You are adding more than I've said ever.

I simply said that by my example of it really happened to me: a standard load + capacity test gave my battery a clean bill of health and was wrong. Then followed up with if it happens to you, consider that the test may have been inaccurate before you replace the starter or alternator chasing the problem.

What will typically happen in such a case (battery tests ok) is somebody will assume that means the problem lies elsewhere. That can be a very costly proposition.

The battery testing formula is of course an estimate. They measure for 15 seconds and estimate the long term capacity of the battery. It uses a *formula* so by definition is an extrapolation of capability. It's more accurate than the DTE in most cases but in some cases clearly the load test is way off.

I would bet 1/100 to 1/1000 odds of repeating the high current low capacity situation I had.

Regardless if people call load capacity or vice versa there are two very different and distinct ratings for batteries that will be on every car battery sold:

CCA and AH.

Cold Cranking Amps and Amp-Hours.

CCA a measure of how fast the battery can deliver charge when cold. This is almost always a calculated estimate because batteries are not fabricated at 0° F and the testers use estimate curves to predict a CCA based on ambient temp and current at that temp.

That is a measurement of LOAD. Not in any way shape or form "capacity"

Capacity of a battery is measured in amp•hours, and can ONLY be measured over time there are no short cuts.

Typically a battery will be subjected to a load such as 5A and the time it takes (hours) to drain to 1.75v/cell will give you the amp•hours (an 80AH battery should give you about 16 hours of 5A discharge for example)

I own hardware for capacity testing batteries: it has FETs and big heat sinks and a fan. It will pull a calibrated drain from a battery and will shut off at a design limit say 3.0v for LiON cell. I've never seen a commerically available tool for testing a car battery capacity only load capability.

I've seen people CALL a load test a capacity test; it assuredly is not. It's a "capability test". Pulling half the CCA from a battery for 15 seconds is a good test of it's capability, it has no relationship in any way to the AH or capacity of a battery.

I am instructing people in the normal care of a suspected end of life battery: the first thing to do if a battery makes itself suspect (old, requires a jump start more than once and no known drain), you of course charge it up (typically a solid drive of an hour is enough to do a test but overnight on a charger is better) then go get a load test.

99 or 99.9% of the time that will be conclusive, most often when an old battery can't start a car after you have left the lights on for 10-30 minutes, coincidentally the amp capability has been diminished and the battery will fail a load test and it's a clear cut case of end of life.

SOME of the time the battery will pass a load test; and some people will even say a "capacity" test (which cannot be done in seconds or minutes but takes large parts of days).

There will absolutely be somebody that chimes in eventually "me too".

I've had MANY MANY MANY batteries fail in this way: countless. Back in the day with NiMH chemistry they used the term "memory effect" to describe the case of a battery that would not lose current nor voltage capability but would lose mAH capacity.

I've only had ONE of many CAR batteries do this. It had at least 3/4 of it's new amp capability but lost maybe 95-98% of it's AH capability.

Put on a charger and it just saw the battery as fully charged, pull 500A out for 5 seconds and it maintained voltage. Pull 1/2A for 20 minutes and voltage dropped to 9.6V.

Jump start and it's fully charged in 20 minutes ready to sit a week and start the car without a problem.

So, yes a very odd case probably 1 in 1000. But if somebody else can realize it's the battery not the alternator or starter and learn from my real world example my job here is done.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)