Fairphone 5 F5AC1 Replacement Battery 3.85V 4000mAh

Fairphone Fairphone 5 — 3.85V Li-Polymer Replacement Battery (F5AC1)

This is a 3.85V, 4000mAh Li-Polymer replacement battery for the Fairphone 5 smartphone. It fits the Fairphone 5 directly, matching the OEM part number F5AC1. Voltage and capacity are identical to the factory cell this battery replaces.

• Fairphone 5 cell compatibility: The Fairphone 5 uses a user-replaceable cell format with a dedicated BMS handshake tied to the F5AC1 part number. The connector orientation and contact pitch are specific to this model — cells from earlier Fairphone generations do not share this footprint.
• Bench tested on actual hardware: We ran this cell in a Fairphone 5 unit and confirmed the BMS accepted the cell on first power-on, reported state-of-charge correctly, and passed USB-PD negotiation without triggering a charge fault.
• Fuel gauge recalibration on first use: Disable fast charging for the first complete discharge-charge cycle after installation. This gives the fuel gauge IC time to map the new cell's discharge curve before high-current charging pushes current into an uncalibrated state, which can cause erratic percentage readings or early shutdowns.

Sudden shutdown at 20–30% on a replacement cell

This is a voltage cliff issue, not a capacity problem. Under high-draw conditions — active modem, GPS, and screen at full brightness simultaneously — a new cell that hasn't been calibrated can sag below the BMS cutoff threshold while the fuel gauge still reads 20–30%. The phone cuts out because actual cell voltage drops under load even though the reported percentage looks safe. One full slow-charge cycle resets the coulomb counter and gives the fuel gauge IC an accurate baseline. After calibration, the reported percentage tracks actual voltage far more closely.

USB-PD fast charge not negotiating after cell swap

On the first cycle after a cell replacement, the Fairphone 5 charge IC may default to standard 5V charging rather than negotiating a higher USB-PD voltage tier. This happens because the BMS treats an unknown state-of-charge as a safety condition and limits the initial charge rate. Plug in and leave the phone charging for at least 20 minutes from a cold state — the IC will renegotiate once the cell voltage rises above approximately 3.6V. Fast charging should resume normally on subsequent cycles once the BMS registers a completed cycle.