iPhone Xs Max Replacement Battery 616-00506 3.8V 3150mAh

Apple iPhone Xs Max — 3.8V Li-Polymer Replacement Battery (616-00506)

This 3.8V, 3150mAh (11.97Wh) Li-Polymer cell is a direct replacement for the Apple iPhone Xs Max, covering model numbers A1921, A2100, A2101, and four additional regional variants. The original 616-00506 cell degrades over charge cycles, causing shortened screen-on time, unexpected shutdowns, and swollen packaging. Swapping this cell restores the power headroom the A12 Bionic SoC and modem require under peak load.

• Xs Max platform fit: The A1921, A2100, and A2101 share the same battery bay dimensions, flex connector pinout, and BMS handshake protocol — all require the 616-00506 form factor. Substituting a cell from the standard Xs will not fit; the Xs Max chassis uses a larger footprint at 105.43 × 62.94mm.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on an Xs Max board. The BMS accepted the cell on first connection, thermal regulation stayed within normal limits, and the charge IC stepped through trickle, CC, and CV phases without interruption.
• Fuel gauge recalibration on first cycle: After installation, disable fast charging and run one complete discharge down to auto-shutdown, then charge to 100% on a standard 5W adapter. This lets the coulomb counter re-map its fuel gauge curve against the new cell before fast-charge currents are applied to an uncalibrated state.

Why the Xs Max reports wrong battery percentage after a cell swap

Apple's fuel gauge IC stores a learned discharge curve from the original cell in non-volatile memory. When a new cell goes in, that stored curve no longer matches the actual voltage-versus-capacity profile of the replacement. The gauge reads the new cell's voltage and maps it against old data, producing percentage figures that are too high or too low. One full discharge-charge cycle on a standard adapter gives the coulomb counter enough data to overwrite the stale curve and report accurately.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC is still calibrated to the old cell and the phone hits a high-current draw spike — typically the LTE modem transmitting or the display at full brightness. Voltage drops faster than the gauge predicts, crossing the hardware protection threshold before the displayed percentage reaches zero. The BMS interprets this as an emergency undervoltage condition and cuts output immediately. Run the recalibration cycle described above; after one full discharge to shutdown and a complete recharge, the cutoff should align with a displayed percentage below 5%.