This Nest Thermostat (2nd and 3rd Gen) replacement battery is what you need to bring your dead thermostat back to life! Battery degradation is an inevitable part of your thermostat's lifespan extend it with this replacement battery compatible with the 2nd and 3rd generation Nest Thermostat. If your thermostat wont turn on, wont hold a charge, or you simply experience poor battery life, this replacement battery may be what you need to fix it. This part is compatible with Nest Learning Thermostat 2nd and 3rd Generation.