Battery test accuracy

SMB

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Oct 30, 2007
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Orlando, FL
As you all know, Oreilly's, AZ and AAP all have free electrical system testing. They can test the battery, alternator and starter without taking it out of your car. My question is: How accurate are these tests? A while back I took my car battery and had it tested at all these places. 2 of them said my battery is good while the 3rd place said it was bad. TIA
 
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Jun 15, 2003
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ME
I figure battery tests fail on the side of condemning a good battery. If their alligator clips don't get a good enough contact etc current won't flow as well, voltage will dip, and the machine will say it's bad. What's their fancy alternator tester, a carbon pile that ups the amps until voltage dips? If so that would also condemn a good one early. If they don't know how to use it, eg run your idle at 2500 RPM or so for full rated amperage, that could factor in. The battery thing is useful if you have a warranty at that store and feel like a new one. WM was good on my warranty when I needed it: The clerk looked slightly puzzled by the tester machine but was "management" and therefore allowed to make the claim.
 
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Aug 12, 2007
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Wilmington, DE
I work for a AAA club. We use a hand-held tester that I guess is similar to the ones used by our competitors. It is greatly influenced by the quality of the connections and, strangely enough, the strength of the batteries in the tester. I have had a battery I use for demonstration purposes show a bad cell, and the next week shows needs charge. It will not start an engine either way.
 
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Dec 19, 2004
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beaver land EH?
 Originally Posted By: SMB
As you all know, Oreilly's, AZ and AAP all have free electrical system testing. They can test the battery, alternator and starter without taking it out of your car. My question is: How accurate are these tests? A while back I took my car battery and had it tested at all these places. 2 of them said my battery is good while the 3rd place said it was bad. TIA
Long questions deserve a short answwer: they are as good as the knowledge of the technician who use them, period. Q. (I can perform 70% of the test with your battery on-car with a hand-held multimeter and be able to tell you your battery condidtion, the rest require a carbon pile tester).
 
Joined
Sep 11, 2004
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snowblind in TX
Hard to beat a VAT40 with a real carbon pile. That being said I used one of the newer computerized battery testers that FoMoCo required techs to use in order to replace a battery under warranty, and it was phenominal. It was small and had two thin wires hooked to the battery. You couldn't pull that many amps through it, but it was accurate and could find a marginal battery. Some of the parts stores still use the little 100 amp "heater" load testers which may catch some problem batts, but not all.
 
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May 7, 2004
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Nokesville, VA
None of these load tests can detect reserve capacity loss. The only way I've found to actually determine how much reserve capacity (amp hours) a battery has is to discharge it with a load and time how long it takes to discharge. I had a deep-cycle marine battery that passed the 2-minute test (a Midtronics Incharge battery tested passed it) and failed the reserve capacity test. The battery in my Saab, when I did a reserve capacity test on it, powered the load for about half the time it should have. Yet it passes load tests just fine. Reserve capacity doesn't much matter for starting the engine, but it does matter for running any electrical loads in the vehicle without the engine running. Half the capacity means you can only run that load for half the time before it won't start the engine.
 
Joined
Jun 3, 2005
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Chicago, IL
 Originally Posted By: brianl703
None of these load tests can detect reserve capacity loss. The only way I've found to actually determine how much reserve capacity (amp hours) a battery has is to discharge it with a load and time how long it takes to discharge. I had a deep-cycle marine battery that passed the 2-minute test (a Midtronics Incharge battery tested passed it) and failed the reserve capacity test. The battery in my Saab, when I did a reserve capacity test on it, powered the load for about half the time it should have. Yet it passes load tests just fine. Reserve capacity doesn't much matter for starting the engine, but it does matter for running any electrical loads in the vehicle without the engine running. Half the capacity means you can only run that load for half the time before it won't start the engine.
This is true. If one of the cells has failed more than the others, or if they are all really bad, you'll fail a load test. But if the cells are all mostly the same and mostly good, you will just get reduced capacity. (I test batteries on UPS units by hooking them up to an old sealed beam headlight. Some quick math on the amp hour rating of the battery versus the wattage of the light and I can tell what kind of shape they are in.) (OK, usually I just guess based on the voltage drop and/or the brightness of the light, and how quickly the light gets up to strength.)
 
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May 7, 2004
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Nokesville, VA
I use an inverter and a desktop PC to test batteries. That makes it easy to time how long the battery lasts and is fairly accurate because the inverter always shuts down when the battery reaches a certain voltage. The way I time it is that I have a program that pings the desktop PC and records when it stops responding to pings, which is when the inverter has shut down.
 
Joined
Mar 2, 2005
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Austin, TX
I had a failing battery that Autozone said was still good. I went down the road and bought a new battery at a store that didn't want to argue. The tests aren't infallible and, oddly, don't seem to indicate that they "have their hand on the scale" in terms of replacement.
 
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