A few months ago, my 7D started throwing up this error, which would refuse to read the battery (even though fully charged) and would actually discharge it rapidly if left inside the camera for a few hours. Searching online for the issue revealed that it’s a common problem for many users, and actually a manufacturing defect in every 7D (MKI), though doesn’t reveal itself to everybody. There is a solution, which I found thanks to the author of this video, which details exactly what the issue is and how to fix it in less than 10 minutes.
My documentation basically follows the exact steps in the video, so hopefully this will help someone as well if you want to know what’s up at a glance.
For this, we will need a tiny phillips screwdriver, and a pry tool (or flathead screwdriver, but a plastic one is preferable to avoid damaging the camera body).
there are only 8 screws that need to be undone to get to the problem spot, marked in circles. Remember which ones go where, they’re not entirely identical (the bottom ones are).
Moving the rear panel back a little, allow the pry tool to leverage the bottom plate out. There is one spot you need to be aware of, under the grip, there is a tab that you need to pull gently. Be careful, it is plastic and can be broken if forced.
The issue of the problem is a screw that got loose and fallen off the pcb indicated in the first photo. It probably provides some kind of ground connection to the chassis, the lack of which causes a rapid battery discharge, and error in communication circuit (I’m not sure myself). The screw in my case was inside, magnetized to the flash capacitor.
Take it out, screw it back in, and follow steps back to reassemble! Voila! The battery wasn’t the problem, camera was.
This *might* void your warranty. My camera was years out of warranty, so I wasn’t concerned, however the entire process doesn’t involve breaking any warranty stickers, so you might get lucky in the future, if you decide to repair this issue yourself while on the warranty period. This would save you up to $400 USD in official Canon repairs.