Asus ROG Phone III C11P1903 Replacement Battery 3.85V 5600mAh

Asus ROG Phone III — 3.85V Li-Polymer Replacement Battery (C11P1903)

This is a 3.85V, 5600mAh Li-Polymer replacement battery for the Asus ROG Phone III (ZS661KS). It fits the ROG Phone 3 line directly and matches the OEM part numbers C11P1903 and 0B200-03720100. Capacity is 21.56Wh — identical to the factory specification.

• ROG Phone III / ZS661KS compatibility: These model variants share the same battery bay dimensions (110.30 × 53.56 × 5.40mm), the same 3.85V nominal voltage rail, and the same BMS connector pinout — so one cell covers the full ROG Phone 3 lineup without modification.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a ZS661KS unit. The BMS handshake completed without error codes, protection cutoffs triggered correctly at the expected voltage floor, and the fuel gauge IC accepted the cell without throwing calibration faults after a full cycle.
• First-cycle fast charge hold-off: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle at standard current. This lets the fuel gauge IC map the new cell's discharge curve before the ROG Phone's high-current fast charge mode pushes elevated current into an uncalibrated cell.

Sudden shutdown at 20–30% on the ROG Phone III after a cell swap

The ROG Phone III runs a Snapdragon 888 paired with a high-refresh AMOLED display — both draw hard current spikes under load. A new cell with an uncalibrated fuel gauge IC can report 25% remaining while the actual cell voltage has already dropped below the BMS cutoff threshold under that load. The phone shuts down not because the battery is empty, but because the voltage sag under peak draw crossed the protection floor before the percentage counter caught up. One full discharge-charge cycle at standard rate recalibrates the coulomb counter against the real cell curve and eliminates the premature cutoff.

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

After installing a new cell, the ROG Phone III's charge IC sometimes falls back to standard 5V charging on the first cycle and refuses to negotiate the fast charge protocol. This happens because the BMS presents slightly different impedance characteristics on a fresh cell, and the charge IC defaults to conservative current until it completes one full handshake cycle. Plug in with the stock charger, let the first charge run to 100% at whatever rate the phone accepts, then unplug and reconnect — fast charge typically re-engages on the second connection once the BMS has completed its initialisation pass.