Redmi BM56 K40 Game Replacement Battery 3.87V 4900mAh

Redmi K40 Game / M2104K10C — 3.87V Li-Polymer Replacement Battery (BM56)

The BM56 is a 3.87V, 4900mAh Li-Polymer cell that fits the Redmi K40 Game (model M2104K10C). It replaces the original battery when capacity has dropped and the phone can no longer hold charge through a full day. Rated at 18.96Wh, it matches the stock cell's dimensions at 85.45 × 62.00 × 5.60mm.

• K40 Game / M2104K10C fit: Both model references share the same chassis, battery bay dimensions, and flex connector pinout. The BM56 seats into either variant without modification — the BMS handshake and connector orientation are identical across the production run.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the K40 Game platform. The BMS accepted the charge protocol without faulting, and the fuel gauge IC completed its initialisation pass on the first full cycle without triggering a low-voltage cutoff.
• First-cycle fast charge hold-off: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate against the new cell's discharge curve before high-current charging pushes current into an uncalibrated cell — preventing early percentage drift.

Why the K40 Game reports wrong battery percentage after a cell swap

The K40 Game uses a coulomb-counter-based fuel gauge IC that builds its model against the original cell's charge and discharge curve over time. When a new cell goes in, that stored model no longer matches the actual cell chemistry. The gauge keeps using the old curve, so percentage readings drift — the phone may show 40% and shut down, or show 15% and keep running. One full discharge to auto-shutdown followed by an uninterrupted charge to 100% forces the IC to rewrite its reference curve against the new cell.

Sudden shutdown at 20–30% on the replacement BM56

This is a voltage-cliff failure, not a capacity fault. Under heavy screen or modem load, the cell's terminal voltage drops faster than the fuel gauge predicts — the phone hits the hardware undervoltage cutoff before the displayed percentage reaches zero. It happens most often when the fuel gauge IC is still calibrated to the old, degraded cell curve. Run two full discharge-charge cycles with fast charging off, then check that the shutdown voltage floor is reading correctly in the battery settings — it should stabilise above 3.5V under load.