Long story short, a "new-to-me" 1DX has some mild battery issues. (Bought from KEH)

Upon inserting the battery for the first time - nothing. Not even the flashing dead battery indicator.

Put it on the charger, didn't do anything for about a half an hour and then started charging.

Went out to shoot a game to test out the new camera - battery worked as normal for about an hour and then started going out. I'd take a few shots, then the dreaded "dead battery thunk" of the shutter and the whole thing would turn off. However, switching it off for 5 seconds and then turning it back on makes it work only for the problem to recur a few shots later.

Can anything be done about the issue, or am I in the mud for a bit? Normally wouldn't be a big deal but I was called to shoot some things in the next couple days and feared I couldn't get a new one on order in time.

Lens used was a 200-400 f4L 1.4x.