Menell2's dealership said to drive the car every day to keep the battery charged. That seems crazy.

I have a Prius...here's how it works. There are two batteries, the ~300v one and a small 12v one. The 300v NiMH battery is kept charged, automatically, around 50% - 75%, because this is the optimum range for longest life. In this range it will last for, IIRC, 60~100 days if left unused.

The 12v battery is just a regular lead-acid car battery that is used to run the accessories, (lights, radio, etc) and the computer that starts the car. If this is dead you can't start the car (no computer to start it and can't validate the security chip in the key), but can easily jump start it just like any other car -- actually easier since so little current is needed. This battery is small because it does not need to provide any power to turn over the engine. However, because it is small, it is possible to have it run down sooner than in other vehicles if you leave your lights on, for example. You can easily leave the car unused for a few weeks and not drain this battery down. There have been some reports of owners that have had problems with this -- maybe a bad/weak 12v battery, maybe too embarassed to admit that they left a light on or door open that caused the dome light to come on (maybe a minor design flaw since it should automatically shut off after a while.)

The problems reported by Menell2 are not typical.

Mike