Hey, I have a 82 Berlinetta with a 305. I recently got a new battery and all my problems started here. The battery I got worked for a few days then one day if i turned the key to crank all power would go out, no lights, no door noise, and no noise of the engine trying to crank. Put the battery charger onto the start setting and she fired up. Idled for a few minutes, then tried to reverse out of my driveway and she died right away. Each time I started it, it would die under any load. Took it to the highway and she died at the lights, the CEL came up and it died half a second after. The next day, it died again so I got a replacement battery which does the same thing. The second I turn the key the battery goes from 11.9v to 1.5v
Im thinking loose ground, what are you guys thinking?
Its rainy so I have a few days to think about where im going with this project.

 

Check grounds, clean any corrosion off. I'd also follow the hot line and check it's terminations as well. If you're still using the original GM battery connectors, you should consider replacing them as they tend to get corroded inside the plastic "covers" after a time. You can get replacement connectors at any auto parts store, most carry GM side post style.

I sincerely doubt that you got two bad batteries in a row, but it doesn't hurt to hook a meter up directly to the battery, and watch the meter while someone else cranks the car. You will probably see that the battery is keeping right at 13.8 where it's supposed to be, re-affirming a wiring fault in the vehicle itself.

 

I will buy the new connectors because mine are rusting now, and I tried to use the meter while my brother was cranking but it drops down to 1-1.5v almost instantly. Occasionally the engine will keep trying to turn as well, key can be out of ignition and you will hear a click, click, click, as the fan turns and then it will all die out to 0v. Melted heavy duty tow truck jumpers and then the fan wouldn't stop spinning. The tow truck couldn't get me started, only my Nascar charger seems to work....
A regular jump does not seem to yield enough power. I am dumbfounded.

 

sounds like a bad battery cable to me