XR Overcharged cell that isn't after battery upgrade (Repaired)
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A friend's board has been reporting overvoltage after a new extended battery was installed. Looking at OWCE all the cells but one is reporting 4.5v which would be dangerously over what it should be (max 4.2v). However the cells are actually all fine suggesting a BMS fault.
TLDR
Check the caps C1- C15 around the main MAXIM chip to the right of the balance connector. These can get broken off or just get damaged when removing the battery harness connector. They may show fine but the connection between the pad and cap might be cracked so touch up the joints if they seems to read fine.It's a 4212 so swapping the BMS wasn't an option (bypass chips were not working for some reason) but swapping everything else showed the same issue. So over to me the BMS came.
Under a first inspection it was fine. testing the passive components and traces between the connector and the MAXIM IC showed no issue and everything checked out when measured. Turns out the conformal coating was hiding a cracked solder joint on the C11 capacitor which is one of the caps that are in the firing line when removing the XT60 for the battery harness. After redoing the solder a little it seemed to behave since prior I needed to push the probe down a little for it to connect.
All that left was testing. I don't have a spare harness so I set up the absolute bare minimum to see the cells through the OWCE app. I've got a spare degraded battery and a 4209 controller I use for testing purposes so I had enough to boot it.
The hex key was to short the 2 pins (blue/black) beside the RX/TX pins (white/green) on the BMS connector. This is to turn it on and off. I didn't fancy wiring in a button. Again, absolute bare minimum to test it :3
Please don't try this, I'm a trained idiot.Anyway...
All happy :3It's currently in the post waiting to be re-installed now with an JW chip and the extended RBXR battery.
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@lia said in XR Overcharged cell that isn't after battery upgrade (Repaired):
Please don't try this, I'm a trained idiot.
lmao
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My onewheel plus started showing 4.2 at full charge instead of 3.6 for the lifepo on only one cell. This gave me some reassurance that the BMS is just false reporting after a certain voltage because i notice it does fine all the way till around 3.6 than cell group 1 just skyrockets to the high 4s which would most likely have exploded the battery several times over if it were true as ive only just noticed and have been charging it to 58.4v for years. Im not as skilled as you so im just gonna leave it as is, overcharge warning has only shut the board off on me one time and never while riding, only when i came to a full stop after running downhill. Thanks for the write up!
