Dead Battery Troubleshooting

So why did your battery die? Before assuming it's dead, make sure the terminals are clean and tight. OK, if that wasn't it and it's dead or at least at a lower charge than it should be, here are some possibilities:
1. Somebody left something on for a long time (radio, lights, something plugged into the lighter socket). In this case, you usually know why the battery went dead; and, if your battery isn't too old, it will probably recover from this incident. Another case worth mentioning is a dead battery after the car was parked a while out of earshot. If you jump the battery and your alarm goes off, it was probably set off a long time ago and ran the battery down. Reset the alarm and go about your business.
2. Your battery really died of natural causes (internal short). An auto parts store that sells batteries can check yours to see if it's really dead and happily sell you a new one. If you want to make sure they aren't just selling you another battery, you can buy and use your own tester. To replace your battery, disconnect the negative terminal first, then the positive. Then remove the hold-down bracket. Note that many vehicles will lose trouble codes, the time, and radio presets, etc. when a battery goes dead or is disconnected.
3. Your alternator has failed and allowed you to drive on the battery until there was no juice left in it. Usually your battery/alt idiot light will provide an indication when your alternator is not charging. If you have a voltmeter in your dash, it should read between 13-15 volts when the alternator is working properly (engine running). If you didn't get an indication but want to check your alternator, you can either use a voltmeter to check the voltage at the battery with the engine running, or you can remove the alternator and have it tested at the parts store. I use my voltmeter set to volts DC and probe the battery with the engine off (no jumper cables) and engine running (again, no jumper cables). If the voltage at the battery is not higher with the engine running, the alternator is not charging the battery. If the voltage is much higher while the engine is running but is less than 13 volts, suspect that your battery has an internal short and have it tested. If the alternator tests good at the store, but you are not getting at least 13 volts to the battery, check for a blown alternator/charge fuse or link in the relay box. I found mine on the side of the battery holder under the cover for the positive terminal. I can just measure the voltage before and after the fuse (engine running) to see if the fuse is blown.
4. You have an electrical short or draw in your car.
This case requires a lot more work and know-how. The first step in searching for a draw is to measure the current at the battery with everything turned off. There should still be a small amount of current to maintain settings, operate the clock, etc., but no more than one or two amps. For the following tests, the battery must be charged or have jumper leads attached to a good battery or charger so that the car has power for the tests. I remove the negative battery terminal and place my ammeter between the loose cable and the battery terminal. A reading higher than 2 amps indicates a draw. If you don't have an ammeter, you can use a 12VDC light bulb, such as a brake light, to estimate the current. (A very crude approach is to just look for a small spark as you touch each fuse to its socket.) Attach wire leads to the bulb and connect them between the negative battery terminal and cable. A dim bulb is ok, but a bright bulb indicates a draw. The draw must next be localized to a fused circuit by pulling each fuse individually and measuring the current at the fuse. (Alternatively, you can leave the test light or ammeter attached between the battery terminal and the cable and monitor it while removing and replacing each fuse. When you pull out the fuse of the offending circuit, the current will drop a lot.) Reattach the battery cable and use your car's owner's manual to locate the fuse boxes. Make a table or diagram of measurements for each fuse using the figures or tables in your manual. Hopefully you will find one and only one circuit that has significant current. If you don't need the devices on that circuit, leave the fuse out for the time being. If I need the bad circuit to work, I get the wiring diagram for that fuse/circuit and start troubleshooting.
This is about as far as it makes any sense for me to coach you without getting some feedback from your tests. So, if you get to this point and have not found the short, hire a professional automotive electrician to resolve your issue.
For more info, see my webpage at byteyourself.byethost7.com/Can...