A car battery can run down for any of several reasons:

• You accidentally left the lights on or some other accessory that pulls power from the battery even when the ignition key is off.
• The battery is not being recharged while the vehicle is being driven (you have a charging problem)
• There is a key off parasitic electrical drain on the battery because a relay is sticking, a module is not shutting down, or there is a shorted diode in the alternator.
• Your battery is old and will not hold a charge anymore. The battery needs to be replaced.