About this replacement
FitBit Charge HR — 3.7V Li-Polymer Replacement Battery (LSSP031420AB)
This is a 3.7V 70mAh lithium-polymer cell that replaces the original LSSP031420AB battery in the FitBit Charge HR activity tracker. It powers the display, continuous heart rate monitor, and all fitness tracking functions. Dimensions are 19.50 × 13.60 × 3.30mm — measure your existing cell before fitting if you are unsure.
- Charge HR cell format: The Charge HR uses a flat pouch-style Li-Polymer cell rather than a cylindrical cell. The compact PCM on this unit manages charge cutoff at 4.2V and discharge cutoff near 3.0V — the same thresholds the original cell ran. No modification to the flex connector is needed.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on Charge HR units. The PCM communicated correctly with the watch's fuel gauge IC, and the display reported charge state accurately after a full initial charge cycle.
- First charge after cell swap: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before wearing it. The fuel gauge IC loses its reference point when the cell is disconnected — it cannot recalibrate correctly without a full charge cycle from near-zero after a swap.
Charge contacts not connecting on the magnetic charger after reassembly
The Charge HR charges through a proprietary magnetic clip that aligns to four contacts on the underside of the band. If the internal assembly shifts slightly during the cell swap, the contact pads can sit 0.5–1mm out of position. The charger will attach magnetically but pass no current — the watch shows no charging indicator. Re-open the case, check that the flex PCB sits flush against the rear housing, and press the clip firmly to both contacts before closing. Confirm charging has started by watching for the battery icon within 30 seconds of connecting.
Watch not pairing to the FitBit app after battery replacement
Removing the battery clears the active BLE session between the Charge HR and the phone. The watch re-initialises its Bluetooth stack on first power-up, but the phone still holds the old pairing record — that mismatch blocks reconnection. On the phone, go to Bluetooth settings and forget the Charge HR device entirely, then remove it from the FitBit app under Account > your tracker > Remove. Re-add the tracker through the app's setup flow as though it were new, and the pairing will complete at the current firmware version.