I would throw the battery on a trickel charge to illiminate the battery and while it's charging check everything else. Check for loose terminals and wires within your charging system. Also visually check them for chaffing and corrosion. Get a multimeter and check your output voltage from the alternator to the regulator, if it's low it's your alternator. Then check from the regulator to the controls, if voltage is low it's your regulator. I just got through troubleshooting my charging system and found a bad exciter and voltage regulator.

 

When ever you have a problem, always check with the simple things first. As others posted clean battery posts and terminals then tighten them down good. Don't forget to do the other ends of the cables and their connections also.

 

Just to close the loop and thanks for all the help, It was the regulator. This is what I was getting to sum it up. Check eng light and low volts. This would happen without warning hot cold...

For the FLH the crome one is cheaper to save a couple of bucks. If you can change a tial light you can handle this. Two plugs, two nuts, and two clips.