What's the USB port on the back of the Nest Learning Thermostat display for?
The USB port on the back on the Nest Learning Thermostat display is primarily for service use, but it can also be used to quickly charge the battery.
The Nest Learning Thermostat normally charges using your HVAC wires, but USB will charge it up faster. You can plug the Nest Thermostat display into a USB port on your computer or a USB wall charger such as those included with a smartphone. You can charge the Nest Thermostat for an hour to fully charge the battery.
The 1st generation Nest Learning Thermostat uses a mini-USB to USB cable.
The 2nd Generation Nest Learning Thermostat uses a micro-USB to USB cable.