fast charge at high amps the bms will always shutoff the charge as soon as the high cells reach 4.20 volts. By slow charging it gives the pack the ability bleed off the high cells and give the low cells time to catch up.

If you still have balancing issues, its usually a cell that has high IR. Just one cell being high IR will cause balancing problems. I encounter it many times when building packs before I got an IR tester.

My "Fast charging" is a meanwell that runs at 1,7A, so not that fast, but I did try a smaller charger that does around 1A.

amberwolf said:
Danger? Not if the BMS is correctly functioning; it will shut off when the lowest group reaches LVC...but the controller's LVC should shut off operation before that happens. (the BMS LVC is a safety shutoff and shouldn't be depended on as the regular shutdown LVC...the controller is designed to do this job; a "48v" controller should have a 41-42v LVC, typically)

pomah said:
After inspection I found two of the series groups to be at 4,15V and the rest to be at around 3,98V, so the pack is not balanced, or?

How are the cells being measured, under what conditions?

Which groups are high?


The bms is connected and the pack is charged over the bms, but I guess it for some reason can not balance the whole pack? But it does cut of the charge so it is alive.

Is it a balancing type of BMS? They are not uncommon. If so, it won't ever balance the pack; it doesnt' have the hardware.

If it is a balancing type, how much balancing current can it shunt away from a cell group? This is usually on the order of 50mA (or less).

If the capacity difference between 3.98v and 4.15v, for your cell model per the manufacturer's spec sheet, is say, 100mAh, then for a 3p group that's 300mAh.

300mAh / 50mA = 6 hours, minimum, to balance the cells. (it will actually take longer as the charging turns on and off and low cells slowly top up and high cells are drained).

I think you are right, I used an older BMS that I had over from another project and since this battery was put together and connected back i december and left unused for that amount of time and still is not balanced my guess is that I need to get a new BMS...
 
pomah said:
I think you are right, I used an older BMS that I had over from another project and since this battery was put together and connected back i december and left unused for that amount of time and still is not balanced my guess is that I need to get a new BMS...
The typical BMS will only balance during charging.

If you have not left it on the charger to do the balancing, for the time required to do so, it will never happen.

There are some that use a balance scheme that *could* balance all the time. You would need to look at your BMS documentation (from the manufacturer or seller, if they provide it) to find out what kind of balancing it does (if any) and under what conditions--but without that, you should assume it only balances (if at all) during charging.


Just leaving the average BMS to sit, pack unused, does not balance anything, it just allows cells to be drained by powering the BMS. (which is part of why I asked which groups were high)


I left the below quoted because there are still unanswered questions that would help us help you, if we had answers to them.


amberwolf said:
pomah said:
After inspection I found two of the series groups to be at 4,15V and the rest to be at around 3,98V, so the pack is not balanced, or?

How are the cells being measured, under what conditions?

Which groups are high?


The bms is connected and the pack is charged over the bms, but I guess it for some reason can not balance the whole pack? But it does cut of the charge so it is alive.

Is it a balancing type of BMS? They are not uncommon. If so, it won't ever balance the pack; it doesnt' have the hardware.

If it is a balancing type, how much balancing current can it shunt away from a cell group? This is usually on the order of 50mA (or less).

If the capacity difference between 3.98v and 4.15v, for your cell model per the manufacturer's spec sheet, is say, 100mAh, then for a 3p group that's 300mAh.

300mAh / 50mA = 6 hours, minimum, to balance the cells. (it will actually take longer as the charging turns on and off and low cells slowly top up and high cells are drained).
 
How are the cells being measured, under what conditions?

Which groups are high?

I took the pack put and measured group by group, I'm not sure if it was 13 and 11 or 1 and 3 that was "high"

It is interesting about the balancing you mention, but for me to understand, when the charger is "green" meaning it is "fully charged", if letting the charger to be plugged in, does the bms still balance? There is no documentation for my BMS, it is something I got a while back when I bought my first battery as a spare. Those bms have worked well for me during 2 batteris over 7 years of usage, so I assumed it would just work.
 
pomah said:
How are the cells being measured, under what conditions?

Which groups are high?

I took the pack put and measured group by group, I'm not sure if it was 13 and 11 or 1 and 3 that was "high"
That probably eliminates the power drain of operating the BMS as an imbalance cause.



It is interesting about the balancing you mention, but for me to understand, when the charger is "green" meaning it is "fully charged", if letting the charger to be plugged in, does the bms still balance? There is no documentation for my BMS, it is something I got a while back when I bought my first battery as a spare. Those bms have worked well for me during 2 batteris over 7 years of usage, so I assumed it would just work.

Unfortunatley you can't go by the charger light for whether the pack is charged or not. In theory, it means the pack has reached a "full voltage" but it doesn't mean the pack is actually done charging, because the charger and the pack do not communicate with each other (except in a very few OEM integrated systems).

So for essentially "all" ebike/scooter/etc battery/charger systems, all the charger's green light means is that the *charger* has reached the conditions the *charger* is designed to light up the green light for. It has no way to know what the state of the battery itself is.


Common ebike/scooter/etc chargers are usually designed to give a "green light" (end of charge indication) once they reach their maximum voltage *and* the current drops below whatever value was chose for that specific charger (a few mA, typically). It's not usually stated on the charger; the only one I have that I can get this information for is the Cycle Satiator from ebikes.ca, because it allows me to set that condition for each profile.

But even in that state, it is usually still outputting the max voltage, and as long as the BMS hasn't turned off the charge port FETs, it will still be allowing that minimal current flow.

This will continue to top off low cells until the BMS reads one higher than the HVC and turns off the charge port, and then it's balancers will drain any cells higher than their turn-on point. When they drop below the HVC point, (or the balancer turn-off point), the BMS will turn the charge port back on.

If the charger detects current higher than it's turn-off threshold, it should then turn back on (charging light on); and the cycle repeats until the BMS finally turns off the charge port completely after all the cells have reached a balance state so none of them drop below the HVC point again.


The exact process depends on the specifics of each BMS and charger designs, but generically, the above is the process.
 
Ok, thank you. At this stage I have tried to run the battery and after around 8ah the bms cut off, at around 40V.

I did not want to power back up or charge the battery, so I have disassembled it and I am now charging each 3p pack by itself, see the amazing invention below :)

PXL_20220517_212108884.jpg
 
So just an update on what is what.

I have recharged every 3p pack by themselves using the setup shown above, it took like 1 week for all the 13 packs.

I have assembled everything back up now and below you can see the charge curve after around 6,6Ah used of power.



So lessons learned:

Do not just install any BMS and assume it will do what you want...

Precharge all the cells before assembling them into a pack, for me it was easy to fix, but for a welded battery that would be harder...

Thank you all for all your help
 
Well it's not over yet :)

BMS cut out today again, got home disassembled the pack and found out this:

1s: 3,6V
2s: 3,6V
3s: 3,6V
4s: 3,4V
5s: 3,6V
6s: 3,4V
7s: 3,6V
8s: 3,6V
9s: 3,6V
10s: 3,6V
11s: 3,6V
12s: 3,6V
13s: 3,0V

I opened the 3p at 3.0 V and measured each cell and found out that one was at 3,6V, one at 3,2 and one at 3,1, so most likely one was not connected and it was running at only 2p.

So reinstalled and measured the voltage after each cell installation and now when I'm sure it is correctly installed connected it to the charger again... So lets see if this is the last time in a while that I have to fix the battery...
 
Hi pomah, I’m considering doing something very similar to what you did - putting together a 13s5p pack using N.E.S.E modules and LG INR21700-M50LT cells (as they are relatively affordable and available at nkon).

In Mooch’s review https://www.e-cigarette-forum.com/...Px9vGjS3m302JuB3fF_BpG6zlcQudLaro7lQmIdLsRz54 he writes that
“The top contact metal is also thinner than most cells and the “legs” coming down from it are narrow so I recommend not using this cell where there is a lot of pressure on the top contact.”

Do you think this might have been a reason one of the cells was not contacting in your pack?
 
Back
Top