I parked my Prius 2010 without a shield for a month without the 12v battery, rain fell at some point too. Before then, I already have a P0A80 which displays check hybrid system sometimes and i clear it with my Carista via Dr Prius each time. However, when i returned the 12v battery and attempted to start the vehicle, it came up with error U0100 and P0A80(always been there) I observed the state of charge(attached) fluctuates from 0% to 10% to 50% e.t.c Stays at 0% most times and kept display Check Hybrid System and won't ignite. I even set it to Maintenance Mode and still show Check hybrid system and wouldn't start the engine. I would like to know if an unused Hybrid battery could drain itself after 30days and also if that can cause error U0100. NOTE: I also tested with a new 12v battery
I had a company vehicle, so I parked my gen 3 for a few weeks. Never again. I had a devil of a time starting it, but once I got is running, I made sure to drive it once a week. Even with a booster, the entire electrical system seemed to stutter, rather than starting. I thought I may have damaged the electronics, which was one of the reasons I got rid of it last year.
You started with a weak hybrid battery. And it sat, slowly discharging.... certainly didn't help. If the 12v battery is weak also, more issues. The weaker cell will try to charge from the stronger cells, draining them. If you need to park it again, remove the safety plug, and disconnect the 12v battery. When you return, reconnect both..... Since there will be no drain from other sources, you shouldn't have an issue. Of course, you need to take care of that weak block. Best to replace the hybrid battery with a Sodium battery....
