Battery balance issue

Starting a thread for this issue. Here is a recap of whats been going on: And in case you don’t wan’t to read all of that shit and just wan’t to know the problem, here you go: My battery is charging to 42V but not all of my cells are getting balanced to 4.2V. B1 is charging to 4.17V and B2-B10 to 4.2V-4.21V. The rest of the info can be found at the bottom of the post :slight_smile:

Here are the possible things that can cause the problem:

  • BMS
  • Bad cell
  • Charger
  • Anything else?

Reply to @b264’s post: How am I able to check if the BMS is not working? Is it correct that there should be some light in the BMS when it’s on? If so then there is nothing. Can I charge my battery safely and still ride my board if the BMS is not working? I bought the BMS (bestech d140) from @thisguyhere so he might be able to help.

The way my battery charges is rising each cell voltage with 0.01 at a time. B1 hits 4.17V the same time B2-B10 hits 4.2V,some even 4.21V, and the battery is now at 42V and fully charged. When leaving the charger on after it has turned green B1 rises to 4.2V after about 24 hours at least and B2-B10 reaches 4.21-4.22V. When unplugging the charger and leaving the battery B1 falls to 4.19V and B2-B10 stays at 4.21V


  • Bestech D140
  • 10S6P Samsung 30Q

If I forgot to mention any important things, please just. Any input will be appreciated

Thanks for reading till the end :slight_smile:


Wall of text critts you for over 9000. Sec…

EDIT: Seems like alot going back and forth, solved, not solved to solved again…

Do you have a proper balance charger? Are you comfortable experimenting with the pack? Do you have a labb supply instead of those damn china chargers?


lol. You read all of the boring stuff in the beginning but it’s really only the stuff at the end that actually matters :joy:

edit: fixed… a bit.

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I have two chargers. Keep in mind that I used the 4A charger on the second charge so I have never been able to check the voltages of each group before using that charger.

2A charger: 4A charger (i’ve only used this charger once and that was before I noticed the voltage different. It was used on the second charge of the battery): 63.12€ |36V 4A ladegerät Ausgang 42V 4A aluminium fall ladegerät Verwendet für 36V Li Ion batterie lade Hight Power smart Ladegerät|charger case for iphone 4|case remotecase cassette - AliExpress

Depends on how :joy: If you mean by discharging each group manually to the same voltage then YES. If you mean by taking the whole pack apart, most likely not, but if it turns out to be the last option, most likely yes.

Unfortunately not. My local library might have one (they got a goddam 3d printer) but it would be a PITA to go back and forth (if they have one. They probably don’t).

nono, balance charge. Like Revoltek och IMAX… not using a BMS here. The charger balances the cells. Will also give you och resistance and how many mAH you put into it.

Visually examining the bad P group might give you a clue if something is loose/looks suspicous.

If you set the lab supply to your packs top Voltage(via BMS obv.) and just wait until the current reading says 0 on the supply theres should be no way you have anything unbalanced.

Guess whaaaat… Unfortunatly no :frowning_face:

And I either dont have a lab supply.

It’s gonna be a bit hard to see that. The battery is shrink wrapped and padded with foam. All connections are cover by fishpaper and kapton tape.

I think it’s most likely the BMS so before starting to take appart the whole battery, someone might have something golden to say about the BMS.

If it doesn’t smoke or you didnt fry it, i doubt its the BMS. Both my bms’s from supower and bestech has worked without any issues. Any of the above given solutions would also exlcude if it was the BMS or not

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0.05v difference is really small, i would consider that balanced.

i noticed the bms doesn’t kick into float charge / balance if drift is greater than 0.1v, so bms should be balance charging on your pack.

this was happening on a pack i serviced for someone on here, bms wouldn’t balance so i manually brought up lower voltage cells as close as possible to remaining cells, then finally it balanced everything perfect.

i did this with a bench power supply where i can set the voltage. so set it to 4.2v, connected to the low volt cell via balance leads. not sure if that’s something you can do.

this is the bench power supply i use:


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I will defenitly look into that! Might be lucky enough to find a really good deal on a bench supply. Wouldn’t I also be able to discharge the cells with more juice down to the same voltage as B1? Sure it would take MUCH longer but I would save some cost.

Thanks a lot for the input guys @thisguyhere @linsus

right, you can go in the opposite direction and get the higher voltage cells down to 4.17. seems you’ve tried that already though, and balancing didn’t work then either…?

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I haven’t tried that yet. I tried discharging my battery by riding it. It made all the cells drop in voltage. They all got down to around 3.8V but B1 was still 0.02V lower than the rest.

I’m sorry TL;DR but I just have a question, and I can’t believe your battery still doesn’t work,

How is your battery right now? The ultimate test would be to actually swap the way it’s wired and swap the first and the second P-groups… and if it changes which one is low, than it’s the BMS … if not, then it’s a cell.

But I know it’s unlikely that would be possible or easy especially if they are welded together.

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The battery seems okay right now. It discharges normal and not any faster than the other cells.

Took the board on a ride today and range seems about right. Haven’t measured the voltages yet.

The p-groups are welded together so unfortunately is it not gonna be possible to do that unless I take the whole pack apart.

I’d just run it then. It’s like the BMS was manufactured at the end of a reel of resistors of something.

Most common electronic parts have a 5% tolerance on the value, and typically devices made sequentially are usually semi-matched to one another.

If the 10,000 piece reel finished and they put a new one on, and your BMS got 9 resistors from the old reel and one from the new reel, it could, in theory, cause this exact symptom because of the 5% tolerance on the resistor manufacturing.

This is just an idea.

If it’s working fine, I would leave it as-is, and monitor it periodically to ensure it’s still operating this way.

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