18650 battery powered temperature sensors would lots many years without to re-charge, for temperature sensors its best to use 3.3v Arduino Pro Mini with lower clock speed to conserve battery. To optimize further i have removed power LED, and status led connected to Arduino mini pin 13, then feed power to Arduino mini through raw pin with 18650 battery. i have these sensors running over 12 months now and battery is still 96%.

Pihome 18650 Battery Powered Temperature Sensors

Pihome 18650 Battery Powered Temperature Sensors

Pihome 18650 Battery Powered Temperature Sensors

Pihome 18650 Battery Powered Temperature Sensors

Pihome 18650 Battery Powered Temperature Sensors

Pihome 18650 Battery Powered Temperature Sensors

Here is MySensors sketch i m using for temperature sensors for low power sensors node.