I gotta neighbor who has an 05 Chevy Duramax that has a short that's draining his battery (down to zero in a couple of days). When the batteries are charged, or once the truck is running, the truck runs great so I don't think its alternator or voltage regulator related. Right after running and within a day or so, the truck still has enough battery juice to start, but more than a day or two and he's dead. He's starting to tear out his hair chasing the problem (he's sort of randomly looking for a short to ground in his wiring) so I thought I'd try to help him out here by asking some of his fellow Duramax guys where or what he might look for. Are there any smoking guns that I can tell him to look for, or is this one of those deals that you just have to chase until you find it. Thanks...

 

Just went through a similar problem with a gas car. Dead cell. Would hold charge for a couple of hours and then not start. I disconnected the battery pos (in this case i would disconnect both) and attached my amp meter across the battery to the pos wire. Let the reading settle and note the amp draw with everything off.

Next I started to open the doors or other things that draw power to see if there was a change in the amp draw. If you find no changes in the amp draw, then it could be that the batteries are just starting to die.

Post the amp draw here and see if someone can confirm if it is in line with others.

What non stock items does he have installed. I worked on a car for a friend of mine several years ago, and it turned out that the battery was getting weak and that there was an amp installed and was powered on at all times.

 

I'd start with any aftermarket accessories. ALARM first. They will drain the batts if installed wrong or if it goes nutty. Then add-on stereo equiptment, air compressors, etc...

After that it becomes a long process. Get a clamp on DC amp meter. Clamp it to the neg batt cable, it shouldn't read anymore than .3a draw. If it is higher, start pulling fuses till it drops off. Then trace that circuit.

Have fun.

 

Do as Rttoys has suggested, I went through this years back on another vehicle. Turned out one of my stereo components was not turning off, bad power relay inside of component.

Unfortunately a DC ammeter will set ya back a few bucks, they aren't as cheap as the a/c units.

 

Check hot wire to starter, may be shorting out at power steering pump.

 

The easiest way to check to narrow down a current drain is to unhook the Neg batt cable and jump a test light to it clip the lead end to the battery post and then the probe end to the neg cable. Then touch the cable back to the post for a few sec to make contact then do not let the post and cable touch and make sure the test light remains connected to both the post and the cable and does not loose connection. Next start pulling fuses and watch the test light it should dim or go out. As you are testing make sure the hood light is either off or pull the bulb out same with the doors make sure they are all shut or the door jam switches closed or you will be chasing the wrong drain. Once you find the fuse that makes the drain go away or the test light dim or go out get a wiring diagram and follow that circuit should be able to find the source of the drain. Now on some vehicles the test light will be dim this is due to things that are a "safe" drain on the battery like the memory in the radio and different capacitors that will hold a charge. But if there is a drain that is making the batts go dead over night then the light should be pretty bright. Like I said it is real important to make sure that the hood light is off, as well as the doors closed you may have to open the doors to get to the inside fuse panel just make sure to close the door jamb switch so the lights are off. Also once you find a drain by pulling fuses note the fuse that made a difference in the light and keep going to make sure there is nothing else. And the Fuses under the hood may make a difference but they sometimes feed multiple circuits to narrow it down it most likely will be an accessory ciruit on the inside fuse panel. Sorry this is so long but wanted to try to be clear on what i was trying to get across. Hope this helps you out keep us posted on the progress and if you have and questions on the testing set up let me know.

 

i suggest having the batteries tested. can be done at most any auto parts store and i think it is a free service.

 

Could even be a bad cell in one of the batteries. GF's car did the same thing. It would start fine on the charger, everything worked, but did not have enough juice to start the car. The longer it would sit, the less voltage the battery would have.

Disconnect the batteries, and watch the voltages. If one drops to zero, it is probably the problem child. If it is a battery, replace them both.

If you do the voltage light test or the DC clamp on meter test, be patient it could take a couple of minutes for the amps to drop.

You can even disconnect both batteries and use one of the cheap meters to bridge the circuit as described in the light test. Do it on the positive side of one of the batteries. The negative side caused the alarm to go off.

 

Both batteries run down-had tested they are good. Everything appears to work right, no add ons.