To test a vehicle battery under load, first make sure the engine is off and the surface charge has been removed by switching on the headlights for about a minute. Connect the tester across the terminals, observing polarity. Apply a load equal to half the battery's cold-cranking amp rating and hold it for fifteen seconds while watching the voltage. A healthy battery will stay above 9.6 volts at room temperature. If the reading sags below that, the battery is failing and no amount of recharging will restore it, because the plates themselves have degraded.