new car battery fails test - which mechanic should I suspect?

I bought a new car battery a year a go from a mechanic. It just “failed” a battery test at the dealer’s. Which is most likely? a) the battery is defective b) the dealer’s tester is defective. What does experience with these two pieces of technology tell us? Thank you. (For now I am ruling out c) defective mechanic, and d) defective car dealer.)

Is the battery starting your car? If no, I’ go with a. If yes, get another test.

Define “failed” in this case.

Exactly what sort of test did they do, and exactly how did it fail?

If you measure the voltage with a voltmeter, the battery should measure somewhere around 12.6 when fully charged. It will measure about 11.8 or so if it is completely dead. If it is measuring significantly lower than either of those, it probably has a dead cell in it. A bad battery would be much more likely than a bad meter.

If they are measuring the cranking amps that the battery can put out, then the battery could be bad or the tester could be bad. Corroded connectors on the tester could prevent enough current from flowing, for example. Sulfated plates inside the battery (a battery problem) could prevent it from supplying enough current as well.

You may have a bad battery, but your battery may not be the root of the problem. If your alternator is weak and isn’t charging the battery properly, then the battery will end up getting too deeply discharged on a regular basis, which in the long run will kill it. Why did you replace the battery a year ago?

Does the battery struggle to start the car?

Have you gotten the alternator tested? It could be that the battery isn’t getting a full charge.

Also, make sure the battery cables are clean and not corroded.

Some guys are not very good at testing batteries and some battery testers tend to be very good for selling batteries. There really is no way to answer your question without testing the battery myself.

f) the poster who asks about faulty devices – but prefaces it with the apparent presumption that the mechanics (who would not have caused the battery or the tester to fail) are to be blamed – is defective.

Dealers are very aggressive about selling parts and service. They may have a very high “pass” threshold for batteries and yours tested below that. It doesn’t mean your battery is bad. I had a shop (not a dealer) tell me my battery was bad and I drove it for over a year before I needed to replace it.

So why were they testing your battery if you just bought it a year ago? Did you ask them to? No? I thought so.


PLEASE, find out IF your mechanic is a problem **BEFORE **accusing him/her of doing a bad job. The problem may be your cars battery, its charging system or its starting system.

First, why was this test done? Second what kind of test was it? Third, was it fully charged for this test? Was it charged with a known good battery charger, or just your car?

Assuming that the dealers tech is honest and knows how to do this test;

If it was just a voltage test, and it had not been charged with a known good battery charger, you have learned nothing besides it is not charged at this time.

If it was a load test, and it had not been charged with a known good battery charger, you have learned nothing besides it is not charged at this time.

Have you noticed a pattern here?

If it was a load test and it had been charged with a known good battery charger, you know that you need a new battery.

If it was a voltage test and it had been charged with a known good battery charger, you know that it will not hold a charge. You need a new battery.

Another pattern here.

Buy a battery with at least as large of CCA (Cold Cranking Amps) as the ORIGINAL battery, not the one that is in your car. Some PO (Previous Owner) may have installed a smaller (cheaper) battery at one time. If so, this may be part of your problem. One thing to note about batteries, the heavier, the better. FWIW, I run a semi battery in my VW bug. It is huge compared to VWs specs, but it always starts and it will crank my engine over for a loooong time if I need it to.

Like engineer_comp_geek, and aceplace57, I want to know why you replaced the battery one year ago. I would also like to know if you left the lights or something on, and drained the battery, and then left it with a low or no charge for a day or two in this freezing weather. If so, there’s your problem. If not, I would like to find out what your problem is. Rarely batteries go bad after only one year, sometimes, but very rarely.

You should check your entire charging system in any case. Buy new cables if there is any signs of corrosion on either of them. Get some at least as big in diameter as the old ones. Now is not the time to buy the cheap ones, neither cables nor batteries.

IHTH, 48.

Most car dealers these days use a capacitance type tester that is pretty close to idiot proof. Enter the CCA and if you are testing in or out of the car and a few seconds later you have a result, often with a printout of the results.
Batteries do go bad even in 1 year. This is not unknown.
As to why the test was done at many shops (not just dealers) it is SOP to test every battery. Yes it sells batteries but just as important it heads off those “my car was just there, why didn’t you find this/tell me?” Phone calls.
One more thought I don’t know where the OP is located. If it is very cold there and the battery is cold it might test marginal or bad due to the cold slowing down the chemical processes.

Defective customer?

A. is my WAG if the test was done correctly.
You allude to having the battery changed by someone besides the dealer, well ask the mechanic for a second opinion. A one year old battery can fail from no fault of yours and be covered by a warranty or from your poor conditions up to but not including freezing and you may still be covered by the warranty but a frozen battery will almost always bulge and certainly not be covered.
Many years ago i brought 2 battery’s back to a discount store and claimed they were bad and they replaced both. Well that winter i found the problem.
When the body shop replaced the hood and grill after damage from a Deer, well the under the hood light didn’t work. Well it did work, but only when the hood was closed and it was obvious when the light kept the frost off that area of the hood.
The Discount store sold the cheap-o Dura Start battery’s and had so many come back they didn’t even test them all the time.