I been having this problem for a least two moths for some reason when I turn my car on a leave it running on the road, I say for 15 min the voltage level light comes on and then the gauge goes to I say 10 volts and then for it goes back to normal sometimes the level shoots all the way to the max then goes back to normal. I have turn the car off when the light and gauge goes on and to 10 volts I start the car back it turns back on. Leave it running for while it goes back to normal please help

Sounds like you want to get your gauge checked, now I have seen altenators overcharged and dropped voltage, but it really sounds like its your gauge!!

The alternator likely needs replacing or rebuilding.