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Fix or workaround for BMS issue

2659 Views 23 Replies 6 Participants Last post by  Duncan
I have been working on installing an aftermarket kit on a Prius to makes it into a plug gable hybrid. I have as input a 16s LiFEPO4 battery.

I have a problem however with the BMS that come with the kit. Cells #12/16 and cell #13/16 both read incorrect voltage (cell 13 reads .2V too high compared to the multi meter reading and cell 12 reads .2V too low). I am worried that the BMS will then "burn" these 2 cells by overcharging cell 12 and over-discharge cell 13.

I was wondering if there is any way I could re-adjust the BMS reading so it properly detect the correct voltage. Otherwise, would it be possible to maybe insert a low resistance on one or many balancing cable to compensate for this reading ?

I could also just buy a cheap BMS circuit without monitor but then I have no idea how I could trust such circuit without feedback.

The kit in question is an Enginer kit (bought a used one)
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I have also read on forum some success story of electric car with absolutely no BMS.
IMHO, at a MINIMUM you should at least monitor half pack voltage vs the other half (without causing an imbalance in the process) and terminate charge or take action accordingly. Maybe I should quit talking about it and make a kit.
good call on inspecting it! I wonder if a battery connection was opened and it reversed through the bms (and it were not designed for it).

here is a very simple balance monitor, appropriate caveats, doesn't control anything, just watch it like a gas gauge for imbalance. don't know if that enginer system works without an official bms or not or what the signals are.

there are a few enginer hacks out there too ie
re: batt-bridge, I went with individual (cheap) voltmeters because the batt-bridge needs a few volts imbalance before it triggers, and knowing the voltage of each pack half to 0.1 volts or so is more useful info.

re: diode drop, it works, only caveat I can think of is if your charger has a "trickle charge" or otherwise tries to top-off the pack repeatedly. If left plugged in it may overcharge since at very low currents there is almost no voltage drop in the diode.

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