[zaarn]

Car batteries are generally a starter battery (minus EV ones). They don't like being closed circuit for extended periods of time.

A low current is usually allowed (200mW is the top end in my experience), but anything over will, over time, sulfinate the plates in the battery and cause it too loose charge and capacity rapidly.

Generally if you want to run a low current device on a lead acid battery, you should use a deep cycle battery with plates designed for it. Starter battery have large, flat plates to carry a lot of current for only a few seconds tops, then be charged up again.

[ash]

It looks like Zipcar should have a separate smartphone-like battery to power GSM and GPS.

[mcguire]

How long does it take to damage the car battery?

How long does it take to discharge the car battery?

[zaarn]

Depends on the load.