The lithium-ion rechargeable battery in the Nest is rated at 3.7 volts, 2.1 Wh (568 mAh).

The Nest receives a constant source of power from your home's thermostat lines, a low-power connection that can't provide enough juice for big operations like Wi-Fi broadcasts and powering the LCD. Keeping this battery topped off means the Nest always has reserve power to spare.