Recovering over discharged cells in a pack when still connected as 10S2P to a BMS

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Sep 3, 2019
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I'm trying to revive a 36v Qualcast battery for a RC mower project. Its battery doesn't charge on the original charger just flash's green after a bit indicating its fully charged. Its a 10S2P pack and I've found half the cell pairs are down to 2.6v and the rest around 3.3v. The battery is the same as in this video although from another brand https://www.youtube.com/watch?v=eHynUZ-jER4
What I want to know is if I'm very carful can I use my Imax B6 on a 1S lilo setting to bring the low cells pairs back up to the same as the rest which will then hopefully allow a 36v charger to charge and balance the pack back up to full capacity. That is if none of the cells have ben irretrievably damaged.
However I want to do this with the BMS still connected and not have to un tab any of the cells or take them out of there parallel pairs. (Not got a spot weld yet.) Does the BMS balance circuit work all the time trying to balance the cells to the same value or does this only occur during charging. Will it be fighting me trying to fight the Imax. Or does it do it similar to my Imax where it only starts balancing once the the first cell gets up to full voltage and then stops it charging any further (dumps/discharges through balance resistor I understand) until all the other cells catch up.
I'll obviously not leave it unattended and in a place where if it does go up it wont burn my property down.
 
That last sentence is a good idea.👍

When I've done it, sometimes the cells themselves are so hard to access that the safest way was to unplug the balance wires out of the BMS, then use two thin insulated wires pushed into the plug and attached to the RC charger. You really want to make sure the wires aren't too thick, and bending the tiny terminals inside the plug when stuck in there. Also, it's only 3 or 4v, but if you accidentally short a pair at the plug with the meter probes or wherever, it's enough to melt and deform the plastic around the terminal and make it not plug back in to the BMS.

The voltage will drop a few tenths after disconnecting from the charger unless you go all the way full with it, so you want to overshot your target voltage a little bit, say like 3.6v if you want it to settle at 3.3v.

And yes, usually the BMS only balances during charging, and usually can only deal with really small imbalances.

I usually try to have another charger with clips, bulk charging the whole thing directly into the battery, bypassing the BMS temporarily.
That way you can monitor the whole pack thru the plug, I've sometimes been adding to the lowest one with the RC charger, at the same time as taking some off the highest ones with an old bike light plugged into different parts of the balance plug, all while bulk charging, or it can take hours longer.
Make it easier to spot a group that shoots instantly up to high voltage also, say from mid 3s up to over 4.2v which usually kills the whole project.

Yours I wouldn't take any voltage off until it's almost full, and just watch if one group shoots way ahead of the others to see if they get to the 4s at the same time. You can always shut off the bulk charging for a while if you need time to catch up on the manual balancing.
 
By coincidence, I just started having to do that to a couple of old 36v batteries I'm reusing for a project. This is a Bosch battery and they use such ridiculously tiny balance wires I'm going to be splicing on a bigger plug and wires cut off of a dead battery.

IMG_20210727_191651421.jpg

Size difference...
IMG_20210727_192207202.jpg
 
I've manged to get the pairs back to roughly similar values 3.5v. I've been using magnetic leads I've made to connect to the metal nickel strips. All the balance connectors are also done in nickel direct to BMS board which is why I can't easily disconnect the BMS without major rework.
Then I thought I'd give the BMS a go and tried connecting a 36v 2amp hoverboard charger as I don't have this official qualcast charger.
https://www.gmvtrade.co.uk/qualcast-36v-lithium-battery-charger-36lc01-clmb3634m.html

I wasn't sure on which of the four pins I should use to charge the battery. The pcb markings are B+D+ NTC CH- P-
The charger has only 3 pins going to CH- NTC and B+d+. Apparently NTC is thermal cut off so thought I'd connect the 36v to the B+/D+ and the CH-.
I may have got the polarity wrong however as the charger released its magic smoke and will no longer work with and the red LED is now on all the time.
I think I'm going to continue to use my imax to charge all the pairs separately back up to 4.1v and try it it the mower. See if it discharges evenly. Then may have to try and get hold of the proper charger. Next step in my project is then trying to switch the lawn mowers 42vdc 10Amp motor on and off via my flysky transmitter. I've got a rc pwm switch but its only rated for 30v 7A so I've a SSR relay on order.
 
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