As you may recall, what originally tripped the Prius error was what basically to a voltage difference between the modules. The sensors in the battery pack report the module voltage (I think in groups of 2? You can see the wires leading to a contact in each section of the bus bar), and the ECU will report an error beyond a certain threshold. I believe it’s a difference in anything greater than 0.3V between any two module groups that will trigger an error, but I’m not positive that’s the threshold.
The significance of this is that cycling the batteries isn’t enough to “reset” the pack. The voltage also needs to be balanced across all the modules. The last charge of the modules left it at a relatively arbitrary voltage, as the charge terminated after reaching the 7250 mAh mark. Here’s what I ended up with:
So the lowest voltage is 7.92V and the highest is 8.09V. That yields a difference of 0.17V – far beyond the tolerance. The modules as designed to drive the Prius are hooked up in series – 28 * 7.2 nominal volts per module = 201.6V. That’s the dangerous part. Welding level voltages. To balance all the modules we hook them up in parallel. One ridiculously giant 7.2V battery, probably run your cell phone for a couple years. This part is not that dangerous. You could have quite a bit of current, but the voltage is low enough that you’d have to trip and accidentally impale yourself on a couple of the contacts for it to really matter.
I read about a couple strategies for doing this, but the wiring harnesses didn’t look very robust, easy, or cheap. I really didn’t want to worry about all those wires. It would be extremely easy to accidentally hook up a module backwards and ruin it. Given that removing the bad module had been relatively easy, I opted for a different strategy.
Flip every second pack, so the terminals are all aligned:
Then run a wire across:
This worked wonderfully! And it was super easy to avoid screwing up, which is a big perk in my book. I left it like this for a little over a day, and it all seemed to work out. The pack equalized at (I didn’t actually write this down, so just going off memory) 7.96V and all good to go!