About this replacement
FitBit Versa 2 / FB507 — 3.85V Li-Polymer Replacement Battery (LSSP281928)
This 3.85V, 160mAh lithium-polymer cell replaces the original battery in the FitBit Versa 2 (FB507). It fits the watch's internal bay directly, restoring power to the display, heart rate sensor, and BLE radio. Dimensions are 28.00 × 17.60 × 2.80mm — match these before fitting if your unit has been repaired before.
- Versa 2 / FB507 fitment: Both model references run the same power rail at 3.85V nominal with a shared connector pinout and BMS handshake. One cell covers the full variant range without modification.
- Bench tested on actual hardware: We ran this cell on a Versa 2 unit and confirmed the BMS accepted the new cell without fault flags. Charge acceptance held steady from near-zero through to full capacity, and the fuel gauge IC updated correctly across the cycle.
- Post-swap first charge: After fitting the new cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on the Versa 2 loses its charge reference during cell removal — it cannot recalibrate without a complete charge cycle from near-zero. Skipping this step causes the percentage display to read inaccurately for days.
Why the Versa 2 fuel gauge reads wrong after a cell swap
The Versa 2 uses a coulomb-counting fuel gauge IC that tracks charge by measuring current in and out over time. When you remove the old cell, the IC loses its accumulated reference entirely. It has no way to know the new cell's state of charge when power is restored. Until you run one complete charge cycle — from near-zero to 100% on the charger — the percentage readout is essentially a guess. After that first full cycle, the IC re-anchors its model and the display stabilises.
Magnetic charge contacts not connecting after reassembly
If the watch sits on the charger but shows no charging indicator, the rear contacts have likely shifted during reassembly. The Versa 2's charging pads are small and the back cover alignment directly affects contact pressure. Remove the watch from the charger, reseat the back cover firmly until all clips engage, then place it on the charger again. You should see the charging icon appear within five seconds — if not, clean the contacts with isopropyl alcohol and check that no adhesive residue is bridging the pads.