Fairphone 2 FP2-BAT01 Replacement Battery 3.7V 1700mAh

Fairphone 2 — 3.7V Li-ion 1700mAh Replacement Battery (FP2-BAT01)

This is a 3.7V, 1700mAh Li-ion replacement battery for the Fairphone 2 smartphone. It matches OEM part number FP2-BAT01 and fits directly into the Fairphone 2's user-accessible battery bay. No tools required — the modular back panel removes by hand.

• Fairphone 2 compatibility: The Fairphone 2 uses a single-cell Li-ion pack at 3.7V nominal with a standard connector and BMS handshake. This cell sits at the same voltage rail and connector pinout as the OEM unit, so the phone's power management IC recognises it without negotiation issues.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the Fairphone 2 platform. The BMS accepted charge from the device's onboard charge IC without triggering protection cutoff, and the phone's fuel gauge IC registered the cell correctly after one full cycle.
• Fuel gauge recalibration after swap: On first use, run one complete discharge down to auto-shutdown, then charge uninterrupted to 100%. This gives the fuel gauge IC a full reference curve against the new cell — skip this step and percentage readings will drift early in the battery's life.

Why the Fairphone 2 reports wrong battery percentage after a cell swap

The Fairphone 2 uses a coulomb counter-based fuel gauge IC that builds its capacity model from the cell it learned during initial use. When you install a new cell, that stored model no longer matches the actual discharge curve of the replacement. The phone will read the old curve against new cell behaviour and report percentages that are too high or too low. One full discharge-to-shutdown followed by an uninterrupted charge to 100% resets the IC's reference points. After that cycle, percentage accuracy normalises.

Sudden shutdown at 20–30% on the Fairphone 2

This is a voltage cliff problem, not a capacity problem. Under sustained load — mobile data, screen-on, GPS active simultaneously — the cell voltage drops sharply below what the fuel gauge predicted. The phone's power management IC sees the voltage fall under 3.2V and shuts down to protect the cell, even though the percentage display still showed charge remaining. It happens most often with aged cells but can also occur in the first few cycles of a new cell before the fuel gauge IC recalibrates. Run one full discharge cycle first, then check whether shutdowns persist — if the cell voltage holds above 3.3V under load, the fuel gauge was the cause, not the cell.