Fairphone 3 F3AC1 Replacement Battery 3.85V 2900mAh

Fairphone 3 / 3+ — 3.85V Li-Polymer Replacement Battery (F3AC1)

This is a 3.85V, 2900mAh Li-Polymer cell for the Fairphone 3 and Fairphone 3+. Both models use the same F3AC1 battery format with the same connector and BMS handshake. The Fairphone 3's tool-free battery bay lets you swap this in without opening a service ticket.

• Fairphone 3 and 3+ compatibility: Both devices share an identical battery bay geometry, connector pinout, and BMS communication protocol — the same cell covers both platforms without modification.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on the Fairphone 3 platform. The BMS handshook correctly on the first cycle, charge current ramped as expected, and the cell held 2900mAh within normal tolerance across repeat passes.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging and run one complete discharge down to auto-shutdown, then charge to 100% without interruption. This lets the fuel gauge IC map its coulomb counter to the new cell's discharge curve before fast charge current is applied to an uncalibrated cell.

Sudden shutdown at 20–30% on the Fairphone 3 after a cell swap

The Fairphone 3's fuel gauge IC holds calibration data from the old cell. When a new cell goes in, the coulomb counter still references the old discharge curve, so it misreads remaining capacity. Under modem or display load, actual cell voltage drops below the BMS cutoff threshold before the OS percentage reaches zero. The phone cuts out because the cell voltage is real — the percentage figure is wrong. One full discharge-to-shutdown followed by an uninterrupted charge to 100% resets the curve and clears the mismatch.

USB-PD fast charge not accepted on the first cycle after replacement

A new cell's internal impedance is higher than a conditioned cell, and the Fairphone 3's charge IC is conservative on first contact with an uncalibrated BMS. Some users see the phone lock to slow 5V/1A charging and refuse to negotiate the fast-charge profile on cycle one. This is a BMS protection response, not a fault with the charger or cable. Complete one full standard charge cycle first — after that, reconnect your USB-PD charger and fast charge should negotiate normally.