What’s causing my battery drain?

Question:

I recently purchased a 1952 Chevy coupe that was hot-rodded in the early 1970s with a 275-hp 327-cid Chevy engine and 4-speed transmission. It has also been converted to a 12-volt electrical system. A modest aftermarket stereo was installed (a radio with cassette player). The car also has an electric fuel pump. The engine is equipped with its original AC Delco generator (not an alternator) and a mechanical AC Delco voltage regulator. The battery is brand new, as are the battery cables.

I soon discovered that the battery was draining, as it would be dead if the car sat for a few days.

My first thought was that the generator or regulator was faulty, but they were both bench tested at a very reliable auto electric shop and deemed to be in good working order.

When the car is running, the battery is getting about 13 volts from the charging system, and the ammeter shows charging activity. However, with the engine off I’ve disconnected the positive lead from the battery and measured current flow of 12 volts, and about 3.1 amps between the battery and the cable. If I turn on the parking lights, the draw is just over 2 amps. A draw of 3 amps with all electrics turned off seems pretty high.

What could be causing this leak, and how do I track it down? I have tried disconnecting the leads to the regulator thinking it may not be working correctly, but that did not affect the current draw. This car has been worked on by many different owners over the years.

Answer:

A quick and easy way to possibly diagnose the source of the battery drain is to disconnect the battery cable and install a test light or voltmeter between the cable and battery terminal. Remove each fuse in the fuse panel one-by-one and observe whether the light goes off or the voltage on the meter drops to zero.

Keep in mind that there may be more than one circuit draining the battery so you should not replace any of the fuses until all of them have been removed. If removing all of the fuses doesn’t stop the current flow from the battery then you need to look at components/circuits that don’t run through the fuse block. This likely includes any non-factory items such as the electric fuel pump and stereo. Also look at the ignition circuit, including the ignition switch.