What’s killing my battery?

June 1, 2012 | By Richard Prince


I can use some assistance with an electrical problem. I’m not a mechanic and have limited knowledge but I have a test light. My assumption is that with the battery connected and the ignition off there should be no electrical draw on the battery. Therefore the battery should not go dead when the car sits without being used. I disconnected the battery and saw, via the test light, that there is a draw that, I am assuming, kills the battery. The battery appears to be about 1-2 years old, holds a charge, and cranks the engine fine. The vehicle is a 1970 Dodge Challenger with a 360 cid V-8, automatic transmission, and MSD ignition.

Something runs its battery down if it remains connected for longer than a week. I’ve only had the car about a year.

While the engine is running, the aftermarket-installed amp meter indicates a steady 12-13 volts output.

This is what I have done so far: I’ve done each of the following things independently to try and isolate the problem. I replaced the rear wiring harness, as it was patched together with multiple splices, with a new one. I replaced all bulbs in the car. One by one I have removed each fuse individually to try and isolate what was causing the draw, to no avail. With all of them removed there was still a draw. I disconnected the alternator, still a draw. I disconnected the starter, still a draw. I disconnected the wiper motor, still a draw. The ground wire is firmly attached to the block. There is an ancillary ground to the battery tray/radiator support.

The only other information I can give is that the courtesy lights do not work nor does the heater blower. All the headlights, turn signals, emergency flashers, and windshield wipers work. The car has an aftermarket radio that only operates with the ignition on. Any suggestions?


When you disconnect a cable from the battery and then connect your test lamp between the cable and battery terminal the lamp will light if there is a current draw somewhere in the car. This does not necessarily mean that there is a problem, however. For example, if your Challenger is equipped with the optional in-dash electric clock, or the optional Rallye gauge package that includes the clock, there will be a constantly “hot” lead that keeps the clock functioning even when the ignition is turned off. Interior courtesy as well as the optional under hood and trunk lights are also normally powered all the time. In a similar vein, the aftermarket radio in your car may have a constant power feed that supplies power even when the ignition is off to keep the tuner’s clock and memory function powered up. None of these “normal” constant electrical draws should be sufficiently strong to kill the car’s battery. You state a few things about the battery—“The battery appears to be about 1-2 years old, holds a charge, and cranks the engine fine”—but none of these observations definitively reveal the battery’s true state of health.

You need to subject the battery to a load test to determine whether it is, in fact, healthy. By the same token, you should test the function of the car’s alternator with an alternator tester instead of relying on the reading that the aftermarket ammeter is giving you. After confirming that both the alternator and battery are good or replacing them if they are not, you should continue the sleuthing you’ve already begun. Systematically disconnect every single electrical component at the point where it connects to the car’s electrical system. Some components may not go through the fuse block, either because they’re incorrectly wired around it or because they have fusible links or other protective devices that eliminate the need to go through the fuse panel.

Bear in mind that your car may have more than one electrical draw, so as you go along disconnecting one electrical device after another you should leave each device disconnected.