About this replacement
Apple MQJU2LL/A Series — 3.82V Li-Polymer Replacement Battery (A1848)
The A1848 is a 3.82V, 270mAh Li-Polymer cell for Apple Watch Series 2. It fits MQJU2LL/A, MQKW2LL/A, MQKV2LL/A, MR2W2LL/A, and 13 additional Series 2 model numbers sharing the same internal layout. This replacement restores power to the display, processor, heart rate sensor, and wireless radio.
- Series 2 model compatibility: All listed Series 2 models use the same A1848 cell, voltage rail, and connector pinout. The BMS handshake and charge IC are identical across the lineup, so one cell covers the full group.
- Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on Series 2 hardware. The BMS responded correctly to the watch's charge IC — no false low-battery flags and no cutoff errors under normal sensor load.
- Post-swap charge requirement: After fitting the new cell, place the watch on its magnetic charger immediately and charge to 100% before wearing it. The Series 2 fuel gauge IC loses its reference point when the original cell is removed. Without a full charge cycle from near-zero, it cannot recalibrate and will report inaccurate percentages throughout the day.
Why Apple Watch Series 2 Battery Percentage Reads Wrong After a Cell Swap
The Series 2 uses a fuel gauge IC that tracks charge state by measuring coulombs in and out relative to a stored reference. When you remove the old cell, that reference is wiped. The new cell arrives partially charged, so the IC has no baseline to work from. It will display whatever percentage it can approximate — often stuck at 0%, 1%, or an inflated number. A full charge to 4.35V resets the reference and lets the IC anchor its calculations correctly.
Watch Not Connecting to iPhone After Battery Replacement
Removing the battery kills the active BLE session between the watch and the paired iPhone. The watch does not automatically re-establish pairing after a hard power loss — it presents as an unknown device on the phone side. Go to the Watch app on iPhone, tap your watch, select "Unpair Apple Watch," then set it up again as a new device. You do not lose health data stored in iPhone Health, only data not yet synced at the time of removal.