Why is My Car Battery Dead?

jump starting engine
Tired of jumpstarting your engine? Figure out why your battery is dead. Whiteway/E+/Getty Images

If your battery has been going dead on you regularly, it's time to find out what's causing this to happen. There are some common issues that are easy to remedy and may solve your dead battery problem. It's a cure for that dreaded feeling -- you have a dead car battery and no amount of wishing, jiggling the key or cursing your car is going to get you on the road any faster. If you've got anybody around, now would be a good time for a jump start. Quick and easy. 

Everybody's battery goes dead from time to time, but the burning question each time is "Why?" Sometimes a battery can go dead because of human error. Did you leave your headlights on? Did you leave your keys in the ignition? Is your trunk open for days at a time? All of these "oopsies" can drain your battery over time. If your starting problem seems sporadic, you should definitely try cleaning your battery terminals. A dirty battery can really affect how much starting power your battery can transfer to your starter. 

Another quick tip involves your interior light, also known as the dome light. Some cars, especially those from the late '80s and early '90s, rely on the interior light circuit to tell the car that you've gotten out and that you're finished with your powered accessories inside the car. For example, on an '87 Porsche 911, the interior switches (power windows, for example) stay active until you open the door, get out of the car, and close it again. It knows you have done this by sensing that the car's interior or dome light has come on, stayed on for a second or two, and then turned off. The interior dome light is activated by a little stitch mounted somewhere near the door opening. On the Porsche in question, it's at the front of the door sill near that steel bar that keeps the door from opening too far and helps it spring back into place when you close the door. These switches are small and usually cheap. And I don't mean inexpensive (though they are often that, too). By cheap I mean low quality, and by low quality I mean they tend to go bad over time. When one of these little switches does go bad, your dome light never comes on, and your car may not know that you've gotten out. So it keeps the window circuit powered up and the radio at the ready for you -- all night long. This is often enough to drain the battery over time. Check to be sure your interior lights are coming on each time you get out. Something as simple as a bad light bulb can even cause this system to fail! 

A common but harder to diagnose reason for a dead battery problem is a stuck relay. Your car's electrical system relies on a series (often dozens) of relays that open and close and control the flow of electricity through the vehicle. Relays are usually fairly reliable, but they do go bad over time and are easily replaceable. When they do go bad, they can be stuck in either the OFF or the ON position. If they get stuck in the ON position, you may have a fuel pump that runs all night, or an auxiliary air pump that never turns off. Things like this can really put a drain on the battery. 

Whatever your theory, it's a good idea to have your battery tested every once in a while to be sure it's putting out enough amperage to reliably start your vehicle on even the coldest morning. Eventually, you will have to replace your car's battery.