GoPro battery problems (again)

I use a “sidecar” battery for my GoPros to get some extra battery life. And when I was using them with GoPro Hero 5s, I was frequently getting 3.5-4 hours of video recording. But at the Round The Mountain race, one of them shut down after less than an hour (I’d turned them both on really early hoping to capture some of the flavor of the start area, and it ended up shutting down when I was warming up). I took them out for another test and I got 99 minutes out of the first and 120 minutes out of the other. In both cases, the sidecar battery appears empty or nearly empty, but there’s plenty of life left in the “built-in” battery. I’m able to turn them on and record another 80-90 minutes after I finished.

I don’t know if it’s an overheating problem, or what. Unfortunately the sidecar requires both it and the camera to be in a clear plastic case, which can’t help with the heat problem. I wonder if I should cover the whole thing in tin foil except the lens area?