Bea-fon M6s Premium Replacement Battery 3.85V 4000mAh

Bea-fon M6s Premium — 3.85V Li-Polymer Replacement Battery (M6s-20)

This 3.85V, 4000mAh lithium-polymer cell replaces the original battery in the Bea-fon M6s Premium and M6s smartphones. It matches the OEM part number M6s-20 and the physical footprint of 86.10 × 63.00 × 4.10mm. Capacity is rated at 15.4Wh, consistent with the factory specification.

• M6s Premium and M6s compatibility: Both models use the same battery bay dimensions, connector pinout, and BMS handshake protocol, so one cell covers either variant without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the M6s platform. The BMS accepted the cell without flags, voltage held steady across screen-on and modem-active loads, and charge termination fired correctly at 4.35V.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one full discharge-to-charge cycle. This lets the fuel gauge IC build a fresh discharge curve against the new cell before high-current charging pushes current into an uncalibrated state.

Sudden shutdown at 20–30% on the M6s after a cell swap

A new lithium-polymer cell has a slightly different voltage-versus-capacity curve than the aged cell the fuel gauge IC was calibrated to. When the phone draws peak current — modem transmit bursts or screen at full brightness — the cell voltage dips briefly below the low-voltage cutoff threshold the IC is watching. The phone reads this as empty and shuts down, even though the cell still holds charge. One full discharge cycle from 100% to automatic shutdown, followed by a full charge, re-anchors the coulomb counter to the new cell's actual curve and clears the false cutoff behaviour.

Phone warm near the battery during the first charge cycle

A fresh high-impedance cell dissipates more heat during initial charging than a broken-in one. The charge IC on the M6s applies the same current profile regardless, which raises internal resistance losses as heat on the first one or two cycles. This is normal and fades as the cell forms. If warmth persists past the third full charge, check that the back cover is seated flat — any gap traps heat against the battery.