Apple Watch SE 2 40mm A2721 Replacement Battery 3.85V

Apple Watch SE 2 40mm — 3.85V Li-Polymer Replacement Battery (A2721)

This is a 3.85V, 200mAh Li-Polymer cell for the Apple Watch SE 2 40mm, using OEM part number A2721. It restores full battery capacity to the watch after the original cell has degraded through repeated charge cycles. Dimensions are 24.00 × 18.80 × 4.30mm — confirm these against your existing cell before fitting.

• Watch SE 2 40mm fit: The SE 2 40mm uses a smaller chassis than the 44mm variant. The A2721 cell is sized specifically for that footprint. Fitting a cell from the 44mm line will not seat correctly and risks pressure damage to the display stack.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on an SE 2 40mm unit. The BMS accepted the cell without fault codes, and the fuel gauge tracked state-of-charge accurately across the full range from near-zero to full.
• Post-swap charging protocol: 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 SE 2 platform cannot calibrate correctly without a full charge cycle from near-zero after a cell swap. Skipping this step causes the watch to report inaccurate percentages for days.

Why the Watch SE 2 reports 0% immediately after a battery swap

The SE 2 uses a fuel gauge IC that tracks charge state relative to a stored reference. When the cell is physically removed, that reference resets to zero. The watch has no way to know the new cell's actual charge state, so it defaults to 0% or refuses to power on. This is not a fault with the replacement cell. Place the watch straight onto its charger and let it reach 100% — the fuel gauge will re-establish its reference point and read correctly from that point forward.

Battery draining in under a day after replacement

A new A2721 cell draining within a single day usually points to a settings combination rather than a cell fault. Always-on display mode running alongside continuous heart rate monitoring produces a sustained current draw that a 200mAh cell handles within its spec, but only with background app refresh limited. Check that wrist detection is enabled — without it, the display stays active indefinitely. If drain persists beyond the first full charge cycle, verify the crown seal seated correctly during reassembly, as a gap allows moisture ingress that elevates standby current. Confirm battery health reads above 80% in the Watch app before ruling out a cell issue.