200mAh TL363844 Battery Replacement Compatible with Nest Learning Thermostat (1st Generation) T100577. Please Note: In most cases, the Nest thermostat can get enough power from the systems heating and cooling wires to charge its internal battery. But in a small number of situations you may need to connect a common or C wire to deliver enough consistent power to your thermostat and keep the unit charged. Here is the URL of a help article for more info: https://nest.com/support/article/When-Nest-needs-a-common-C-wire#a-typically-Why