About this replacement
Redmi K30 Ultra 5G — 3.85V Li-Polymer Replacement Battery (BM4U)
This is a 3.85V, 4300mAh (16.56Wh) Li-Polymer cell built to the BM4U specification. It fits the Redmi K30 Ultra 5G (model M2006J10C). Install this when the original cell no longer holds charge across a normal day of use.
- K30 Ultra 5G and M2006J10C compatibility: Both model references draw from the same voltage rail and use the same BM4U connector pinout. The BMS handshake is identical across these variants, so one cell covers both.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the K30 Ultra 5G platform. The BMS accepted charging without fault flags, 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 for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging begins pushing current into an uncalibrated cell.
Sudden shutdown at 20–30% on the replacement cell
This happens because the fuel gauge IC is still running the discharge curve from the old, degraded cell. The new cell has a steeper voltage drop under modem and display load than the IC expects at that state of charge. When the terminal voltage dips below the protection threshold during a high-draw moment, the BMS cuts power before the percentage readout reaches zero. Run one full discharge to below 5% and a full charge to 100% without interruption — this forces the coulomb counter to reset against the actual cell capacity.
USB fast charge not initiating after installation
On the first charge cycle after a cell swap, the charge IC may default to standard 5V input and refuse to negotiate the proprietary fast-charge protocol. This is a protective behaviour — the charge controller treats an unrecognised BMS state as a reason to limit current. Plug in using the original Redmi charger and cable, let the first full cycle complete at standard rate, and fast charging should negotiate correctly from the second cycle onward. If it does not, check that the battery connector is fully seated — a partial connection raises contact resistance enough to trip the protocol handshake.