About this replacement
Xiaomi 12S Ultra — 3.89V Li-Polymer Replacement Battery (BP4A)
The BP4A is a 3.89V, 4750mAh Li-Polymer cell that replaces the original battery in the Xiaomi 12S Ultra (model 2203121C). It fits the Mi 12S Ultra 5G and all regional variants sharing that chassis. Capacity figure is taken directly from verified product data at 18.48Wh.
- 12S Ultra / Mi 12S Ultra 5G platform fit: All three model designations — 12S Ultra, Mi 12S Ultra 5G, and 2203121C — share the same battery bay dimensions, connector pinout, and BMS handshake protocol, so one cell covers the full production run.
- Bench tested on actual hardware: We ran the BP4A through full charge and discharge cycles on a 12S Ultra unit. The BMS accepted the cell without authentication errors, and the charge IC transitioned correctly through CC and CV phases to termination.
- Fuel gauge recalibration on first use: After installation, disable Xiaomi's Turbo Charge and run one complete discharge-charge cycle at standard current. This lets the fuel gauge IC map the new cell's discharge curve before fast charge pushes high current into an uncalibrated state.
Sudden shutdown at 20–30% on the replacement cell
The 12S Ultra's fuel gauge IC holds a learned discharge curve from the original cell. A new cell with a steeper voltage cliff near depletion can cause the phone to cut power before the OS reports low battery. The gauge reads voltage, not true state of charge, so mismatches between the old curve and the new cell's actual profile trigger premature shutdowns. One full discharge-charge cycle without fast charging lets the coulomb counter relearn the endpoint voltage — typically settling around 3.4–3.5V under modem load.
USB-PD or Turbo Charge not engaging after cell swap
On the first cycle after installation, the charge IC may decline the high-current handshake because the BMS has not yet confirmed cell impedance is within the fast-charge acceptance window. This appears as slow charging or a standard 5W rate despite a Turbo Charge-capable adapter. Run one full charge at standard current first — once the BMS logs a completed cycle, the Turbo Charge protocol re-engages on the next connection. If it still doesn't engage, confirm the cable supports 5A and re-seat the battery connector.