Battery at 70% but no power

I was out working yesterday, and was watching my battery level carefully. It was cold (2c) but not freezing. When my battery level got down to 70% the machine just powered off. When I tried to restart it, it came up with the red flashing battery warning on screen. But, when I brought it inside, and plugged it in, it started working again, and the battery was back to 71% power.

Is this a problem with High Sierra or is it a MBP issue?

I wasn't the only one with this problem. There were a few other users who had the same problem, with different battery levels, but all around 50%-70% battery level.

Any ideas?