About this replacement
Amazfit ARES / A1908 — 3.8V Li-Polymer Replacement Battery (PL402120T)
This is a 3.8V, 200mAh lithium-polymer cell for the Amazfit ARES smartwatch (model A1908). It replaces the original PL402120T cell that powers the watch's fitness tracking, heart rate monitoring, and notification functions. Swap it when the original cell no longer holds a usable charge between sessions.
- ARES / A1908 compatibility: Both model names refer to the same hardware platform. The ARES shares its chassis, connector, and BMS handshake with the A1908 designation — one cell fits both without modification.
- Bench tested on actual hardware: We ran this cell through a full charge cycle on the ARES platform. The fuel gauge IC accepted the new cell, BMS protection thresholds triggered correctly at low voltage, and charge termination cut off at the expected 4.35V ceiling.
- Post-swap charging requirement: After fitting the new cell, place the watch on its charger immediately and charge to 100% before first use. The fuel gauge IC on this platform cannot calibrate its reference point without a full charge cycle from near-zero following a cell swap — skipping this step causes the percentage readout to report inaccurate figures for days.
Why the ARES fuel gauge shows a random percentage after a cell swap
The ARES uses a coulomb-counting fuel gauge IC that tracks charge by measuring current in and out of the cell. When you remove the old cell, the IC loses its reference entirely. It wakes up with the new cell at an unknown state of charge and guesses. The fix is a full uninterrupted charge to 100% immediately after reassembly — this resets the IC's full-charge reference point. Until that cycle completes, any percentage shown on screen is not accurate.
Magnetic charger not making contact after reassembly
If the watch sits on its magnetic charger but the charging icon never appears, the charge contacts on the case back have likely shifted slightly during reassembly. Remove the watch from the charger, check that the rear cover is fully seated and flush with the case, then re-place it on the charger with the alignment pins centred. If the icon still does not appear, clean the contacts on both the watch and charger with a dry cloth — oxidation on the gold pads breaks the circuit before any current flows.