Battery drain – How to test
So you’ve checked out the battery and the alternator but the battery is still dead in the morning? Well, you’ve got a case of the parasitic drain blues. Something is draining your battery and your job is to find the culprit. You’ll need a clamp-on inductive ammeter like the one shown in the picture. If you can’t get your hands on one, you can use an ordinary 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.
We’ll start with the clamp-on ammeter because it is the easiest to use. Cla
mp the meter 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. 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.
For more information on this repair or any others for your vehicle, buy an online subscription to either Alldatadiy.com or eautorepair.net. Click on this link to compare the two services: Compare Alldata and Eautorepair. If you just need information for a single repair and want to save money, eautorepair offers a lower price 1-week subscription for only $11.99. Or, if you'll be working on this vehicle in the future, you can buy a 1-year subscription (Alldatadiy.com for $26.99, or eautorepair.net $29.99)
© 2007 Rick Muscoplat
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Such informative post you got here, well explained indeed. Appreciate the effort a lot. Nicely done. More power.
The last thing you want is to come out in the moring to go to work and find a dead battery. I HATE That. Good advice.. Great article.