Prius (Gen 1) :: Manually Charging HV / Traction Battery?

I don't own a Prius, but my parents own two identical 2002 models (each with 120k miles).

My mother's car wasn't driven for nearly a month (hip replacement), and when they tried to start it last week, the ready light would blink 3 times - but no cranking.

After taking it to the dealer, they were quoted $4600 to replace the traction battery and ECU. We did the mandatory research on rebuilt batteries, but with only a vague diagnosis from the dealer, we were not convinced that this was the (only) problem.

So, my brother and I decided to swap the ECU, then the battery from my father's working Prius to my mother's - thereby proving the issue. Upon removing the 'dead' battery from mother's car, I decided to open the module, and check each battery. We measured anywhere from 3.0 to 6.4V on the individual cells - and 7.0V to 11V on the paired cells. It was clear that the batteries had discharged unevenly. The total charge remaining was about 151V - still above the 40% SOC minimum I'd read about.

We then bought 5 x 10A chargers from Walmart, and connected each to a pair of modules. Set the charge to rapid (10A) and waited 30 minutes (or until the charger indicated full). After 2 hours, all 19 pair were charged fully - with each reporting 15.7V +/- 0.5V. The total power was about 295V.

We reassembled the battery, re-installed it, and the car started immediately. We let it sit running, until it cycled off on its own. About 5 min later, the ICE fired again to charge the 12V battery - but it threw a P3006 error code. It showed a PS + MAIN + battery icon on the MFD. Using an OBDII scan tool, we cleared the error, and drove the car for a few miles. The error code did not return during that drive - but did one more time after letting it cycle off/on automatically. After clearing the code a second time, it never repeated again.

My assumption is that the 300V HV charge was above the 80% SOC and it didn't like that. But after a few cranking cycles, and the tendency for the battery to discharge back to it's rated output, the error no longer repeated.