What causes a car battery to lose its charge?

Updated March 20, 2017

Car batteries are essential to the function of motor vehicles. They provide electricity to the vehicle to power the ignition system, the starter motor, the lights and other accessories. Car batteries are charged by the vehicle's alternator, and will charge while the vehicle is running. There are several reasons why the battery will lose its charge.

Alternator Malfunction

When car batteries are connected properly, the battery will charge while the vehicle is in use. The car's alternator is the device used to charge the battery while the engine is running. If the alternator is malfunctioning, the battery will drain without a charge for future use. To ensure that the alternator works properly, make sure the connection between the battery and alternator is secure. A mechanic can check the alternator with a voltage meter or multimeter.

Current Drain

Older cars whose battery is properly connected and in working order may have a current drain problem. Normally, when you shut off the car, the battery load should drop. When you shut off an older car, the "key-off load" is typically little to none. However, on newer vehicles with computer systems on board, "key-off load" may not drop enough. The load can range from 20 to as much as 400 milliamps. Excessive current drain may be caused by lights remaining on, or computer modules that won't shut off.

Extreme Weather Conditions

One measurement of battery power is "cold amperage." The higher the cold amperage, the easier the battery will be to start a car in cold conditions. However, if the amperage is too low, the battery may not be powerful enough to start the car in cold conditions. So, although the battery may not have lost its charge, you may think it has because of its failure to start your car. The battery may not be sufficient for your climate region.

Damage to Battery Casing

The battery's casing may become damaged because of a car accident or not being sufficiently secured in the engine. If the casing becomes damaged, it may lead to toxic leaks of sulphuric acid or other chemicals. The battery will then lose its charge.

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About the Author

Nicholas Smith has written political articles for, "The Daily Californian" and other publications since 2004. He is a former commissioner with the city of Berkeley, Calif. He holds a Bachelor of Arts in political science from the University of California-Berkeley and a Juris Doctor from St. John's University School of Law.