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Questions and Answers

Batteries And Battery Testing

Q. Vincent, I had a problem with a battery in my Chevy Conversion van. It was only about one year old and it did not keep a charge. I only use the van every week or two and thought maybe there was too much drain on the battery. I checked the residual flow with the van off and it was about 30 ma. This did not seem to be too much of a resting draw to do anything.

Batteries And Battery Testing

When the battery was tested at the shop with a load tester it tested good. I thought maybe it was my alternator, however the voltage is up and doesn't seem to waiver when the fans and the lights are put on.

I also ran through your test with the volt meter and found some odd voltages between cells. One was about 1.5 volts and another one was 3.5 volts. Total voltage is OK. I purchased a new battery and all voltages are around 2.1 volts just like you said they should be. OK, I finally gave up on the old battery and bought a new one, but how do you know the battery is bad from the cell to cell voltage? Hydrometer readings were good and all about the same when charged up.

I really wanted to read the alternator amps to make sure it was putting out enough current. Maybe one of the rectifiers was out and I was getting half wave charging. I put my oscilloscope on the output of the alternator and it seemed like I was getting square waves. This is hard when you have such a great capacitor as a battery in parallel with it. My dad gave me one of those inductive amp meters that you place on top of the alternator and that showed it was putting out about 75 amps. I tried it on my Nissan Maxima it read about the same. I really don't think it was the alternator but I don't know anything about these ammeters and they only cost about $7.00 in JC Whitney.

It has been about a month now and so far the van is good. I didn't have the problem when the old battery was new last year either. The first battery was purchased at K-Mart and the auto parts guy says it was made in 1999 so it is already old. It also had a bulge on the one end. What I don't get is that the battery is out of the van now and holding a good charge. It just bothers me. If you could tell me why the cell to cell voltage matters so much, I'd really appreciate it. Maybe it has some kind of internal short or something. 30 ma for 24 hours a day is about .7 amp hours/day or about 5 amp hours/week. The old battery was a 1000 cranking amp battery.

Anyway, any info you have would be appreciated. I really like your site. It is the best that I could find out there. Thanks, John

A. Batteries are mad up of a series of cells that are connected inside the battery case. A normal cell in a good battery will be about 2.1 or 2.2 volts. When you put six cells together, you get a 12 volt battery. The connections that go from cell to cell are attached to the plates that make up the cells. Sometimes this connection can go bad. Remember the fluid inside the battery is sulfuric acid, very corrosive stuff.

Batteries And Battery Testing

If the connection opens, the cell will read about 0 volts, depending on how bad the connection. If the cells short out, then you will get a reading of more than 2.1 volts, if two cells are shorted you will get about 4.2 volts, again, depending on how bad the connection. So what the cell test tells you is how good the internal connections of the battery are. Opens and shorts will make the battery go dead or not hold a charge.

An ammeter is a necessary tool in checking a charging system. A good voltage reading is nice, but if you don't have the amps, volts are worthless. When you test the output of an alternator you hook up your ammeter. Mine wraps around the battery cable so I see what is actually getting to the battery. Then you put a load on the system and make engine rpms about 2,200 to 2.500 rpm. This is where the alternator works most efficiently.

Now the amp reading should be pretty close to what the alternator is rated at. If it is a 75 amp alternator, the amp reading should be between 70 and 75 amps or above. If it is a 125 amp alternator, the amp reading should be 120 to 125 amps or above. If a 75 amp alternator is putting out 50 amps, it will test as good, but weak. I replace the alternator if it is below half it's rated amp output.

Additional Information provided courtesy of ALLDATA

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