Fitbit Alta HR FB407 Replacement Battery 3.7V 45mAh

FitBit Alta HR / Alta Activity — 3.7V Li-Polymer Replacement Battery (FB407)

This 3.7V 45mAh Li-Polymer cell replaces the original FB407 battery inside the Fitbit Alta HR (FB407) and Alta Activity fitness tracker. It fits the slim 21.10 x 10.10 x 3.30mm cavity and connects to the same flex ribbon used in the stock unit. Swap it when the original cell no longer holds enough charge to get through a full day of tracking.

• Alta HR and Alta Activity compatibility: Both trackers share the FB407 battery footprint, connector pitch, and BMS handshake protocol — so one cell covers both platforms without modification.
• Bench tested on actual hardware: We cycled this cell through the Alta HR's heart rate monitoring and notification stack. The BMS accepted the new cell without fault flags and held steady voltage across continuous HR polling intervals.
• Post-swap charging requirement: After fitting the new cell, place the Alta HR on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC loses its reference point when the original cell is removed — it cannot recalibrate correctly without a full charge cycle from near-zero after a cell swap.

Charge contact alignment after Alta HR reassembly

The Alta HR charges through four small pogo-style contacts on its underside. During a battery swap, the internal flex assembly can shift slightly, which moves the contact pads out of alignment with the magnetic charger pins. If the charger LED does not light within 30 seconds of docking, remove the tracker, inspect the contact surface for debris or offset, and reseat the tracker firmly against the charger. A stable connection reads 5V across the charge contacts.

Watch shows 0% and will not power on after battery swap

This is a fuel gauge IC calibration failure, not a faulty cell. When the original battery is disconnected, the IC loses its stored state-of-charge reference entirely. The tracker then reads the new cell's resting voltage as 0% because it has no baseline to map it against. Plug the Alta HR into its charger and leave it connected until the display confirms 100% — the IC resets its reference at that point and the tracker will boot normally.