Car electrical problems.

December 30, 2007 4:06 PM

Can I diagnose my electrical problems with a small multi-meter?

A few days ago, I drove a few miles to my friend's house, and when I went to start the car after 20 minutes, the battery was dead. AAA came and jumped it, and the guy with the charger declared it was not the alternator - after he got it running, the meter on the charger showed there was juice going where it should be, apparently. Then last night it would not start again - I was able to use my little charger to get it going, but only after letting the portable charger get really juiced up.

The battery is less than a year old. Is there any way to test to see if it is not charging up as much as it should be with a small radio shack multimeter? Today I drove 22 miles, hung out for an hour, and then drove the same distance back - Should it be fully charged at this point? Can I detect if anything is actively draining it?

posted by mzurer to travel & transportation (7 comments total)

Where is your battery from? If it's from a larger retailer, have them check it (they will probably do it for free). Although it probably isn't the battery, it doesn't hurt to have it checked. Then you need to have your alternator checked. The alternator can be generating enough electricity to run the car, but if the diodes are bad, it can also allow the battery to discharge as soon as the car is shut off. I suspect this is the case. An alternator can be checked by many (most?) auto parts stores; I've had pretty good luck wiht AutoZone. Check with them to determine whether you have to remove the alternator or they can check it with it on the car.

We had a "new" battery go bad recently; one cell went bad and the whole battery was shot. You never know. That's why they put warranties on them.
posted by Doohickie at 4:21 PM on December 30, 2007

In my car the diodes and brushes are in the same unit, which is inexpensive and can be replaced quite easily if you have adequate access to the alternator. Fifteen bucks and two screws.
posted by weapons-grade pandemonium at 5:30 PM on December 30, 2007

You probably have a bulb on or some sort of short. Good places to look include the light in the glove box (does the switch on the door really still turn it off when closed) etc.
posted by caddis at 5:33 PM on December 30, 2007

Yes, you can detect if it is being drained. Put the meter into amperage mode and place it between one terminal of the battery and the wire which connects to that terminal. If there is current flowing then there is a short or something on draining your battery. (Modern cars have a lot of electronics so there is probably a very small trickle current even under normal circumstances. You are looking for something of significance here.)
posted by caddis at 5:37 PM on December 30, 2007

You can diagnose some battery problems with a multimeter.

Here's a page describing how to do some basic tests.

The only thing I've done that's not described there is just measuring the voltage being produced by the battery without any load (with the + lead to the car disconnected), after it's been charged for a while: if you get less than 12V, and in particular if you get less than 12V by a significant margin, one or more of the cells inside the battery have shorted/died and it's time to replace it.
posted by Kadin2048 at 5:42 PM on December 30, 2007

Oh, and here is a chart showing the "open circuit" (car is disconnected) voltage that a typical 12V car battery should have at various levels of charge. 12.65V is considered 100% down to 12.06V is 25%. If after a lot of driving around you're not at 100%, something is wrong -- but determining whether it's the battery or the charging system will require some of the tests mentioned in my previous link.
posted by Kadin2048 at 5:46 PM on December 30, 2007

Another thing to consider is the starter may be going bad; if it has not failed completely the starter will take more power than usual to turn and so will act very similar to a dead battery. I speak from recent experience, but here is another description from Click and Clack.
posted by TedW at 5:56 AM on December 31, 2007