A vehicle battery is key to starting and operating the car or truck. If there is a draw, or drain, on the battery when the vehicle is not operating, the battery can run down and you may have problems starting the motor. When a car is not running, the draw on the battery should be virtually nothing, although a clock or digital radio may draw small amounts of electricity. Any draw on the battery, other than these minimal discharges, can indicate a problem.
Have a backup supply of power for the vehicle while you test the battery. (This is only necessary if the battery is removed.) Many modern vehicles store data necessary for the operation of some accessories in special digital memory that may be lost if all electrical power is lost. This can cause some small inconveniences, such as having to reset radio presets, or more serious problems with electric windows and climate control that may need to be reset by an authorized dealer. You can connect a backup battery through the cigarette lighter.
Plug the backup power supply into the cigarette lighter and disconnect one cable from the battery. Attach a digital multimeter between the battery post and the disconnected cable. Once this connection is made, the backup battery is removed from the circuit. Set the DMM to milliamps with a 100 to 1000 scale. Readings of more than 50 milliamps after the vehicle has been shut down for more than one hour may indicate an unusual drain on the battery. Consult the vehicle's specifications for electrical information. Reconnect the backup battery while removing the DMM and reconnecting the disconnected battery cable.
Locate the power drain. The drain could be from the car's electrical system or from accessories, such as cell phones or games, left plugged into power receptacles. Remove any possible power drains then repeat the testing cycle.
