Redmi K50 Ultra BM5J Replacement Battery 3.87V 4800mAh

Redmi K50 Ultra / 12T / 12T Pro — 3.87V Li-Polymer Replacement Battery (BM5J)

This is a 3.87V, 4800mAh (18.58Wh) Li-Polymer cell built to replace the BM5J battery in the Redmi K50 Ultra, 12T, and 12T Pro. All three models share the same battery bay dimensions and BMS handshake protocol, so one cell covers the full lineup. If your phone shuts down unexpectedly, charges to a lower percentage each cycle, or takes noticeably longer to reach full charge, the cell is the likely cause.

• K50 Ultra, 12T, and 12T Pro fitment: Xiaomi grouped these three models on the same mainboard platform, which means they share an identical battery connector, the same voltage rail, and the same BMS authentication sequence. One BM5J cell fits all three without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a K50 Ultra unit, confirming the BMS handshake completed cleanly, fast-charge protocols were accepted after the first full cycle, and the protection circuit tripped correctly at the low-voltage cutoff threshold.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle at standard speed. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated cell.

Sudden shutdown at 20–30% on the Redmi K50 Ultra after a cell swap

This happens because the fuel gauge IC is still reading the discharge curve of the old, degraded cell. The new cell has a steeper voltage cliff at lower charge states than the worn cell did, so the phone's reported percentage and the actual usable voltage no longer match. Under high-load events — 5G modem handoff, screen-on brightness spikes — the cell voltage drops below the protection threshold faster than the OS expects, triggering an emergency shutdown. One full, uninterrupted discharge-charge cycle at standard speed forces the coulomb counter to recalibrate against the new cell, and the premature shutdowns typically stop.

USB-PD or Xiaomi fast charge not activating after battery replacement

On the first charge cycle after a BM5J swap, the charge IC on the mainboard sometimes withholds fast-charge negotiation until it confirms the new cell's impedance profile is within expected parameters. This is a protective hold — not a fault. Plug into the original Xiaomi charger, let the phone charge to 100% at standard rate without interruption, then unplug and replug. Fast charging typically re-enables on the second or third connection once the BMS has logged a baseline for the new cell. If it still does not trigger, verify the cable supports the required current — minimum 3A for Xiaomi's proprietary protocol.