What can cause a car battery to drain?

Over time, we came across many problems where a car battery would drain quickly, sometimes even overnight. Modern cars are full of electronics and problems with drained batteries become more frequent. Let's look at why this happens, common problems causing a battery to drain and ways to fix them.

How it works: A good car battery has plenty of capacity to keep the charge for many days. In normal circumstances a car should start without any problem even after being parked for up to several weeks. Often, however, one of the electrical devices in a car draws excessive current that drains the battery sooner.

Electric device plugged into cigarette lighter socketWhat can cause this? Besides the instances where we simply leave the lights on, here are several examples: First, air fresheners, radar detectors, phone chargers, dash cams and other devices that plug into the cigarette lighter often cause parasitic current draw. This happens in cars where the cigarette lighter is powered even with the ignition off.  We also have seen a number of aftermarket remote start units, alarm and audio systems pulling excessive current.


 In one car, a door switch was damaged and the door ajar light was staying on on the dash draining the battery.  We know many cases where water leaked into the electric components, shorting the wiring and draining the battery. A sticky relay is another common culprit for dead batteries. For example, A/C compressor clutch relays are known to go bad in some Honda and Acura vehicles. The Bluetooth and satellite radio modules have been mentioned as culprits by several car owners. In some cases, an internal fault or damage inside the battery can also cause it to drain quickly.

In the next photo below, an aftermarket remote start unit have caused excessive parasitic current draw and the battery went dead after three days. Parasitic current draw test How is this problem diagnosed? Your mechanic may recommend the parasitic current draw test. The test involves measuring the battery current draw after all electrical devices are turned off. In most cars, various electronic modules may stay active for up to 20-30 minutes after the car was turned off. This means the parasitic current draw must be monitored for a longer period of time.

Typically, the current draw should be less than 40-50 mA (milliamps) after the car was shut down for 20-30 minutes. Specifications may vary for different cars. In many cars the normal current draw is as low as 15-20 milliamps (in the photo).Normal battery current drawIf the parasitic current draw stays higher, the faulty electrical circuit can be identified by pulling the fuses out one by one and monitoring the current. If the current drops after the fuse is pulled, the faulty component is on the circuit protected by that particular fuse.

This process could be time consuming, because there could be several components powered from the same fuse. In this case, each component is disconnected one by one until the faulty circuit is found.