About this replacement
Apple Watch Series 1 — 3.82V Li-Polymer Replacement Battery (A1850)
This 3.82V, 350mAh lithium-polymer cell replaces the original A1850 battery in Apple Watch Series 1. It fits MQL12LL/A, MQL22LL/A, MQL42LL/A, MQL02LL/A, and 22 additional Series 1 variants. Swap it when the original cell no longer holds a charge or the watch shuts down under light load.
- Series 1 compatibility: All Series 1 models share the same 27.50 × 18.50 × 5.30mm cell footprint, the same flex connector tab, and the same BMS handshake parameters — so the A1850 part number covers the full Series 1 lineup regardless of case size or finish.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on Series 1 hardware. The BMS accepted the new cell without error flags, and the fuel gauge re-registered correctly after a full charge cycle from near-zero.
- Fuel gauge reset after fitting: Place the watch on its magnetic charger immediately after seating the new cell and charge to 100% before first use. The fuel gauge IC on Series 1 loses its reference when the old cell is disconnected. Without a full charge from near-zero, it reports inaccurate state-of-charge from the first wake.
Why Series 1 shuts down at 20–30% after a cell swap
The Series 1 fuel gauge IC stores a charge table tied to the previous cell's impedance curve. When you fit a new cell, that table is stale. The IC reads voltage correctly but maps it to the wrong state-of-charge — so it triggers a low-battery shutdown while actual cell voltage is still above 3.5V. A full charge cycle from near-zero rewrites the table to match the new cell's curve and stops the early cutoff.
Magnetic charger not registering after reassembly
If the back cover shifts even slightly during reassembly, the charge contacts on the watch body no longer sit flush against the magnetic puck. The watch will not show the charging icon and the cell stays at whatever voltage it woke at. Press the back cover firmly until it clicks flat, then re-seat the charger. If the icon still does not appear, check that the flex connector tab is fully latched — a half-seated connector lets the watch power on but breaks the charge circuit.