I own a 2017 Prius V4.. bought it new. Never given me any trouble and have something I'm pleased to report. A month ago the wife and I headed for the airport to catch a flight out of town for vacation. We tried to leave early enough to drop the pets off for boarding, park at the airport and arrive 2hours early for TSA. As Murphy's Law dictates, everything went wrong and in all the rush, frustration, and getting into the terminal, I discovered when we got back that I left one of the map lights on. We were out of town for a total of 9 days, so the light was on the entire time. When I got everything loaded back into the car and discovered the light was still on, I felt the car might not start out of fear the 12V battery, already 5 years old, was nearly dead. To my surprise, when I stepped on the brake pedal and hit the Start button, everything came on as usual and the hybrid system came on. Gads! was I relieved. I recall reading many posts here of 12V batteries dying after a few years, and even my dealer suggested I get it replaced at my last oil change. So this good fortune more than made up for all the delays I ran into trying to get us into the airport terminal when we left.
HOWEVER......that kind of discharge likely did some damage to your 5 year old battery. Probably should shop for a new one NOW before it does fail.
The dealer's battery will have a 84 month prorated, warranty, so it is the 'best' battery if you are keeping long term (until 2028) If you are not thinking long term, Amazon has an AC Delco ACDB24R for under $240 with a 24 month warranty
I have heard that the computer in a Prius V will trickle juice out of the 1.3 kiloWattHour NiMh traction battery, if it sees that the 12v battery getting low. Yes the 12 volt battery is small by non-hybrid standards (46 WattHours lead acid), but that traction battery has 28 times the capacity. The 12 volt battery could be kept alive for a very long time with just a dome light draining it. You might not have been as lucky if your trip was say 29 days. The trickle charge doesn't go on forever. Once the traction battery depleted, you are pretty screwed. A simple jump start could take a while before the "ready" light will come on to let you drive the car. That smallish 12 volt battery seems to cost a pretty penny too (at least $200). I don't understand why it is so expensive. The very rare odd shaped 20Ah motorcycle battery from my 85 shadow only costs $75 (Acid sold separately of course).
NOT true. That only happens when the car is "running"; that is, in the READY mode. It does not happen when the car is OFF.
