Apple iPhone Xs Max 616-00506 Replacement Battery 3.8V 3700mAh

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

This 3.8V, 3700mAh Li-Polymer cell replaces the original power unit in the iPhone Xs Max (A1921, A2100, A2101 and related variants). It matches the OEM form factor at 105.43 × 62.94 × 4.25mm, fitting the battery bay without modification. Voltage and capacity align with Apple's original 616-00506 specification.

• Xs Max model coverage: The A1921, A2100, A2101 and associated variants all share the same battery bay geometry, connector pinout, and BMS handshake protocol — one cell covers the full Xs Max production run across regional SKUs.
• Bench tested on actual hardware: We ran this cell through the Xs Max charge IC on a production unit. The BMS accepted the USB-PD negotiation cleanly, held charge termination at the correct voltage, and the fuel gauge IC registered full capacity without manual intervention.
• First-cycle fast charge tip: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's discharge curve before high-current fast charging pushes current into an uncalibrated coulomb counter.

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

Apple's fuel gauge IC uses a coulomb counter calibrated against the original cell's charge and discharge curve over time. When you install a new cell, that stored curve no longer matches reality. The IC keeps referencing old data, so percentage readings drift — often showing full charge when the cell is not, or dropping suddenly. One complete discharge to automatic shutdown followed by a full uninterrupted charge cycle forces the IC to re-anchor its reference points to the new cell.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem, display, or both spike current demand faster than the cell's internal resistance allows voltage to recover. The phone's power management IC reads a momentary voltage drop below its cutoff threshold and shuts down, even though the reported state of charge still shows 20–30%. It is not a faulty cell — it is an uncalibrated fuel gauge misreporting true charge state. Run two full discharge-charge cycles and confirm the BMS is not flagging a low-voltage lockout by checking Settings → Battery → Battery Health before the third cycle.