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. 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

Use a digital battery tester to check the condition of the car best car battery testerbattery 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.

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

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Posted on by Rick Muscoplat


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