Apple Watch Series 5 A2277 Replacement Battery 3.82V 240mAh

Apple Watch SE 40mm — 3.82V Li-Polymer Replacement Battery (A2277)

This is a 3.82V, 240mAh lithium-polymer cell replacing Apple part A2277. It fits the Apple Watch SE 1st generation 40mm (MWWP2LLA, A2351, A2353) and Apple Watch Series 5 40mm variants. Swap this cell when the original shows rapid drain, fails to hold charge, or the watch shuts down under normal use.

• SE 40mm and Series 5 40mm shared cell: Both platforms use the same 40mm chassis dimensions, connector pitch, and BMS handshake protocol, which is why Apple used one part number — A2277 — across both watch generations. The voltage rail and flex connector are identical.
• Bench tested on actual hardware: We seated this cell in a Series 5 40mm chassis and confirmed BMS communication, stable 3.82V open-circuit voltage, and correct charge termination through a full cycle on a magnetic charger. No charge-rejection faults were triggered.
• Post-swap charge sequence on watchOS: After fitting this cell, place the watch on its magnetic charger immediately and run it to 100% before first use. The fuel gauge IC loses its reference point when the original cell is disconnected — it cannot calibrate accurately without a full charge cycle from near-zero after a swap.

Why the Watch SE 40mm reads 0% and refuses to boot after a cell swap

The Apple Watch fuel gauge IC — a coulomb counter on the S5 and S7 SiP — stores charge-state data referenced to the original cell's impedance profile. When you disconnect the old cell, that reference is lost. The IC has no valid data, defaults to zero, and the watch may refuse to boot rather than risk running the display and sensors into an unknown low-voltage state. This is not a fault with the replacement cell. Placing the watch on its charger immediately after reassembly lets the IC re-establish a baseline. Once it reads a confirmed full charge, normal boot and gauge behaviour returns.

Always-on display and continuous heart rate monitoring draining the new cell in under a day

The always-on display on Series 5 and SE models draws current continuously from the LTPO panel, and the optical heart rate sensor pulses the LED array at up to 32 samples per second in workout mode — both are significant loads on a 240mAh cell. If drain feels heavier than expected in the first week, the sensor is also calibrating its signal against the new cell's internal resistance, which can cause slightly elevated current draw. Disable always-on display temporarily and set heart rate monitoring to automatic rather than continuous. If drain normalises after seven days, the calibration period has passed; if not, check that a background app is not holding a continuous workout session open in the Watch app.