A slow battery drain can be immensely frustrating. Sometimes, the problem “sneaks up” on you and leaves you stranded. Other times, you can’t enjoy your vehicle without pulling out your charger. Since diagnosing a slow battery drain isn’t always easy, we’ve put together a basic process you can use to try and determine the cause of a slow battery drain.

The first step? Make sure your battery is good and that it is completely charged. This is sometimes easier said than done, as basic battery testing tools are notoriously unreliable. A simple voltmeter, for example, can often indicate that a failed battery is working properly, as it can’t determine the condition of the cells inside the battery. A load test or (even better) a conductance test is best.

Next, you want to see if there’s a drain on your battery somewhere in your system. Take the positive battery cable off your battery, and using a good working 12V test light (like this one) attach one end of the test light to the battery and the other end to the positive battery cable.  If the test light is lit with all accessories turned off and the doors are shut so that the dome light will not come on, than you have a drain on the battery.

If you have a drain, you want to use the test light and a simple process of elimination.

• Completely disconnect the alternator wiring, then conduct the same test with your light as described above. If the light is out, there’s a problem in the alternator (or the alternator is wired incorrectly). You’ll want to see what we’ve written below about properly wiring the alternator if this is your situation.
• If the alternator isn’t the problem, the next step is to remove one fuse at a time from the fuse block, checking each time you remove one fuse to see if the test light has gone out.  When you remove the fuse that makes the test light go out, you’ve found the circuit that is causing your battery drain.

Proper Alternator Wiring

The way a standard (GM) internal regulator alternator is wired is as follows:

1. The large 8 gauge red wire is attached to the threaded stud on the back of the alternator.
2. Where the plastic connector plugs into the alternator case, the numbers “1” and “2” are cast into the metal case.  The #1 position gets the brown wire.  The #2 position you simply loop #2 position to the threaded stud on the back of the alternator with a minimum of a 14 gauge wire.

If you’ve got the alternator wired up properly and you’ve got a drain, than there’s an internal issue. But often times the wiring is to blame.