[fyr145]

Could be any number of issues. Lets look at a few of these.

1. Voltage regulator: Most late model units today have the voltage regulator built inside the alternator. To rule this out, you could simply get one of those load testers and a meter and measure the output of the alternator. While your at it, check the tension of the serpentine belt to make sure that you're getting the proper revolutions to even generate any power. Maybe also look at the pulley on the alternator and ensure that its not worn out and making any noises. You can find out how to make certain connections for specific tests on You Tube.

2 Battery: You could have a bad cell in the battery. How old is your battery? (Here in AZ, batteries only last an average of 3-4 years due to the excessive heat.) But large electrical loads from powerful aftermarket audio equipment or using lots of off road lights can also drain a battery. If the battery is not taking a charge, then either the alternator is not sufficiently charging the battery OR the battery is not taking the charge provided by the alternator.

3. Bad or loose Ground(s): Another remote possibility is a ground wire is not secure to the frame. Check all ground locations starting with the engine, then the negative cable on the battery, and so on.

These issues is where I would start to begin to rule systems out. As with any electrical gremlins.......its always best to start with one system at a time and go from the easiest to check to the more difficult ones. Be systematic in your approach.