Dr. Prius "Warning: Weak Module Detected"

Hey everyone! Just wanted to share my experience recently after rebuilding my Prius HV battery.

I rebuilt the pack using modules with very similar internal resistance and capacities. No warning lights now, car drives perfectly.

I ran a test with Dr Prius after installing and got 83% life left. But then... "Warning: Weak Block Detected:1"

I messaged Jack (Creator of Dr Prius?) to confirm that the "1" indicates the block number and not simply the number of weak blocks. He explained that a weak block is one that begins the test with the highest voltage and ends at the lowest. And yes. The 1 indicates block 1 is weak.

I removed the case from the pack again and was confused to find that in block 1 were my two strongest modules (initially looking at my spreadsheet I thought block 1 ended up being my weakest modules, but that was block 14).

I had a melted orange sensor wire plug on the battery ECU, which I had replaced, but hadn't replaced the ECU, just cleaned the corroded pin. It was primarily a pin that connects to sensing wire 0 or 1 that was corroded, so I thought maybe that was the issue. I swapped the ECU to see if that would resolve it. It didn't. Still same issue in Dr Prius.

Finally, I took a closer look at my spreadsheets and realized I had mixed up modules when installing- because my first row in the spreadsheets is a label row, all the module numbers are offset by 1 from the row numbers. So I had mistakenly put a bad module that I meant to discard into the pack in place of a good one. Luckily it was #1 and was very simple to swap out.

I ran the test again and finally did not receive the Weak Block message! But since I hadn't charged the last module to match the pack voltage, it was sitting a bit low. I removed the bus bars again and gave the new module a short charge to bring it's voltage up to close to the surrounding modules. Anyone know of the modules will even out their voltages automatically? For the rest of the pack I discharged all modules to 7.5v and that worked really well. But hoping just swapping in 1 module won't be a problem with slightly different voltage.

The car seems very happy now! All internal resistances look identical, voltages stay pretty close, and the car drives quite a while on the battery.

It was a really fun experience rebuilding this battery.

Just wanted to post this since I couldn't find any info on the "Weak Block Detected" message here in Prius Chat.

I've been reading Prius chat for years and am just now posting! Such great information on here. Thanks everyone.

We have three gen2 Prius right now. Two with about 350k miles, both on original batteries, and this car with 195k with a freshly rebuilt pack.

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