Poco C40 BN66 Replacement Battery 3.87V 5800mAh

Poco C40 / 220333QPG — 3.87V Li-Polymer Replacement Battery (BN66)

This is a 5800mAh Li-Polymer replacement battery for the Poco C40 (model 220333QPG). It runs at 3.87V nominal and outputs 22.45Wh, matching the original cell spec. The BN66 part number covers both C40 hardware variants on the same board revision.

• C40 and 220333QPG compatibility: Both device identifiers reference the same PCB layout, connector pinout, and battery contact placement. The flex connector and BMS communication lines are identical across the two designations, so one cell fits both.
• Bench tested on actual hardware: We ran this cell on a C40 unit and confirmed BMS handshake, charge acceptance from the PMIC, and correct voltage reporting at each SOC step. The cell held stable voltage under display and modem load without tripping BMS overcurrent protection.
• Fuel gauge recalibration on first use: After installing this cell, disable fast charging and run one full discharge-to-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated counter — without it, percentage readings drift early.

Sudden shutdown at 20–30% on the C40 after a cell swap

The C40's fuel gauge IC retains the discharge curve of the original cell after a replacement. When the phone expects a voltage cliff at one SOC point but the new cell hits it earlier, the PMIC cuts power before the display shows 0%. This is not a defective cell — it is a calibration mismatch between the coulomb counter and the new chemistry. Run one full discharge without fast charging, then charge to 100% uninterrupted. After that cycle, the gauge re-anchors to the new cell and shutdown shifts back to the correct low-voltage threshold near 3.4V per cell.

USB fast charge not activating on the first cycle after replacement

On the first charge after installation, the C40's PMIC may default to standard 5V charging rather than negotiating the higher voltage fast-charge profile. This happens because the BMS on a new cell starts in a conservative state until it logs one complete charge cycle. Connect to the original charger and let the first charge complete fully without interrupting it. After that cycle, fast charging resumes at the normal negotiated rate — confirm by checking the charging speed notification at around 3.87–4.0V cell voltage.