The Dead Car Battery: Why Does Your Battery Keeping Going Dead?

It was time to track down that pesky problem that had been draining my truck battery for several days now. Many believe that when the key is turned off the power flow in a vehicle stops. This isn't quite true. The alternator has a very small draw at all times. And that fancy sound system? It has a clock...and that clock needs a power supply too. So there is another draw. Over time, either of these draws would cause a battery to go flat. But this problem I was looking into was drawing the battery down over the course of about two days.

I had previously found that the alternator had indeed gone bad and had replaced the unit. And no sooner had I done that than I had found the battery was still going flat..so it was time to find that drain on the system. The easiest way I have found to do this is through the ' pull fuse ' method. This is quite easy. You take a voltmeter and take a reading across the battery posts. There should be no power on in the vehicle at all...everything turned off and key in the off position. A normal reading may between 12 and 14 volts. Record this reading in a notebook.

Now..go pull a fuse in the fuse box. Just one..and go back and take another reading at the voltmeter. Was there any change? No? Pull another fuse. Now you have two fuses out. Take another reading. At some point...that reading will change. It will show an increased reading. You have just found which circuit is causing the drain. With the drain removed the available battery voltage will go up.

I lucked out on my fourth fuse. That fuse was connected to the external lighting system. The taillights and license plate light to be specific. Now that I knew which circuit was causing the problem...it was now time to find exactly what in that circuit was causing the problem. An inspection of the license plate light revealed corrosion between the contacts inside the socket for the bulb. This was indeed the problem. The corrosion was acting as a conductive bridge causing energy to flow constantly to ground..thus draining the battery. But..this didn't make sense I thought. That particular light isn't on unless the lights are first turned on. So there is a switch in the circuit to stop current flow. Can the switch be bad? I replaced the fuse in the circuit and saw from the meter that the drain had reappeared.

And little while later I had the offending switch in my hand and sure enough...it had heat damage. No doubt it was constantly making some sort of partial connection inside. Thereby offering current flow to the corroded socket and draining the battery. I picked up a new switch and a new socket a couple days later and replaced the old ones. A check with the meter showed the current flow had stopped and the problem was fixed.