BP-RE1 FREETEL Samurai Rei Compatible Battery 3.8V 2700mAh

FREETEL Samurai Rei FTJ161B — 3.8V Li-Polymer Replacement Battery (BP-RE1)

This is a 3.8V, 2700mAh lithium-polymer replacement battery for the FREETEL Samurai Rei smartphone. It fits model variants including FTJ161B, FTJ161B-REI, and FTJ161B-REI-BL. It replaces OEM part number BP-RE1 and restores power to phones that no longer hold a useful charge.

• Samurai Rei variant coverage: The FTJ161B, FTJ161B-REI, and FTJ161B-REI-BL all share the same battery bay dimensions, connector pinout, and BMS handshake protocol — one cell fits the full variant range without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Samurai Rei platform. The BMS accepted the cell on first connection, completed a full charge cycle without interruption, and thermal output stayed within normal range throughout.
• First-cycle calibration on the Samurai Rei: After installing this cell, disable fast charging and run one complete discharge-charge cycle at standard rate. The fuel gauge IC in the FTJ161B calibrates its coulomb counter against the cell's actual discharge curve — skipping this step causes the percentage indicator to read inaccurately for weeks.

Why the Samurai Rei reports wrong battery percentage after a cell swap

The FTJ161B uses a fuel gauge IC that tracks charge state by counting coulombs against a stored discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual cell chemistry. The IC keeps using the old reference, so the percentage on screen drifts from reality. One full slow-charge cycle — from 0% to 100% without fast charging — forces the IC to re-map the curve against the new cell and restore accurate readings.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under load — typically when the modem fires at full power during a call or the screen ramps brightness. A lithium-polymer cell below 3.5V per cell cannot sustain voltage under that current draw, and the BMS cuts output to protect the cell. The phone reads 25% on the gauge but the cell is already at its load-collapse threshold. Run that first full calibration cycle and confirm the phone shuts down at or below 3.4V under load — if cutoffs still happen above 3.6V, the BMS needs a full re-initialisation by discharging to automatic shutdown, then charging uninterrupted to 100%.