Fairphone 4 F4AC1 Replacement Battery 3.85V 3900mAh

Fairphone 4 — 3.85V Li-Polymer Replacement Battery (F4AC1)

This 3.85V, 3900mAh Li-Polymer cell replaces the OEM F4AC1 battery in the Fairphone 4 smartphone. The Fairphone 4's modular back panel lets you swap the battery without tools or a service centre. Capacity figures come from our product data — 15.02Wh at nominal voltage.

• Fairphone 4 modular bay fit: The F4AC1 cell uses a specific connector pinout and BMS handshake that the Fairphone 4's charge IC expects. The physical dimensions — 91.50 × 46.70 × 6.80mm — match the battery bay exactly, so the cell seats and latches without modification.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the Fairphone 4 platform. The BMS accepted the cell, reported state-of-charge correctly after one full cycle, and the charge IC held the correct 4.4V upper cutoff throughout.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging and run one full discharge down to automatic shutdown, then charge to 100% on standard speed. This lets the coulomb counter recalibrate its discharge curve against the new cell before fast-charge current is applied to an uncalibrated state-of-charge model.

Sudden shutdown at 20–30% on the Fairphone 4 after a cell swap

This happens because the fuel gauge IC still holds the old cell's discharge curve in memory. A new cell has a steeper voltage cliff under load — when the modem transmits or the screen peaks, voltage drops faster than the IC expects. The phone interprets that voltage drop as a dead cell and cuts power. One complete discharge-charge cycle with fast charging off forces the coulomb counter to map the new curve and eliminates the false cutoff.

USB-PD fast charging not activating on the first cycle after replacement

On first insertion, the new cell's BMS has not completed its initialisation handshake with the Fairphone 4's charge IC. The phone defaults to standard 5V charging rather than negotiating the higher USB-PD voltage. This is not a fault with the cell. Charge fully once at standard speed — on the second cycle, USB-PD negotiation completes normally and fast charge activates.