About this replacement
Nest Learning Thermostat 2nd & 3rd Generation — 3.7V Li-Polymer Replacement Battery (TL284443)
This 3.7V 380mAh Li-Polymer cell replaces the internal backup battery in the Nest Learning Thermostat 2nd Generation and 3rd Generation, including models T3007ES and T3008US. It maintains settings, schedule data, and basic operation during brief power interruptions from the HVAC system. When the original cell degrades, the thermostat loses its ability to bridge those gaps and may reset or go offline unexpectedly.
- 2nd and 3rd Generation compatibility: Both generations share the same internal cell footprint, connector pinout, and BMS communication protocol — the 48.60 x 44.10 x 2.90mm form factor fits both without modification.
- Bench tested on actual hardware: We cycled this cell through the thermostat's charge management circuit and confirmed the BMS accepted the cell, balanced charge current correctly, and logged no fault codes during the process.
- Post-install reconnection window: After seating this cell and reassembling the thermostat, allow at least 60 seconds before expecting Wi-Fi reconnection — the device runs a full network re-join sequence after any internal power interruption.
Why the Nest thermostat loses settings after an HVAC power cut
The Nest Learning Thermostat draws primary power from the HVAC common wire (C-wire) or harvests it from the heating and cooling control wires. The internal Li-Polymer cell acts as a bridge when that supply drops — during a tripped breaker, a furnace lockout, or a system switchover. A degraded cell can no longer hold enough charge to cover even a short gap, so the thermostat reboots and loses its clock, Wi-Fi credentials, and schedule. Replacing the cell restores that buffer capacity and stops the reset loop.
Low battery alert showing immediately after installing a new cell
If the Nest app throws a low battery warning within minutes of a new cell install, the replacement cell likely discharged below the thermostat's alert threshold during storage. The thermostat expects the backup cell to sit above approximately 3.6V before it clears the warning. Place the thermostat back on its base and leave the HVAC system energised for 30–60 minutes — the charge circuit will top up the cell from the C-wire supply. Check the Nest app once the cell reads above 3.7V and the alert should clear automatically.