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.)