Battery light is coming on and going off. the voltige meter dropped and lights dim but light goes off and volts return to normal

but when I turn the key the radio goes off and ??? When you turn the key to start ? Do the dome light's dim ,the instrument cluster lights go off or dim ? Battery , battery connections , battery cables . Voltage drop test the main battery + an ground cables .
Starter Voltage Drop
The Trainer 47 What measuring voltage on the ground side means

Charging problem

I think you may have a dead cell in you battery

I think the battery is shot

Is it over 5 years old replace it
then test the alternator output

Doubt the alterntor is bad the battery
won't hold a charge that is why your
lights are so dim,too much resistance
in battery cells

Doing voltage drop testing will show where the problem is ! Watch these videos an get yourself a volt meter an do some testing .
Mastering Voltage Drop Testing with Pete Meier and Jerry Truglia

replace your voltage regulator...hope it did'nt fry your battery ??

Generally the light will come on when the charging rate drops below normal, though sometimes it can actually go above. If wiring is intact, drive-belt is tight and battery tests good (age does not matter), the problem may be either a diode or regulator beginning to fail inside the alternator.
To get a good "ballpark" idea of how your system is working, put a volt meter on the battery and run it. Voltage should be about 13,5 volts then turn on all accessories. volts should not drop below 12,5 and should not slowly drop below 12volts.

Well if lights are not staying on then you have a draw of power somewhere. Easy way to find out is volt meter. You take off negative cable and put one end of meter to battery post and other to wire dont let the wire touch post or anyother metal of vehicle. Then go through your fuse box and pull fuse's one at a time until you see voltage on meter drop. Meter should should a (-)negative voltage at start and get smaller when right fuse is pulled take notes and pull all one at a time and replace back in orginal spot. If you find one that really lowers the negative voltage draw on meter good chance you found where problem is. Id start by checking all interior lights first, including glovebox.

Its not unusual to have a sudden drop of volts. especially if you have something running like your a/c or heater that requires a lot of power to run. it will also do it when your fan kicks in. anything that requires power is going to kick down your voltage. i would not worry about it unless the battery starts to die overnight.

Connect a volt meter to the neg. and pos. of the battery. Have an assistant start your car and watch to see how far the voltage drops. If it drops more than 2 volts, for example, hook up the meter, you should see 12 volts. By starting the car and it drops below 10 volts during start up, your starter is slowly going out. It taking more amperage to start it than normal.