What's Draining Your Battery?
Most vehicles draw some battery current when the key is off, thanks to the clock and the internal memory of engine computers, body-control modules, and radio presets. Altogether, they draw a very small amount of current. Fifty milliamps would be a safe upper limit for this, though many vehicles will draw less. If you're not sure, look up the correct rating in the service manual.
To measure the car-off current draw, you'll need a multimeter capable of reading current, preferably one with a 10- or 20-amp capacity, but a 200 milliamp lower scale. Start with a fully charged battery. Either make sure the doors are closed or wedge the door switch shut. Turning off the dome light isn't good enough—on many cars, an open door will activate several circuits.
Unplug any power-draining cables from the lighter socket, such as a cellphone charger or GPS. Even if the device itself is unplugged from the charger, the plug may still consume a few milliamps of current. Got an ear-bleedin' stereo amp in the trunk? Pull the fuse, because it may be in standby mode rather than completely shut down.
One caution: If your radio or antitheft system requires you to input a code after the power is interrupted, better hunt it down now. It's likely that you'll need it. Don't let the dealer entice you to bring the car in and pay him to input it. The code should have been included with the owner's manual when you purchased the car.
Start hunting by putting your ammeter in series with the battery's ground circuit. Disconnect the battery's ground cable and wire the ammeter in series between the battery terminal and the cable. Start with the meter on the highest range, probably 10 or 20 amps. Warning! Doing something silly, like trying to start the car or turn on the headlights—anything that draws more than the meter's rated capacity—can blow the meter's fuse.
Once you have determined that the current drain you're reading is safe, gradually reduce the meter's scale to the appropriate low range, probably 2 amps or 200 mA. You are now reading the parasitic drain on the battery. Some vehicles will show as little as 10 mA residual drain. Others, probably high-end cars with lots of high-end gadgets, will draw more.
An important note: Some devices, like alarms and automatic-dimming lights, will draw substantial amounts up to 20 minutes after they're deactivated. So if the reading is high, wait a few minutes to see if it changes.
If you can't do this yourself, looks like a visit to the garage or get another to troubleshoot. Good luck.
