Fairphone F1B101 Replacement Battery 3.7V 1800mAh

Fairphone FP1 Series — 3.7V Li-ion Replacement Battery (F1B101)

This 3.7V, 1800mAh Li-ion battery replaces the original cell in the Fairphone FP1, F1B101, and F1B201. It fits the modular battery bay and connects via the original pogo-pin contact strip. Capacity is 6.66Wh — identical to factory spec.

• FP1, F1B101, and F1B201 compatibility: All three share the same 51.30 × 50.80 × 5.60mm cell envelope, 3.7V nominal rail, and pogo-pin connector layout. The BMS handshake protocol is also consistent across these variants, so the replacement cell communicates correctly with the charge IC on each board revision.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on an FP1 unit. The BMS accepted the new cell without faults, charge termination triggered cleanly at 4.2V, and low-voltage cutoff engaged as expected before cell damage could occur.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging begins on an uncalibrated coulomb counter — which prevents erratic percentage readings early on.

Sudden shutdown at 20–30% on the Fairphone FP1

The FP1's fuel gauge IC was calibrated to the original cell's internal resistance profile. A new cell has lower impedance, so the reported state-of-charge doesn't match actual voltage under load. When the modem transmits or the screen peaks in brightness, voltage drops sharply — the BMS trips the low-voltage cutoff while the OS still reads 25%. One full discharge-to-cutoff cycle followed by an uninterrupted charge to 100% resets the coulomb counter and closes this gap.

Phone not powering on after the battery sat in storage

If the replacement cell discharged below 2.5V during storage, the BMS enters lockout mode and blocks charge current to protect the cell from damage. The phone will show no response when connected to a charger. Connect to a wall adapter — not USB — and leave it for 30–45 minutes without attempting to power on. The BMS trickle-charges the cell back above the reinitialisation threshold, typically around 2.9V, at which point normal charge current resumes.