Battery (All): Terminology: Self-discharge
WARNING: This page is about a different car, the 2002 Volvo C70, 2001 Volvo C70, 2000 Volvo C70, 1999 Volvo C70, and 1998 Volvo C70. However, it is still accessible from the selected car via links, so may be relevant.
A battery that is not in use will discharge itself over time. This is know as self-discharge and is quicker in hot environments. The battery must be maintained by recharging at regular intervals to avoid excessive discharge.