Unbalanced 10s Liion Battery with BMS

I built a 10s4p battery last year with 25R cells and hooked up a SuPower BMS. The battery probably has 100-150 cycles on it. It hasn’t been charging up to 100%. Usually it only gets to 89-90% before my charger turns off. I checked the cells’ individual voltages and here’s what I got. At the time I think my battery indicator was saying that I had 54%.

3.7 3.9 3.8 3.8 4.1 4 4.1 4 4.1 4

Is the BMS bad? What should I do? @barajabali in this thread suggested that the pack should be discharged until completely dead, and then charged. Is that not bad for the battery?

Thanks in advance.

What are the cell voltages after you charge it? How long did it sit on green when you measured?

Its a problem with how SuPower bms’ work. They over charge the cells to 4.25 and then balance down from that. The problem is that if one cell is higher than the rest it will hit 4.25v first and stop charging any other cells. So in order to get all cells to the same voltage you have to go through this cycle of getting to 4.25v and wait for it to balance. Rinse and repeat until you have a balanced battery. Pretty crappy way to balance if you ask me.


Hello, this is what has been happening:

  1. Your cells got unbalanced, I bet you had nickel strips connecting the individual packs. BTDT
  2. The sum of all 10 packs give you the total battery voltage, if some have lower state of charge they will not charge fully and the higher state of charge won’t go over 4.3v, that’s why you get a lower capacity after fully charge your board.
  3. Some cells have been overcharging, this is dangerous for the cells during charging as they were being charged more than 4.2v, this could lead to eventual fire.
  4. If you have riden long enough to deplete the battery to zero (2.8v each pack usually) the lower charge packs could have gone down even lower than 2.5V (any battery going down less to 2.5v needs to be replaced), you might even see 20% of battery remaining on the indicator.
  5. Lowering any battery excessively might not kill the battery immediately but will wear it out pretty fast.

I’ve been runing with my current Trampa board BMS less, one of the indicators to perform balance procedure on the packs is precisely this, checking the SOC of the battery fully charged, if it’s not 100% consistently open the enclosure and check for unbalanced packs urgently

Thanks for all the responses so far.

@b264 I’ll check that in the morning. It will have been sitting green for 10-12 hours.

@chinzw So you’re saying that I should look into getting a different BMS? I think a lot of people use SuPower.

@Eboosted Makes sense. Any remedy for an unbalanced pack?

How are you connecting each battery pack? I had way too many unbalanced packs before, my assumption is that a nickel strip has too much resistance to connect packs properly, since I started using silicone wire to connect the packs they have been balanced to milivolt levels even with no bms, the Samsung 30q also have a reputation to keep balance over time.

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I used 12AWG grounding battery braid for the parallel connections and 10AWG silicone wire for the series connectors.

Try to post a picture.

The easiest way to balance each pack is to connect a load, it could be a car light bulb an electric fan or any other load, bring the SOC of the higher cells down to the lowest pack.

The garage your battery with the enclosure on, so you could monitor the individual voltage of each pack, you should get 100% if all cells are still in good condition.

The cell voltages after I charge it are:

4.1 4.1 4.2 4.1 4.4 4.5 4.5 4.4 4.5 4.4

:confused: It sat on green for several hours.

You need to bleed those that are over 4.2 down with a small 12V incandescent light bulb or something

Then once none are above 4.2, carefully remove the BMS and throw it in the trash can.

It’s likely your cells are damaged.

Put a new BMS on the battery.

What charger are you using? I’m not sure why your charger would go over 42 volts. Are you using something besides a lithium-ion charger?

I don’t think the charger went over 42V because the total voltage was 43.2. I’m using a charger I picked up from here from @JLabs

Ok, I’ll bleed those down and trash the BMS. So SuPower is no good? Is there a BMS that you would recommend? Maybe a Bestech one?

Do you run the discharge current through the BMS or have it bypassed?

I bypass it

If you bypass it the Bestech 10S “D190” is a good one that I know works.

After you get your cells bled down to 4.2V each you should briefly connect the charger again long enough to measure what voltage it is sending out to the battery. It seems to be more than 42V. Don’t leave it connected for any non-negligible amount of time.

If I measure the voltage coming from the charger without it being plugged into my battery, would that be an accurate reading?

No it won’t. The charger will probably not put out its full voltage without being connected. Certainly try, but probably not.

I don’t get it. Are you using a 6s charger?

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wtf… this is frightening and the reason I’m so scared of BMS. It looks like BMS is bypassed for charging and you’re just bulk charging. Are you sure BMS is in the charger’s path? It should have cut off the incoming power when any cell hits the per-cell over-voltage-protection, I think usually 4.25v. This is the BMS’s primary job (protect cells from dangerous conditions, protect whole pack from dangerous conditions)

If you’re connected properly, this BMS killed your pack.

supower bms has a good reputation…

Also, if your pack needs anything other than trivial amount of balancing, these bms with the tiny per-cell bleed current cannot keep packs in balance. Given the voltages you showed, your pack is likely not manageable by BMS anymore.

It’s likely this is a result of the BMS not working and/or not being connected correctly, but whichever it is, it’s been happening for a long time. They have been drifting for a long time.

Not using a BMS is more risky than using one. It just appears this one isn’t working.

@markyoe Do you have photos of how the BMS is wired?

Charger not plugged into battery: 45.8V Charger plugged into battery: 45V I also tested a different charger that I borrowed from a friend and it was putting out 44.5V not plugged into the battery…

I’ll take pic of the wiring and try to find the diagram I followed.

@Jumpman No, he had a couple 10s chargers available in addition to a 6s one.