Xiaomi Yi just shuts off

I have just been out mapping, and suddenly realized that both of my Xiaomi Yi had shut off. They had been running for about 30 minutes using the ash script that takes a photo, sleeps for 1 sec and starts over. I started them and they worked fine. Then they shut off a little later. I started them and they worked fine, then one shut off later. There was plenty of battery.

Has anyone experienced this?

Last Sunday I drove for about 3 hours without any issue. They got extra power from a power bank with 3 outlets.

I never had this problem. Only some freezes sometime, for a few seconds.
I think that i have less freezes since I take 8mp pictures (more pictures per minutes)

Do you think it could be a heat problem?

The cameras did not seem to be any kind of hot. Personally I think more of a problem with system resources compared to what I do. The issue is just, that I only try what everybody else seems to do without any issue.