I store my Z for the winter, and wanted to make sure the battery didn't go dead. So I did allot of research and ended up buying a product called the BatteryMinder.
This thing is state of the art! Even if I open the door for half a minute, the BatteryMinder light goes on, and it brings the battery back up to full charge. It's that sensitive, and I was impressed. And it keeps the plates from getting a sulfer build up on them, (or at least put's it off for a much longer time) and that's what shortens battery life, and also doesn't allow the battery to be fully charged, but you can research that if you're interested.
The web address where I bought mine is:
Batteries - Laptop, Camcorder, Cordless Phone, Cell Phone, Digital
I bought the model 12117, and it's great.
If you're interested, go take a look.
.
.
.