Nest Learning Thermostat 3.7V Backup Battery 3701-0001-01

Nest Learning Thermostat T200577 — 3.7V Li-Polymer Replacement Battery (3701-0001-01)

This 3.7V, 450mAh lithium-polymer cell replaces the internal backup battery in the Nest Learning Thermostat. It fits the T200577, T200377, T200477, T200777, and six additional Learning Thermostat variants sharing OEM part numbers 3701-0001-01 and P11GY1-01-S01. The battery maintains the thermostat's clock, schedule memory, and programmed settings when main power is disconnected.

• T200-series compatibility: These Learning Thermostat variants share the same battery bay geometry, voltage rail, and BMS handshake. The 3701-0001-01 part number covers all of them — the same cell services the full T200 lineup without modification.
• Bench tested on actual hardware: We cycled this cell through simulated mains-loss events on a T200577 unit. The BMS accepted charge normally, held voltage above the thermostat's low-battery threshold, and passed the settings-retention check across repeated power interruptions.
• Post-installation reconnect window: After swapping this battery, allow 60 seconds before expecting the thermostat to resume normal operation. The Learning Thermostat runs a network re-join and clock-sync sequence after any power interruption — cutting this short can trigger a false low-battery alert or connectivity error in the app.

Why the Nest Learning Thermostat loses its schedule after a power cut

The Learning Thermostat stores schedule data and time in volatile memory that depends on the backup cell to survive mains loss. When the original battery degrades past roughly 3.0V under load, the thermostat can no longer sustain memory during an outage — even a short one. The result is a reset clock and wiped schedule when power returns. Replacing the cell restores that buffer; a healthy cell holds above 3.5V at rest.

Low battery alert showing immediately after fitting a new cell

A low-battery notification right after installation usually means the replacement cell shipped at storage voltage — typically around 3.6–3.7V — which sits just below the thermostat's alert threshold. The thermostat reads this as a depleted cell rather than a freshly installed one. Connect the thermostat to mains power and let it charge the new cell for at least 30 minutes. Once the cell reaches approximately 3.85V, the alert clears on its own.