I have a 03 Dakota 4.7. was driving home the other night and noticed my headlights pulsing slightly. then noticed cluster was too and battery gauge was at zero. I automatically figured bad battery. got home and parked and out of curiosity tried starting. it started perfectly normal and gauge was up. put a charger on the battery anyways. next day went for work, started fine, then about 20min in the gauge drops again. I'm wondering if it's still a bad battery, or would it be a sensor/relay problem?

 

the battery gauge illuminates due to low charging voltage. next time it happens check the alternator output.

you should have 13.5 to 15.5 volts at the battery and alternator. if you got 12.6 or less...your reading the battery voltage...its not charging.

 

it was bad alternator, thanks