2004 Prius, last 4 cells of battery were dead & battery fault

I have a 2004 Prius that was giving a battery fault. I tested the cells and the last 4 cells on the positive end were very low. Around 0.1 volt. But the rest were normal, around 7.2 volts.

moisture had gotten in and there is corrosion but the cells seemed to be in good contact.

I connected them to a bench power supply and charged them up to 8 volts. The next day they have all held charge and the power they absorbed was normal. No heating.

I will load test them but I haven’t yet.

Most problems, even if they seem odd have come up before. So I was thinking of reassembling the pack now that the cells hold voltage. But I still don’t know why it bled down just the last 4 cells on the positive end. Why would that happen.

The issue started after someone stole the catalytic converter and the car was sitting for 6 weeks before I welded in a new one. The car ran fine for a few drives then all the faults came on during a drive.

I suspect the water leaking in was a long issue (fixing that too) but with the car running normally it didn’t build up as much due to the heater. So I think moisture may be related. But I can’t think of a clear reason it would kill just the last 4 cells. I expected one to test out as bad.

maybe it’s a problem with the balance/sensing taps?

 

I've done the load test. It looks like they are bad. Charged them up to 8V, and under 2A load they fell to 6V in 2-4 minutes, putting out only 0.4 to 0.8 watt hours. I used a bench top programmable load so it was all measured. I don't know how quick it should have dropped to 6V but I know it should have been closer to an hour than a few minutes. After removing the load they rose back up to 7.2 volts after a minute, but they are presumably dead.

It seems odd that all four in a row went bad. Maybe one of them caused the others to drain and ruined them because they sat dead.

 

I think at this point I've confirmed that 4 of the modules are bad and the rest are holding a good capacity. They seem to level out at about 7.2V.

I've been searching but it's hard to find, what does the BMS need to detect to not give a battery fault? If I get modules that are mismatched will it raise an error, or is it ok as long as they are within parameters? I noticed there is a wire, presumably a voltage probe, every 2 modules.

I'm not sure if there is a way to test if the BMS is bad also. I'm wondering of connecting a bench power supply to stand in for the 4 missing modules (it can do 30 and they would be about 28.8) would atleast confirm with power on it would work. I'm guessing it would need to apply intermediate voltages to the voltage probe/balancing wires.