About this replacement
Apple Watch 38mm — 3.8V Li-Polymer Replacement Battery (A1578)
This is a 3.8V, 200mAh Li-Polymer cell that replaces the original A1578 battery inside the Apple Watch 38mm. It also fits the Watch 42mm and models A1553 and A1554. Swap it in when the original cell no longer holds a charge across a full day of use.
- 38mm and 42mm fitment: Both watch sizes use the same A1578 cell spec — same voltage rail, same connector pinout, and the same BMS handshake with the S1 or S1P processor module. One part covers both.
- Bench tested on actual hardware: We ran this cell on a Watch 38mm unit and confirmed the BMS accepted the new cell without fault codes. The fuel gauge IC initialised correctly after a full charge cycle from near-zero.
- Post-swap charging requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on the Watch platform loses its voltage reference when the old cell is removed — without a full charge cycle from near-zero, the percentage readout will be inaccurate and the watch may shut down unexpectedly at mid-charge levels.
Why the Apple Watch BLE session drops after a battery swap
Removing the battery cuts power to the W1 or W2 wireless chip, which discards its active Bluetooth session with the paired iPhone. The watch reboots as an unknown peripheral. The iPhone holds the old pairing record but the watch no longer matches it. Re-pair through the Watch app on the iPhone — Settings → General → Reset → Erase Apple Watch Content and Settings — then set up as new.
Watch draining to zero within one day after cell replacement
If battery life collapses to under 24 hours after the swap, always-on display combined with continuous heart rate polling is usually the cause. The new cell has a slightly different internal impedance than the aged original, and the sensor stack draws harder against it during the first several charge cycles until the BMS recalibrates. Check that Theatre Mode and Wrist Detection are both enabled, then run three full charge-to-discharge cycles. If drain persists past cycle three, confirm the display assembly was reseated fully — a lifted gasket keeps the screen backlight at full draw.