BMS will not balance

I was hoping that the collective knowledge of the forum could help me resolve a problem I have been having.

About a month ago I accidently switch the balancing leads for 2 of my P groups on my 10s4p battery. This discharged one P group to an unrecoverable level so I replaced that P group which was all fine and dandy accept my BMS will not charge the battery anymore. I have tried two different BMS’s. A besttech HCX-D223V1, and one of these off eBay.

The behaviour of both BMS’s was the same, I’ve also tried two different chargers ( both designed for hoverboards) and replaced the charge port and rewired everything for the new BMS as it’s now charge only. The behaviour I am experiencing is that it will charge for 5 minutes and then, I assume, stop charging indicated by the green light on the charger.

I’ve measured the p pack voltages at the cell level and at the BMS connection and they are the same indicating to me that the BMS is seeing the right voltage. I also tried leaving it ‘on charge’ with the charger displaying it’s green light and recorded the cell voltages at cell level again to check if it did anything and it appears not. Here is my table of results:

Cell col is initial voltage at cell level.
BMS col is voltage from the balance leads.
True col is voltage from balance leads adjusted for individual cell ( for example bms10 - bms9 = True10).
After charge is the voltage measured at cell level after ~3 hours on charge.

As far as I am aware the disparity in voltage level is exactly the scenario a BMS is used to rectify. I thought that maybe the cells are too out of balance for it to work but I couldn’t find any information to suggest there was a limit on this.

Does anyone have any idea’s about what I am missing here?

your cells are really far off.
try to charge the individual p-packs one by one up tp 4.20V than discharge them till 3.6V or something around there and see if the BMS will work than and how much your packs get out of blance during the charge/discharge cycle.
The p group with 3.5V could have a bad cell in it.
Some BMS just balance during the time of charge. as soon as the charger switch off the bms stop balancing as well.
means as soon as your pack get more then 0.2V isch drift inbetween the p groups the bms will not manage to balance the groups out.


P group 5 is the new group, which is why it has such a high disparity. As the battery is a year old I doubt my packs will maintain a 0.02v difference concidering p group 5 is brand new…

you can´t just slap in a new p group on storage charge voltage to a near fully charged pack and hope the bms will manage.
charge you p groups individually as I said till they have the same voltage. After that the bms should be able to balance the pack in future if there is no other issue in the old cells.


Alright thanks, I guess I have misunderstood how BMS’s balance cells. I will have to do some more reading. Thanks for your help.

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Yeah I would suggest them being between 0.1v of each other if possible, it would probably manage that just fine but some of the differences are quite a bit, and since there are differences across the board, if it could balance it, it would take quite some time.

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I’ll stick all the p groups below 4.09 on a charger this evening and see how it goes.

Rember you need a cc cv charger some so called cheep China ones do not do this and just switch of if the current draw drops off below a certain point not alowing the BMS to work properly

With large differences it can take days to balance a battery as thay work by adding a tiny load to the highest voltage p group. others work by only looking at the voltage difference in the p group next to it so you end up with a cascade effect across the battery as it discharged the higher p group as it charging. Some will only discharge a couple of p groups at a time. There are some of types iv come across so fare.

I presume you are using the correct voltage charger for your battery.

can you show me how you would charge discharge the p groups individually?

The simplest way is to unplug the BMS balance connector and plug a single cell charger (or a resistor of appropriate value) between the wires corresponding to the group you want to charge (or discharge).

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suppose i’m using a balance charger how would i connect it. the positive lead would connect to the corresponding p group and the negative lead goes to the battery negative(xt60 or the actual negative post on the battery)

If you’re only trying to charge or discharge a group or contiguous set of groups in the middle of the pack (not starting at either end), then all your connections will have to be made either through the balance plug, or directly to the cells themselves. You will not be able to use either of the main discharge leads.

If your group or groups start or end with one of the leads you can use it, but it doesn’t really matter.

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ok, yes i have a dead number 4 p group. I have to discharge the other 11 down to 3.6 before i transport it for repair. so to be clear i’ll have to connect both negative and positive through the balance plug, got it…thx a million

Depending on how “dead” the number 4 group is, you may find it faster/easier to just charge it back up to nominal, then drain the whole pack all at once. Most balance chargers are limited to a few watts or tens of watts of discharging power because they just convert all the energy to heat. That means that a several-hundred-Wh pack will take quite a few hours to discharge that way.

If the dead P group is just leaky (takes a charge but drains by itself over time) but doesn’t drain super fast or generate heat, what I’d do it charge that group up close to the rest and then drain the whole pack as a whole with a large resistive load.

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the battery will not take a charge anymore and i think the nickle is broke it burned through the shrink wrap. the last voltage reading said 16% 41.7v or something like that before i unplugged everything.

Alright, that sounds bad enough not to mess with it too much then.

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do you think its safe to discharge from the balance plug as long as i stay away from that group also can a battery short by just sitting there? :grimacing:

You should be fine discharging the other groups through the balance lead. Just bear in mind the wires are small so don’t try to draw like 5+ amps through them.

I think you’ll be fine .


thx a lot for your help. I plan to discharge 1 or maybe 1.5 amps tops.

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