Max.K created this Arduino Pro Mini compatible development board for low power applications:LP Mini
ideal for data logging over extended periods of time [..] can run for months or even years on a coin cell
Hardware
- Microcontroller: Atmega328p with Arduino bootloader
- Real Time Clock: MCP79410
- Voltage Regulator: MCP1700
- FDN340P MOSFET
Features
- 2.8 µA current consumption in sleep mode
- Compatible to the Arduino Pro Mini (except for pin 2 and 10)
- A real time clock keeps the time and is used to wake the Atmega from sleep mode
- Sleep current is reduced by using a more efficient voltage regulator
- A simple Mosfet is used to turn external devices on and off if the IO pins cannot provide enough current
OSH Park 
You might be interested in check this project: https://www.hackster.io/Talk2/temp-and-humidity-sensor-with-a-cr2032-for-over-1-year-580114
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