Dead Batteries and Locating an Electrical Draw

Q: Why would my battery die in a day or two?

A: There are a few things that can do this. The first and most obvious is the condition of the battery. If a battery is old, or is in a car that seldom gets used and is stored for great lengths of time, the internals can sulfate and go bad. When this happens, the battery will appear to take a charge, but will not hold it. You’ll want to recharge you battery on a slow setting and try to bring it back up and see how it reacts. A good battery will generally read between 13.6 and 14.6 volts at rest. Once completely charged, do a draw test (if you have that capability) and see how the battery reacts. If the battery drops below 9-10 volts on a heavy draw and stays below where it was before you performed the test, there’s a good chance that you need a new battery. Generally, if a battery is over about 4 years old and continues going dead, it’s safe bet that the battery has outlived its useful life. This can happen almost overnight in the fall or winter. A batter that has given you no indication that it has any problems will appear to die “overnight”. As the days and nights get colder, the useful cranking amps of a battery begin to diminish as the electrons move more slowly in the cold and you can actually get a break in the internal cells of the battery as well. When this happens, it’s time to buy a new battery.

The other thing that can happen, is a heavy draw on the battery when it is at rest, can discharge the battery almost overnight. The logical thing to do, is once again, recharge the battery in a slow fashion. It is best to remove the negative cable to do this and charge the battery either in or out of the vehicle. After the battery has been brought back up to the proper charge level, and before hooking the negative cable back up, take a test light and connect it between the negative cable and negative battery post. If your test lamp lights up, than you have a draw on your system. The brighter the light, the heavier the draw. If the light comes on dimly, something such as a clock or digital radio can cause a slight draw and give you this scenario. This will eventually kill the battery, but will usually take a very long time to do so, especially if the battery is in good operating condition.

If you get a bright and persistent light, this is an indication of a draw heavy enough to quickly kill your battery. Locating the source of the draw is easier than you might think. With the test light still hooked up as outlined above between the negative post on the negative cable and with a brightly illuminated light, start removing fuses one at a time. This will generally occur on a “battery” circuit or one that is hot all the time (things like a lighter, courtesy lamps, stop lamps, dome lamp, etc.). When the light goes off, you have found the offending circuit. Once you have determined the problem circuit, see what is connected into that circuit and start unplugging those items one at a time (This method also works on a circuit that blows a fuse). Again, when the light goes out, you have found your culprit. Fix and repair it as necessary.

Now even with all the fuses pulled, the light continues to stay on. That tells us that the problem is being caused by a “battery unfused circuit”. Things like a voltage regulator, alternator, ignition switch, headlight switch, and even some lighters will be on an unfused circuit. Again, start unhooking these items one at a time being careful not to ground them out (as being unfused, these leads and will cause quite a mess if grounded out). Internally regulated alternators, while usually a great product, are notorious for developing an open diode inside of them that diverts 12 volts to ground and drains a battery overnight. They will charge a battery, operate perfectly, and give you no indication of any problem. Disconnecting the alternator and having the test lamp go out finds this problem for you.