About this replacement
FitBit Blaze / FB502 — 3.7V Li-Polymer Replacement Battery (LSSP321830)
This is a 3.7V, 160mAh lithium-polymer cell built to the LSSP321830 specification. It fits the FitBit Blaze smartwatch (FB502) and powers the display, heart rate sensor, and onboard fitness tracking. Physical dimensions are 30.00 × 18.60 × 3.25mm — confirm these against your existing cell before fitting.
- Blaze / FB502 platform fit: Both Blaze model references share the same PCB layout, charging circuit, and LSSP321830 connector pinout. The fuel gauge IC communicates over the same voltage rail on both, so one cell covers the full FB502 lineup without modification.
- Bench tested on actual hardware: We ran this cell through the Blaze charging circuit and confirmed the BMS accepted the new cell without error flags. The fuel gauge IC re-initialised correctly after a full charge cycle from near-zero following swap.
- Post-swap charging on the Blaze: After fitting the new cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The Blaze fuel gauge IC cannot set an accurate reference point until it completes one full charge cycle from near-zero after a cell swap — skipping this step causes persistent incorrect percentage readings.
Why the Blaze shows 0% immediately after a battery swap
The Blaze uses a coulomb-counting fuel gauge IC that tracks charge state relative to a stored reference. Removing the old cell wipes that reference entirely. When the new cell lands at any partial voltage, the IC has no baseline and defaults to 0% or shuts the display off. This is not a faulty cell. Place the watch on charge immediately after reassembly, let it reach 100% without interruption, and the IC will write a new reference — percentage readings return to normal after that first full cycle.
Magnetic charge contacts not connecting after reassembly
The Blaze back panel seats on four alignment tabs. If any tab shifts during cell replacement, the rear cover sits fractionally proud and the magnetic charger cannot make firm contact with the pogo pins. The watch will appear to charge intermittently or not at all. Press each corner of the rear cover down firmly until all tabs click, then re-seat on the charger — you should see the charging indicator appear within 10 seconds. If contact is still intermittent, check the pogo pins on the charger for debris and clean with a dry cloth before re-seating.