Wiring an Alternator

Troubleshooting a slowly draining battery

If you find your battery going flat after only a day or two, and you know that it's not the battery's fault, chances are you have a parasitic electrical leak/drain that's stealing your amps. While it's possible to use an ammeter to look for current draws on all the possible circuits, I think it's easier to use the following method.

The first step is to remove the negative battery cable. Next find a regular 12 volt light with a couple of leads attached. It doesn't have to be big or bright; there won't be much power going through it. Then connect one lead to the negative battery post and the other to the negative battery cable like so:

Testing battery drain

The idea is that if *anything* in your vehicle is drawing power, it has to go through the light first, which will cause it to glow. The more power being drawn, the more power that has to go through your light, and brighter it will glow. It's best to do this test in somewhat dim lighting conditions to make it easier to see the relatively faint glow.

To start your test, make sure your vehicle is off (keys out of the ignition is best) and all doors are closed. If you have to leave your hood up to access the fuses, make sure to remove any bulbs that turned on. Now go to your fusebox, and start removing fuses, one at a time. You're looking for the fuse that when pulled, causes the light to go out (or at least dim significantly). That will be the circuit that is drawing power when your car is off.

Obviously this is easier if you have two people, one to watch the light and one to pull the fuses.

Some of the common causes of parasitic drains I've seen are light switches that get stuck on, e.g. glovebox or mercury hood switches; power antennas; and alternators. The main alternator power wire isn't fused, so if your light is still glowing after removing all the fuses, try removing the wires at the alternator.