When wet cell batteries (I'm ignorant about AGM and other advanced types) are charged the chemical reaction takes place from the anode and cathode connections and over time permeates the whole battery, so that immediately after charging a voltmeter will read falsely high. Apparently the charged state chemistry diminishes from the bottom up, which is why without a load the top of the cells will register a good charge while the cells at depth are weaker. A heavy load will quickly reverse the chemistry at the top of the battery and drop it to the lower average, generating the lower true readings.
I think.
Thank you.. I know load testing is one of the better ways (Actually a multi-test is best but those testers are expensive) Sometimes that simple carbon pile job will clear a bad battery or catch one the many dollar tester at say Auto Zone missed....
I do believe in testing under load.
Another example of a "load Test".. I have a couple of automotive test lights.
It's basially an ice pick with a 12 volt bulb and a wire coming out the handle... I clip the wire either to a known good ground or to a long "Extension" wire clipped to a known good ground.
Touch the tip to a known 12 volt source and if the light is not bright. Fix it (The tester).
Then I an test blade fuses in place touch one end then the other (The windows offer access
BRIGHT BRIGHT good. Dark Dark no test Bright Dark or Dark bright = Blown
I had a wire that went High Resistance.. Voltmeter shoed good.. Light dark... Found the bad spot and did a bit of surgery.. Worked well after that (Still does in fact)