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Battery Drain Issues, Diagnosis


Persistent battery drains are a serious annoyance for car owners, but are particularly difficult (and tedious) for a mechanic to locate and repair. Quite often the drain will be in an inaccessible place, there will be more than one drain source, or the issue just never seems to resolve. Small, parasitic drains are the most difficult to find whereas big, problem-causing ones are usually the easiest.

Finding a parasitic drain begins with testing the battery and its charging system. This may seem obvious, as most people with a dead battery or persistently dead battery will assume that it’s the battery or charging system that’s at fault. Yet a test of this system is often skipped because the battery or alternator are “new.” New parts can go bad as well.

Since battery and charging system tests are usually free of charge at most auto parts stores, it’s easy to have them checked. Many shops will also do these tests for free. Once the battery and alternator are ruled out as suspects in the battery drain issue, that leaves only an electrical fault in the car as a possible culprit.

To find a battery drain, you’ll need a multimeter and some patience. A lot of patience. Disconnect the negative battery cable and tie it off so it is not grounded or able to touch the battery or its terminals. Set the multimeter to read amps rather than milliamps. Then put the negative pin on the negative battery post and the positive pin on the negative battery cable end. The multimeter is now completing the vehicle’s electrical circuit, allowing it to ground. The car will likely have its computers switch on for a few minutes. Wait that out until they shut back down. The multimeter should drop well below 25 milliamps and stay there without fluctuating much. If it moves or stays above about 25 mA, then you have a drain.

With the multimeter still connected to the battery, begin removing fuses in the engine’s fuse panel one at a time. Start with the smaller ones and work up to the larger amp sizes. Replace the fuse once you’ve gotten a multimeter reading. When the multimeter lowers its reading, you will know which system is draining the battery based on the fuse or relay you’ve pulled. Refer to your shop manual or repair manual for the car to find which systems should be using battery power and how much.

Once you know the circuit that is causing the drain, replace its fuse/relay and begin testing devices on that circuit. Check switches/buttons, remove lamps or other items, or disconnect items and watch that meter. Check and replace as you go. Eventually, you will find the problem. Or at least get a very good idea of where it is. Then individual components can be tested for continuity, unexpected grounding, etc.

It’s a tedious, time-consuming job, but it must be done if the problem continues to cause battery drain. This leads to premature battery replacements and could mean being stranded somewhere with a dead battery.