Dead battery?

[PCGUY112887]

The battery in my 3500 GP seems to be acting up. It sat for a month or 2 disconnected from everything and it still had a 75% charge when I first hooked it up to the new engine to start it. It's started and has run for maybe 5min total. When I first primed the engine (ran the starter with no fuel or ignition fuses to prime with oil), when I was done priming I put the battery on a charger for a bit to ensure it had enough charge to fully start the engine.

Few days ago of not really messing with the car (not driving or anything, just sitting there) the battery went dead (no lights or anything). Put it on the charger and it said 50%, let it go to 100% and later that evening it went down to 75% again over a matter of a few hours.

I notice that the car can no longer keep any power. Even if it's actually on the charger, if I flip on the headlights the interior lights and headlights/taillights go very very dim.

It acts like the battery is dead, but the battery was just fine I don't see why it would go dead. Also seems like something could be draining power from it, however I don't know what could be... it was just working fine I haven't changed anything.

[IsaacHayes]

I would remove the battery from the car, charge it up, let it sit for a few hours and see if it drops back down to 75%. If it does that means the battery is dead and time for a new one. It would be self discharging inside at a high rate. If you have it off the car and it stays charged up fully for a long time, then it's safe to assume the car is drawing current when off.

Sounds to me like it's just time for a new one. My battery is barely hanging on. When I start the car it drops to 8volts but still fires up (then the voltage goes up once the alt starts spinning)! Then if I have the A/C on it drops very low to 11-10.5v. Other than starting and the A/C being on, the voltage looks normal, but the alt is helping out with that. And with the car off the voltage is fine. It's just got a high internal resistance from being old and is time for a new one. It sags too much under load.

I'm pretty sure thats your problem as well, but you could test it like I said to make sure the car isn't sucking power when off.