I left the car in the garage undriven and unplugged for 25 days in warm/hot temperatures. The results: The lithium battery was at 42% or less when I left it. When I turned it on, it was at 41% or more, so less than 1% drain in 25 days. The OEM lead–acid battery had no problem starting the car.
I did something similar in 2018 when the car was a year old. Went on a 60 day cruise with the traction battery fully charged, but put a battery tender on the 12V battery. I wanted to see how much charge the traction battery would lose, so I just checked the mileage available on the HUD before I left and when I came back. It was a negligible change; about .2 miles if I remember correctly. After that, I learned from this forum to never park it for a long time with the traction battery fully charged. I know checking the HUD mileage available isn't very scientific, but it was gratifying that it didn't lose much charge in two months.
That's right—~ 30–40% is the general industry recommendation for lithium-battery storage. Toyota appears to recommend 0%, which is like ~ 10% counting the HV reserve, which is probably a relic from non-plug-in Prius, as you don't want to store a non-plug-in Prius with a lot of charge remaining in the HV battery but have the ICE come on before you store it, which Toyota also recommended for the Prius Prime somehow, even though ~ 10% charge (0% on MFD) may not be ideal for Prius Prime storage.
My solar roof equipped EU Prime, when left parked outside, takes care of the 12V and charges the traction battery up to 80% in around a week, depending on the starting point SOC, the season and sun exposition. 80% SOC is equivalent to 54 km of extimated EV range being the maximum extimated EV range in the summer around 67 km. I guess then that such SOC value is considered safe by the OEM for the traction battery health.
Don't leave your car plugged in when you go away for vacation. It'll drain your 12 volt battery. If that battery is old, it'll drain it enough so the car won't start.
Unfortunately there’s a good chance your 12 voltle drain if you leave it unattended for a long period. The real solution is to leave it unplugged and put a battery tender/trickle charger on the 12 volt battery when you’re away.
