Solved Source For Replacement Battery For The Nest Thermostat V2 0 Nest Learning Thermostat 2Nd Generation

Chosen Solution

Battery as detailed in this iFixit teardown. Teardown URL Picture of battery

This unit uses a Lithium-Ion rechargeable battery. It is charged by the control lines or an auxiliary charger. It should no need replacing. If it does then you need to contact NEST for a replacement unit under their product warranty. Update (04/02/2017) The 3rd & 4th gen units uses an Energizer Ultimate Lithium (L91) AA battery. The 1st & 2nd gen units require a custom battery which is only available though NEST.