Motorola RAZR 50 Ultra 5G QR30 Replacement Battery 3.88V 2800mAh

Motorola RAZR 50 Ultra 5G (XT2451) — 3.88V Li-Polymer Replacement Battery (QR30)

This is a 3.88V, 2800mAh Li-Polymer cell built to replace the OEM QR30 battery in the Motorola RAZR 50 Ultra 5G 5th generation smartphone. It fits the XT2451-1 and XT2451-2 variants and the broader Razr 50 Ultra line. Swap it when the original cell no longer holds capacity or causes unexpected shutdowns under load.

• RAZR 50 Ultra 5G compatibility: The XT2451 series runs a single battery bay with a flat Li-Polymer cell and a BMS that handshakes with Motorola's charge IC over a dedicated data line. This replacement carries the same connector and BMS profile, so the charge controller accepts the cell without throwing a fault.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the XT2451 platform, confirming the BMS communication line stays active, the charge IC steps through trickle, CC, and CV phases correctly, and no thermal events occur at full charge rate.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one full discharge-to-charge cycle at standard rate. The fuel gauge IC is still calibrated to the old cell's discharge curve. One full cycle at low current lets the coulomb counter re-anchor its state-of-charge model against the new cell before fast charging pushes higher current into an uncalibrated gauge.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff problem, not a capacity problem. The fuel gauge IC reports 20–30% remaining, but under the combined load of the 5G modem and the foldable display, the cell voltage drops below the BMS cutoff threshold before the gauge reaches 0%. The BMS then trips and the phone shuts off. One full discharge-charge cycle lets the coulomb counter remap the voltage-to-capacity curve on the new cell, which shifts the reported percentage back into alignment with actual cell voltage. If the shutdowns continue after two full cycles, check that the BMS data line connector is fully seated at 0V bias offset.

USB-PD fast charge not accepted on the first cycle after replacement

After a cell swap, Motorola's charge IC sometimes defaults to standard 5V charging on the first cycle because the BMS on the new cell has not yet completed its initial handshake with the power delivery controller. This is normal. Unplug the charger, wait 10 seconds, and reconnect — the PD negotiation restarts and the fast charge profile is usually accepted on the second connection. If fast charge still does not engage after two attempts, complete one full standard-rate cycle first, then retry fast charging at the start of the next charge session.