Any previous issues with 12v battery failing, being replaced, followed by quick HV batt failure?

Hi!

I was just wondering if this has every happened to anyone? I noticed some strange behaviour (as I monitor quite a few parameters on my car); my 12v battery is near to death, and it seems to mess with the battery ECU in some unusual ways. I was wondering if anyone has had 12v battery 'die', get replaced, then the HV battery died very soon after? Some will have simply due to statistics, but I'm wondering if there is any 'pattern' at all...

 

The reason I asked, is that my 12v battery in my 2008 Gen 2 is actually the original still, 276000km, and has been drained down about twice. It drops down to 10.x volts quickly and stays there, and can easily go to high 8s, low 9 volt range. I can only boot up my car if I have the fans and all lights off - otherwise it doesn't boot up the computers.

It has, in the last few days, started getting a bit worse. Specifically, the MFD sometimes reboots (boots up for maybe 1 second, then reboots) during initial power-up (no signs of failing MFD, but it just starts up due to the CPU brownout protection in all the Prius computers).

What I have noticed is that the battery ECU has also been 'rebooting' during these times (the odometer trip meters haven't been reset though). When the battery ECU reboots, it resets the SoC to 60% (well, 59.98%).

The second time this happened was after arriving at work, having driven down a long hill. I had a SoC of 81%, and sat for a few mins to stabilise temperature etc, before power-down. SoC was about 79%. When I restarted it rebooted to 59.98% and then 'charged' to 78%, then put the HV battery fan on '4', and kept powering up the ICE with no fuel to discharge the battery, even at around 75% SoC. I used EV mode to discharge down, but it was still 'stuck', not recognising the fact the battery was trying to overcharge, and it wouldn't allow EV mode despite 70% SoC, normal temperature, warm engine etc. I fixed the issue by draining the battery down to about 45% SoC, then unplugging the 12v battery, so it 'reset' to 60%, and after this it has worked fine (with some initial 'adjustments' to the reported SoC occurring - maybe 5% correction at the limit of the range).

The first time this happened was after I had arrived at home on a damp, rainy day, so had been going up a very steep hill with air-con and lights. The SoC was about 45% on power-down. On power-up the next morning, it was 59.98%. I recognised what had happened, but the car drained the battery down so that the cell capacities were into the 13v range, even at low load. In the end I coasted for a good while, until the engine warmed up enough to charge, then did my down-hill commute. I charged it more 'gently' and used friction brakes, and when I got to about 68% SoC, it 'zoomed' up to 80% SoC even though I put it in neutral - so it corrected the issue.

In summary, due to 'resets' of the battery ECU, my car tried to severely discharge my battery the first time, and the second time, tried to severely overcharge it. My battery is worn enough as it is. The resets are due to a terribly worn 12v battery. I could have killed my battery (definitely on the first time by over-discharging it) if I hadn't had live data running at the time. If I didn't have live monitoring, the two events above could have been enough to tip my battery over the 'edge' if I hadn't intervened each time. In the short term, I am planning my drivers and have a 58-61% SoC whenever I power down! But I am finally ordering that new 12v tomorrow...