KJ completely dead out of the blue?

This afternoon my girlfriend stopped to get gas. She shut the vehicle off and then when she went to restart it she got nothing. The vehicle was completely dead. The entire electrical system just died. By the time I showed up(15minutes later) the lights on the dash would start flickering when you put the key in the run position. When I tried to start it I got nothing. It wouldn't even crank. I end up towing to my stepfather's house with my JK.

I did some real quick troubleshooting but nothing too in-depth. The battery has 12.5v at rest. The battery cables are good and secure and also read 12.5v so I know there's contact. When I had my girlfriend attempt to start the vehicle while I was under the hood I noticed chattering coming from the power distribution box. The fuel pump relay was chattering. I swapped it around with the wiper hi/lo relay and it did the same thing.

Does anybody have an insight as to what I should be looking for?

One more thing that might be note worthy. This Jeep has had the BCM replaced in the past by a Jeep dealer.

You need a good 12 volts to start any 12 volt starter I dont think 12.5 at rest is going to make it under a load take the battery in and get it load tested.

sounds like the battery

I think you hit the nail right on the head with the load test. The battery voltage is fine at rest but not under load. I put my OBDII scanner on it and I saw two pending codes; P0700 and P0562. P0562 is "low system voltage". I threw the battery cables on it and let it sit for a few minutes. Then it reluctantly started up and everything seemed fine. I'm going to put a new battery in it tomorrow.

Agree with battery prognosis - not enough juice.
Lancer also had his stop in mid travel recently - battery was defintely on the out as well as a 15A fuse but I don't know exactly which one.
Perhaps a PM to him may confirm which fuse.
The battery and the fuse went out together.

I'm surprised there isn't a low voltage cut off built into the BCM. I was afraid something was going to get fried with the way it was acting.

You could have a look at fuse # 28 and ISO relay #30...both Starter (Power distribution centre)).