Xiaomi Redmi 7A BN49 Replacement Battery 3.85V 3900mAh

Xiaomi Redmi 7A — 3.85V Li-Polymer Replacement Battery (BN49)

This is a 3900mAh Li-Polymer replacement battery for the Xiaomi Redmi 7A smartphone. It fits models M1903C3EI, MZB7995IN, MZB7798IN, and 15 additional regional variants that share the BN49 form factor. Voltage is 3.85V nominal, matching the original charge IC and fuel gauge configuration.

• Redmi 7A variant coverage: All listed models use the same BN49 cell footprint, connector pinout, and BMS handshake. Xiaomi distributed the 7A across multiple regional SKUs — M1903C3EI for India, MZB variants for other markets — but the battery interface is identical across all of them.
• Bench tested on actual hardware: We ran this cell through a full charge-discharge cycle on a Redmi 7A unit. The BMS accepted the charge IC handshake without error, and the fuel gauge IC began tracking state-of-charge from the first cycle. No thermal event occurred during the charge phase.
• Fuel gauge recalibration on first use: After installing this cell, disable fast charging and run one complete discharge-charge cycle at standard rate. This lets the fuel gauge IC map its coulomb counter against the new cell's discharge curve before high-current charging pushes current into an uncalibrated register.

Sudden shutdown at 20–30% on the Redmi 7A after a cell swap

This happens because the fuel gauge IC is still using the discharge curve it learned from the old, degraded cell. The new cell has a steeper voltage cliff at a different state-of-charge point, so the IC reports 25% remaining while the actual cell voltage is already approaching the BMS cutoff threshold. Under modem or display load, that remaining voltage collapses faster than the gauge predicts. One full uninterrupted discharge-charge cycle at standard rate forces the coulomb counter to remap against the new cell's actual curve.

Redmi 7A not powering on after sitting in storage with a flat battery

Li-Polymer cells that discharge below approximately 2.5V trigger a BMS lockout to prevent cell damage. The phone shows nothing — no charging indicator, no vibration — because the protection circuit is open. Connect the phone to a low-current charger (a standard 5V/1A USB adapter works better here than a fast charger) and leave it for 20–30 minutes without pressing the power button. Once the cell recovers above the BMS re-enable threshold, the charge IC takes over and the charging animation will appear. Do not attempt a forced boot before the indicator shows.