Hello! please help.i have a 1992 civic. last night the battery light suddendly came on and i drove it for another hours until i got home the car die off and all the lights, radio,and horn are dead. than today i started the car its fine,12.30volts,but when i test the voltage it keeps dropping really fast until it die off again

 

12.3V is extremely low - you should read 14v with the car running. Replace the alternator.

 

I agree. You should test the output of the alternator while running. Anything less than 14v means your alternator is going south...