Redmi Note 5A BN31 Replacement Battery 3.85V 3000mAh

Redmi Note 5A — 3.85V Li-Polymer Replacement Battery (BN31)

This is a 3.85V, 3000mAh Li-Polymer cell for the Redmi Note 5A smartphone. It fits the standard Note 5A as well as the Note 5A Global Dual SIM TD-LTE, MDG6S, and MDG6 variants. Swap it in when the original cell can no longer hold a charge or causes unexpected shutdowns.

• Note 5A variant compatibility: All listed variants share the same battery bay dimensions, 3.85V supply rail, and BN31 connector pinout — the BMS handshake and flex connector are identical across MDG6, MDG6S, and the Global Dual SIM TD-LTE board revision.
• Bench tested on actual hardware: We cycled this cell through the Note 5A charge IC and confirmed the BMS accepted the charging protocol without fault flags. Capacity was logged across three full cycles and tracked within spec tolerance.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-charge cycle before resuming normal use. This lets the fuel gauge IC map the new cell's discharge curve — skipping this step is the leading cause of erratic percentage readings and premature low-battery cutoff on fresh installs.

Why the Note 5A reports wrong battery percentage after a cell swap

The Note 5A uses a coulomb-counting fuel gauge IC that builds its capacity model over charge cycles. When you fit a new cell, the IC still references the old degraded cell's curve — so the percentage shown is calculated against the wrong baseline. This causes the display to read 40% when the actual state of charge is much lower, or jump several points mid-use. One full discharge to cutoff followed by an uninterrupted charge to 100% forces the IC to remap against the new cell's actual capacity. After that cycle, percentage accuracy stabilises.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem radio or display draws a current spike that pulls cell voltage below the BMS cutoff threshold — even though the fuel gauge still shows charge remaining. A fresh cell with an uncalibrated fuel gauge IC is particularly vulnerable to this because the reported percentage doesn't reflect the true voltage under load. The BMS interprets the voltage sag as a depleted cell and cuts output to protect the cell. Run the recalibration cycle first, then check that the replacement cell voltage reads at least 3.7V under screen-on load using a battery diagnostics app.