A few days ago I had my Defrosters on front + rear, air on max, wiper blades going full blast, and headlights on and about 2 minutes into having that all on i noticed my Check gauges light came on and my battery voltage was 0. everything was still working though.

I tested it again and I saw the battery voltage drop some then suddenly drop to 0. as soon as i turn the stuff off a few seconds later it goes back up to normal readings.

Its always at normal readings also with the lights and radio on but when the air was on max and defrosters and wiper blades on it drops slowly then takes a plunge to 0.

Would this be a bad battery or do i need a new alternator?

 

Verify any and all suspect IP voltmeter readings with a good multimeter at the battery terminals before doing any additional troubleshooting.

I've gone through a test procedure that will cover the starter motor, battery, and alternator all in one go - search around here, and you should find it (because, to be honest, I really don't feel like typing that all out again...)

 

He is right, get a multimeter and check the voltage when it says 0 volts. If it was 0 volts nothing would be running

 

got it figured out. Used a battery tester to test it out. It was good while nothing was running except the XJ. then as I turned the lights, wipers AC full blast the meter started to drop on the battery tester and on the dash inside slowly but not to 0. It shouldn't have dropped at all. Then I used the alternator tester on it and it showed that I have a bad diode.