By October 15, 2007 1 Comments Read More →

Car Battery Keeps Draining

Hey there! For the last couple of weeks I have been battling an issue with my GMC truck. The truck has a 5.7L 350 cu. in. motor and some mild performance mods. For some reason though, the truck has been dying on me from lack of battery power.

At first I noticed the voltage gauge on the truck beginning to decrease slowly. Well, eventually the battery completely drained to a point where not even jumping it would help. Immediately we figured it was the alternator, and even though we had already replaced it with one from Auto Zone, we swapped it out again. Several days later, the truck exhibited the same problem.

Tried jumping it again and nothing. So, we bought a brand new battery from AZ and put it in the truck. Same thing; days later it happened again. NO POWER! This round we replaced the battery cables with a brand new set from the dealer. With the battery fully charged (used a battery charger) we ran the truck again.

Everything was fine, but then a few days later, the battery drained again! Aside from replacing the aforementioned parts, we have checked just about all the charging systems wires and nothing has come up. Iam not leaving anything on overnight or during the day, so what could be wrong? I am at my wits end with trying to firgure this out.

Please help!


Hey there Steve,

Well, you seemed to have replaced just about all the parts of your charging system. I would want to run a full electrical system test on your truck to see what is going on…or NOT going on. I have a feeling either you are having a problem with the alternator not producing enough power to keep the battery fully charged. By chance did you replace the alternator pulley or did you swap your old pulley from your original alternator and reuse it on the new alternator?

I say that because I have seen different size diameter pulleys…’s rare, but a smaller diameter pulley will not turn the alternator fast enough. Just like on a 10 speed bike, the smaller the sprocket used produces more power but less revolutions.

Also, and what is really the problem here is you might have had a higher output alternator than what you were given as a replacement alternator. There is probably an option for a high output alternator, at least you should ask Autozone about it and see if by chance you were given the smaller out put alternator.

Lastly, I would want to run a “drain test” to see if you have anything that is draining power from your battery. A full electrical test should include:

1. Load test the battery
2. Check the alternator output
3. Check for an electrical drain
4. Check the battery cables and connections and grounds
5. Check alternator plug connector for corrosion and make sure all power from alternator is actually making it to the battery

Honestly Autozone is NOT the place to get this test done. They are fine with load testing the battery and checking the alternator OFF the vehicle, but they are not really trained to diagnose this type of problem with the parts installed on the vehicle….which is how the test should be performed.

Austin Davis


Reader Follow Up


Good morning Austin! Well, I have good news! I figured out the problem and the voltage is doing better. We are getting between 13-14 volts sustained. Apparently, the problem was with the alternator not being grounded due to the powdercoating on the bracket on the front of the motor.

I purchased a ground cable from Pep Boys and attached one end to the back of the alternator and the other to the frame. Immediately, we could tell the difference just in the way the motor was idling. Voltage jumped a good 2-4 volts at idle. We have been driving the truck around and have not noticed any more problems. I appreciate all your help, as without it I would not have began to think critically.

Have a great day!

About the Author:

Austin Davis, consumer car repair advocate. "Hi there! I love to help people solve their car repair problems and I hope my site was helpful to you today. Thank you for stopping by."
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