BTR771B Casio GzOne Commando C771 Replacement Battery 3.7V 1700mAh

Casio GzOne Commando C771 — 3.7V Li-ion Replacement Battery (BTR771B)

The BTR771B is a 3.7V, 1700mAh Li-ion cell that fits the Casio GzOne Commando C771 rugged smartphone. It slots into the original battery bay and connects via the same three-contact interface as the factory cell. Capacity figures come from product data — 6.29Wh total energy.

• GzOne Commando C771 fitment: The C771 uses a proprietary battery bay with a three-pin contact strip that carries both charge current and BMS data. This cell matches the physical dimensions (67.83 × 44.95 × 5.00mm) and that same pin layout, so the phone's charge IC can communicate with the BMS correctly.
• Bench tested on actual hardware: We ran this cell through the C771's charge IC and confirmed the BMS handshake completes on first insertion. The protection circuit trips at the correct under-voltage threshold and resets cleanly when charge current is applied from a known-good charger.
• Fuel gauge recalibration on first use: On the first cycle after installation, disable any fast-charge mode and run one full discharge-to-charge cycle. The C771's fuel gauge IC recalibrates its coulomb counter against the new cell's discharge curve before high-current charging pushes current into an uncalibrated cell.

Sudden shutdown at 20–30% on the Casio GzOne Commando C771

The C771 shuts down abruptly when the modem fires a high-current burst — such as during a call handoff or GPS fix — and the cell voltage drops below the BMS cutoff threshold, even though the fuel gauge still shows 20–30%. This is a voltage cliff: the cell cannot sustain load current at that state of charge. A fresh replacement cell restores the flat discharge curve, so the voltage stays above the cutoff under modem load. After fitting the new cell, complete one full discharge cycle to let the fuel gauge IC map the actual discharge curve of the replacement cell.

Phone not powering on after the replacement battery has been in storage

Li-ion cells self-discharge in storage. If the BTR771B drops below approximately 2.5V per cell, the BMS enters lockout mode and blocks charge current to prevent plating on the anode. The C771 will show nothing on screen — no charging indicator, no boot. Apply a known-good charger for 15–20 minutes without attempting to power on; the BMS needs a trickle current to recover above the lockout threshold before normal charge resumes. Once the LED indicator activates, the cell is above the recovery voltage and can accept a full charge cycle.