A car battery drain -- also known as a parasitic battery drain -- is a common problem for many drivers; it occurs when a car's electrical system is functioning improperly. Most modern cars feature a computer system that continues to use some power after the car has been turned off. A battery drain occurs when this system pulls too much power from the battery, leaving it discharged. Diagnosing an auto battery drain is a relatively straightforward process, although it does require some persistence.
Recharge your battery to full voltage using a car battery charger. This may take up to several hours.
Disconnect the battery charger when it has completed charging. Turn the car on and let it idle.
Check the battery with a voltmeter. Set it to 20 volts. Connect the red probe to the battery's positive terminal and the black probe to the negative terminal. The reading should measure between 12.4 and 12.8 volts.
Turn off the car. Disconnect the voltmeter and let the car sit overnight.
Check the car's voltage using the voltmeter. If it still measures in the 12.4 to 12.8 volt range, your battery is good and you most likely don't have a battery drain. If the charge has dropped significantly, either your battery is going bad or you have a battery drain.
Take your car to a licensed mechanic to determine and repair the root cause of the parasitic battery drain. You might also inspect your car's fuses and relays to determine the cause. Use the owner's manual to determine which fuse corresponds to which vehicle system, and replace the fuse accordingly.
