Q:I have a 2001 Ford Explorer. My wife and I recently flew to Mexico and when we returned and went to use the car, the battery was dead. I used a battery charger and it now works fine, but I'd like to identify the problem. My two-year-old daughter sometimes turns on the interior light and I think she may have hit it and it stayed on the whole time we were gone and drained the battery. Could a small interior light drain a car battery in nine days? -Vernon H., Minneapolis
A: In modern cars there are lots of things that can draw current even when the car is off. This current is often devoted to computer memory for things like the vehicle clock, climate control, keyless entry, trip computer and seats that remember various driver settings. If you have a car alarm, it likewise draws a small amount of current. If they are the only things consuming power - your system has no shorts creating additional draw - average drain for a modern vehicle is in the 50 milliamp range or 50/1000s of an amp. (Drain may be higher for the first 20 minutes to an hour after you turn off the engine.)
To sustain a 50-milliamp draw, the electrical system requires one amp every 20 hours. Nine full days (9x24 hours) is 216 hours. Thus, if this is all the drain that is occurring in your Explorer, it should have consumed less than 11 amps (1 amp in 20 hours means 11 amps in 220 hours). Even if the average draw from your system were 100 milliamps, you'd have consumed less than 22 amps in 9 full days.
A typical car battery is rated around 45 to 100 amp hours (Ah), which means, with the 45 Ah example, it can sustain a draw of 1 amp for 45 hours if fully charged, or 100 milliamps (100/1000s or 1/10th of an amp) for 450 hours.
A non-LED interior light draws much more power than 50 milliamps. It could draw in the range of .725 amps or 725 milliamps. To burn for 100 hours, that light would need 72.5 amps and to burn 200 hours it would need over 140, handily exceeding a typical car battery's capacity.
If your daughter has a history of turning on that light and your vehicle has functioned properly since, it may well have been the source of the dead battery. Other problems can cause "parasitic drain," however, including shorts in the electrical system. If the battery is still within its service life, it goes dead again and you're sure that no interior light was left on, you have parasitic drain in the system - a short somewhere, including a possible stuck relay or a bad module, such as for an anti-lock braking system or keyless entry. If you have something plugged into a power receptacle in the vehicle, this too could be bleeding off charge.
There are a few things you could do if your daughter likes to turn on the interior light. The obvious one is to check the light each time you get out, but of course life is hectic, especially with kids, so this is not something you will always remember to do. Another option, if you like tinkering and gadgetry, is to look around for an LED interior light to replace the standard one. These typically shine at least as brightly and consume much less energy. Quality LED lights should last a long time, too; some people who have bought the very cheapest stuff report poorer reliability.