How to troubleshoot car battery?

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). Any 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 holddown bracket. A typical holddown bracket/bolt is shown below.

Note that many vehicles will lose codes and presets, etc. when a battery goes dead or is disconnected. If your battery still has power and you want to avoid losing codes and presets, use a second battery to jump the car while you replace your battery. Hook the jumper cables to terminals wherever your 2 main battery cables attach to the car and be careful not to bump them off until the new battery is connected.

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 14-15.5 volts when the alternator is working properly (engine running). If you have an ammeter in your dash, it should be on the positive side with the car 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. If the alternator tests good, but you are not getting 15 volts to the battery, check for a blown alternator fusible link in the relay box.

4. You have an electrical short or draw in your car. :-(
Before getting into a thorough testing of your circuits, check the most common cause of a short--the alternator. The alternator contains diodes to convert the AC power to DC. When a diode fails, it opens a path to drain the battery and can do it quickly.

Note: always unhook the negative battery cable before taking any hot leads off the alternator or starter and wrap these leads with electrical tape before reconnecting the battery.

If your battery is draining quickly, try unhooking the wires from alternator and leave the car with the battery connected overnight. If the battery survives the night but wouldn't before, replace the alternator. If it drained again as before, please continue reading.

This case requires a lot more work and know-how. I will now describe the first phase of troubleshooting. If you have a high current ammeter, you can check the circuits at the fuse boxes to see which ones are drawing current. If you don't know where your cabin fuse box is, check your manual or common locations such as under the dash on either side, the outside corner of the dash, or behind the glovebox (open box fully and press tabs to open past the stops). There is also usually a relay box in the engine compartment that contains the larger fuses. A charged battery or a charger needs to be hooked up to the car to troubleshoot, but leave the ignition turned off. Your ammeter should be able to handle 25 amps for this chore. If you don't have an ammeter, use the "spark or test light test." Remove each fuse one at a time, and look/listen carefully for a spark when reinserted. If the circuit is drawing current, it should make a small spark when the fuse touches the fuse socket. Note each circuit that generates a spark. A better method, if you have or are willing to buy a 12 volt test light, is to test each fuse circuit with a test light. While not quantifiable like an ammeter, the test light will glow brighter when there is a bigger load/current and is easier to see than a spark. To use a test light, just push each of the leads into one side of the fuse socket until they both touch the terminals. Alternately, you can disconnect the negative battery terminal and hook the test light between the cable and the battery post. The light will stay on until you remove the fuse of the troubled circuit.
There are some circuits that are supposed to draw current, even when the key is off. For example, the courtesy light may spark if you have the door open during the test. Any circuit that should not draw current when the key is off is now a candidate for further troubleshooting in phase 2. Complete the ammeter, test light, or spark test for all fuses in the cabin as well as in the engine compartment fuse box. Below is a typical fuse panel.

If none of your fuses generate a spark/light, either you don't have a short, the short is too small to make a spark, or the short is in the battery or something that doesn't have a fuse. In the second case, you can reperform the test using a low-range ammeter. If you are stuck here, shoot us a message at FixYa explaining the problem and what you have done so far.

For phase 2 troubleshooting, you will need to identify all of the wires and components that use the sparking/lighting fuse. Write down everything it says about those fuses on the guide on the fuse door or in your owner's manual. You can then start checking each of those components

But before you do that, think about whether or not that circuit should be energized with the key off. If not, perhaps the problem is a stuck relay or bad ignition switch that is energizing circuits that should be open. It makes good sense to find the relay--if there is one--that energizes that circuit when the key is turned on. If you can pull the relay out of the fuse box and the spark/light goes away, you are holding a stuck relay--replace it. Check all of the sparking circuits in this way. An example would be a fuel pump that continues to run after the key is shut off--replace the relay!

Now, if once you have checked any associated relays and found them all good, it's time to check the components for the sparking fuse circuits. If you can find the connector attaching to those components, remove them and see it there is a spark at the connector terminals when the connector is reattached. If there is a spark at the connector, that component is using current and likely has an internal short. If there is no spark, leave the connector off and recheck the fuse for a spark. If the fuse still sparks, the short is in a different component that is powered thru that fuse, or it is in the wires that run to that component.

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