Rick's Free Auto Repair Advice

Dead car battery

What causes a dead car battery?

If you have a dead car battery first thing every morning, you might have a parasitic battery drain and you should conduct a car battery drain test. See this post for more information on how to test for a battery drain. Or, you may have a charging system problem. Start by checking your charging system and ruling out a battery drain issue.

Start by testing the car charging system

best car battery tester

Solar BA9 battery and charging system tester

Use a digital battery tester to check the condition of the car battery and the charging system. Attach the meter to the car battery and enter the battery type and cold cranking amps listed on the battery label. Then press the test button and read the results. If the battery tests, good, move on to the charging system test. If that tests good, move on to the next test to check for a battery drain issue.

The tester shown here is MUCH more reliable than the older “toaster” style testers that just put a load on the battery. A load test used to be the gold standard, but not anymore. The load tester show here is called a “toaster” because when you press the test button it energizes heating coils to simulate a load on the battery. If the battery maintains at leas 9.6 volts during the test, it’s considered good. The coils get hot enough to make toast, so bring some bread and butter when you use this tester.

The Solar BA9 tester, on the other hand performs a resistance and capacitance test as well as a simulated load test. There are far more reliable in determining the condition of your battery. The tester can also test your charging system.

dead car battery

“Toaster” style battery tester

Symptoms of a bad car battery

1) If your  battery is dead shortly after the engine is intentionally turned off that’s the sign of a bad battery.
2) If your battery is dead 8 to 24 hours after shutdown, it still might be a bad battery. But it could also be dead due to a drain on the system.
3) If your battery is dead in 24 hours to several days after the vehicle sits unused, the likelihood of a battery drain rises dramatically.
4) If the engine starts immediately  away after a jump start that’s the sign of a bad battery.
5) If the engine cranks slowly shortly after shutting down the engine, that can be the sign of a faulty cable or a bad starter.
6) If you hear a rapid clicking sound coming from the starter, that’s the sign of low voltage. It’s often caused by a poor connection at the battery posts.

How to find what’s draining your car battery

Here’s how to do a car battery drain test. You’ll need a low current clamp-on inductive ammeter like the one shown in the picture.  If you can’t get your hands on one, you can use an inductive ammeterordinary multi-meter.  But be warned, most multi meters have a 10 amp fuse in them, so you have to take some precautions before using it.

Start with car battery drain test by clamping the inductive ammeter over the negative battery cables.  The way I’ve shown it in the photo is WRONG!  I couldn’t get the meter to clamp over all the cables and still see the meter’s screen in the camera.  So I had to clamp it on only one cable.  Make sure you clamp it over ALL the cables on the negative terminal.

Next, turn on the meter and read the amps.  The maximum allowable battery drain is 50 MILLI-amps.  That’s the current needed to keep the computer alive.  If you show more than that, don’t panic.  There are many circuits in the car that have to “time out” before they stop drawing current.  Let your meter sit for about 15 minutes.  If you still have a high reading, it’s time to start pulling fuses.

Pull the fuses one at a time and record the reading on the meter.  If the current draw goes down, you’ve isolated the circuit that’s draining power.  You’ll need a wiring diagram of that circuit to troubleshoot from that point on.  After you pull each fuse and re-insert it, you may be powering up a circuit that’s already gone to “sleep.”  If that happens, you’ll have to wait for that circuit to time-out again.

If you use a regular multi-meter, you’ll have to connect it in series with the negative battery cable to do the car battery drain test.  That means you have to remove the battery cable and sandwich the meter leads in-between the terminal and the post.  Once you have it attached, DO NOT TRY TO START THE CAR.  If you’re lucky, you may just pop the 10-amp fuse in the meter.  If you’re not so lucky, you will fry the meter.

One final note:   Every time you open the door to get to your fuse box, the interior lights will increase the current reading on the meter.  One way to get around this problem is to pull the fuse to the interior lights.  However, if your car’s interior lights are operated by a computer and the computer is on another circuit, you might still see a current surge when you open the door.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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