Been having the battery light on my dash for a year. So I did some measuring today. Before I started the engine, I measured 12.9 volts. But when the engine was running it went to 12.1-2 volts which is low, but after a single blip on the gas it went up to 14.5 volts and stayed there. Could this be the regulator or the alternator?
How old is the serpentine belt?
Tester
I’m not sure
Do people replace serpentine belts? Its there and it spins the system 
Yes.
When they know it’s worn.
Tester