Battery (All): Terminology: Self-discharge
WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. 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.