Redmi BN42 Smartphone Replacement Battery 3.85V 4000mAh

Redmi 4 Standard Edition — 3.85V Li-Polymer Replacement Battery (BN42)

This is a 4000mAh Li-Polymer cell at 3.85V, replacing the original BN42 battery in the Redmi 4 Standard Edition. It fits the standard single-SIM and Dual SIM variants, including the TD-LTE model (2016090). Swap it in when the original cell no longer holds a charge or causes unexpected shutdowns.

• Covers 4 Standard Edition variants: The standard, Dual SIM, Dual SIM TD-LTE, and 2016090 board revision all use the same BN42 footprint, connector pinout, and BMS handshake. One cell fits all four without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a Redmi 4 board. The BMS accepted the cell, thermal cutoff triggered correctly at high charge current, and the fuel gauge IC read within expected range after one full cycle.
• Fuel gauge recalibration on first use: On first install, disable fast charging and run one full discharge down to auto-off, then charge to 100% uninterrupted. This lets the fuel gauge IC map the new cell's discharge curve before it takes high-current input — skipping this step causes the percentage readout to drift for the first week.

Why the Redmi 4 reports wrong battery percentage after a cell swap

The Redmi 4 uses a coulomb counter that builds its discharge model against the original cell's impedance curve. When a new cell goes in, that stored model no longer matches reality — especially in the 30–15% range where cell voltage drops steeply. The firmware keeps predicting remaining capacity based on old data, so the percentage reading lags or jumps. One complete discharge-charge cycle without interruption forces the fuel gauge IC to rebuild its reference curve against the new cell, and readings stabilise after that.

Sudden shutdown at 20–25% on the replacement cell

This is a voltage cliff issue, not a capacity problem. Under combined modem, screen, and CPU load, the cell voltage briefly dips below the BMS cutoff threshold — even though the fuel gauge still shows 20% or more remaining. Aged cells hit this cliff earlier because internal resistance rises, but a brand-new cell can trigger the same fault if the fuel gauge IC is uncalibrated and holds the phone active past the safe voltage floor. Run the recalibration cycle first; if shutdowns continue, check that the replacement cell voltage reads above 3.6V under load using a USB power meter or the built-in battery diagnostics at *#*#6485#*#*.