I own a fully restored 1983 t bird heritage that has gone through 3 batteries in 5 months. I bought it and the battery went bad in a couple of weeks. I bought a new battery and after about three weeks it went dead again over night. I put a multi-meter on it and it read 12 volts but when I tried to start it, it droped to three volts. I thought is was just a defetive battery so I exchanged it and has now happened again. I have checked the alternator & the regulator and they are fine. What can destroy a new battery in just a couple of weeks?

 

Do you have good clean connections on your battery post and cable ends? With everything off, have you checked for parasitic drain on the battery (something not shutting off)? With everything off, put a amp meter on the positive battery lead and see if there is any draw. Being a 1983, there's not much of a computer on it so there should be no amp draw or just a few milliamps. If you do have amperage draw, start pulling fuses until it goes away. That would be your culprit.

 

I had similar problems with my '85 van. Turned out to be a bad positive cable.