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 are more frequent. Let's look at 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 electric charge for many days.
Electric device plugged into cigarette lighter socket In this car a charger adaptor has been left plugged in overnight and drained the battery
In normal circumstances a car should start without problems after being parked for at least two-three weeks. If the battery is good, but it drains sooner, this means that one of the electrical devices in a car draws excessive current.

What can cause this? Besides the instances where we simply leave the lights on, here are a few examples: First, air fresheners, radar detectors, phone chargers, dash cams and other accessories that plug into the cigarette lighter socket can drain the battery. This happens in cars where the cigarette lighter is powered even with the ignition off. We have also seen a number of aftermarket remote starter units, alarm and audio systems drawing excessive current. If a car has an aftermarket remote starter or other accessory, it needs to be checked first. Door switches also common to cause this problem.  In one car, a door switch has been damaged and the door ajar light was staying on on the dash draining the battery.
 We came across a few cases where water leaked into the electrical components, shorting the wiring and draining the battery. A sticky relay is another common culprit for dead batteries. For example, the A/C compressor clutch relay is known to go bad in some Honda and Acura vehicles. The Bluetooth and satellite radio modules can cause excessive current draw too. A battery itself could be the culprit, if it has an internal fault or external leak.

Parasitic current draw test Parasitic current draw test. In this car, the current draw is greater than specified.
How is this problem diagnosed? First, check if there are any lights staying on in the car, or if there is anything plugged into the accessory power plug. See if the dome light and the trunk light are off. Check the instrument panel. Does the door ajar light or some other warning light stay on? In most cars, only the security light is normally blinking on the dash when the car is turned off and locked. If nothing obvious found, 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 has been turned off. This means the parasitic current draw must be monitored after 20-30 minutes, when all control modules in a car went into the "sleep" mode.

Typically, the current should drop to less than 40-50 mA (milliamps) after 10-20 minutes since the car was turned off. Specifications may vary. In many cars the normal current draw is as low as 15-20 milliamps (in the photo).
Normal battery current draw In this car, the current draw is normal (8 miliamps).
If the 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 protected by the same fuse. In this case, each component is disconnected one by one until the faulty circuit is found.