Nest Learning Thermostat 1st Gen Backup Battery 3.7V 200mAh

Nest Learning Thermostat 1st Generation — 3.7V Li-Polymer Replacement Battery (TL363844)

This is a 3.7V, 200mAh Li-Polymer battery for the Nest Learning Thermostat 1st Generation (T100577). It replaces OEM part TL363844. The battery keeps thermostat settings and scheduling intact during a mains power interruption.

• 1st Generation Nest compatibility: The T100577 uses a compact Li-Polymer cell at 3.7V — a spec unique to this generation. Later Nest models run a different cell format and voltage rail, so this battery is specific to the 1st Gen board only.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the T100577 board. The BMS accepted the cell without fault codes, held the charge correctly, and the thermostat retained its schedule and Wi-Fi credentials across the test cycle.
• Post-swap network reconnect: After fitting this battery, leave the thermostat undisturbed for at least 60 seconds. The 1st Gen Nest runs a Wi-Fi re-join sequence after any power interruption — touching the screen or cycling power again during this window interrupts the handshake and delays reconnection.

Why the 1st Gen Nest loses its schedule after a battery failure

The T100577 stores its learned schedule and climate data in volatile memory backed by this cell. When the battery drops below approximately 3.0V, the board can no longer sustain that memory during a mains outage. The thermostat reboots with a blank schedule because the write cycle never completed cleanly. Replacing the cell before it reaches deep discharge prevents this data loss entirely.

Low battery alert showing immediately after fitting a new cell

A new cell that has been in storage for several months may arrive at a resting voltage below the T100577's low-battery alert threshold, typically around 3.5V. The thermostat reads that voltage on insertion and immediately flags a low battery warning — even though the cell is new. This is a storage voltage issue, not a faulty battery. The thermostat's onboard charging circuit will top the cell up within a few hours; the alert will clear once voltage climbs above 3.6V.