Some of these devices report in 1/9ths of battery life (I’m assuming it’s the device and not SmartThings). 77% is 7/9 (it should round to 78%, but it doesn’t).
My oldest of these sensors is just 6 weeks old and BatteryMonitor shows it is at 77% also. All my others report 88%. It’s too short a period to really tell how long the (single CR2) battery will last.
On a different note, one of my 6 Iris Motion Sensors just failed – it was reporting motion constantly. It was still updating the temperature, but it was stuck on motion for 3 days. I’m exchanging it tomorrow.