Pirtûka Hilberînê:

The EM501577 evaluates the battery’s ability to crank an engine.
The tester draws current from the battery while measuring its voltage level.
The voltage level of a good battery will remain relatively steady under load, but a defective battery will show a rapid loss in voltage.
Battery size (CCA rating) and temperature will affect test results.
Battery tester: Evaluates the battery’s ability to crank an engine.