I have the same problem. Our FMB920's are shutting down after about 30-60 min, they are connected to a 9V battery with 800mAh. Ignition and movement source are configured on Accelerometer. When I disconnect and connect them again, they are working again and I can see that the internal battery has enough power (60%). These trackers are working 3hours a day only and configured on Deep Sleep after a timeout of 15min.
Do they need a minimal 12V power supply to operate stable?