The issues with the 12v accessory battery are that it's easy to flatten the battery (run it down) by either not driving the car for 2+ weeks, or if you leave the headlights on overnight or a door ajar overnight. Once the battery has been flattened once, the lenght of time it can sit unused (undriven) shortens to maybe a week or less. (You can still jumpstart/boost the battery like any other car battery in this situation, though.)Magnus said:
If you feel like replacing the battery here is the instructions someone else already posted:
http://www.mindspring.com/~dparrish/Pri ... Mods2.html
I think the battery leads (cables) are different on the Prius compared to most other cars, so you'll have to modify the ends a bit for a different battery.