CARTMAN 12V Car Battery & Alternator Tester - Test Battery Condition & Alternator Charging (LED indication) LED display indicates condition, for 12V battery only. Alternator state check, battery state check, check before charge and overload display. Alternator check : accelerate your engine to 2000 rpm when the green light is lit ,charging circuit is ok. Battery check: indicate the percentage charging in the battery . Charger check: when green light is lit, charging is complete.