TIROL 12V LED Digital Battery Alternator Tester 6 Led Lights Display Indicates Condition LED display indicates condition, for 12V battery only. Charger check: when green light is lit, charging is complete. Battery check: indicate the percentage charging in the battery . 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.