This is a known issue. If you dig through the archives, you should find many threads on this topic. For example: What do do when away for for weeks
The short answer: "You should never leave any car unattended for 5 months, but you can't leave a Prius unattended for even 5 weeks. Now you need a new battery."
Here's the long answer:
- Modern cars have many parasitic drains on the 12V battery (e.g. computers, security systems, auto-shut-off headlights). These draw power even when the car is off and the key is out. People who store cars for several months (e.g. classic car owners that don't want to drive them in the snow) will put a battery tender on the 12V battery. If you don't want to do that (or can't, such as in an airport parking lot), you should disconnect the 12V battery.
- A Prius can only last a few weeks, because its 12V accessory battery is very small (because it doesn't need to crank the gas engine), so it goes dead faster than the larger 12V starter battery on non-hybrid cars.
- Once a 12V lead-acid battery goes dead and is allowed to sit in a discharged state for a long time, physical changes occur in the lead plates, reducing its ability to hold a charge. You can still charge it to 100% full, but it will go dead a lot sooner than before. This cannot be repaired, so you need to buy a new battery (and you should take better care of this one).
- Until you get a new battery, you can jump start this one the usual way, but there are simpler ways. The Prius uses very little current from the 12V accessory battery when you turn the key, so you can get by with any small 12V battery. See Using tire inflator to jump-start: data gathered
. Rumor has it you can just attach a 9V battery with clip leads, but I haven't tested that.
Douglas (2002 Prius, 2006 Highlander Hybrid, Wisconsin)