It uses a TMP36 to read temperature and it is optimised for power consumption.
It uses watch dog that wakes every 8s and increments a counter. Once the counter reaches a certain value (e.g. equivalent to 5 mins), it will read and send temperature to the gateway.
Every hour the voltage is also read and sent.

According to the calculator here, https://www.geekstips.com/battery-life-calculator-sleep-mode/, a cell coin battery with a capacity of 240mAh should last for ~ 3 months (108 days), given that:

Current consumption of device during sleep is 4.5uA
Device wakes up every 5 mins (12 times/hour)
Current consumption of device during wake is 10mA
Duration of wake time is 1000 ms