About this replacement
Casio GzOne Rock C731 / C751 Series — 3.7V Li-ion Replacement Battery (BTR731B)
This is a 3.7V, 1000mAh Li-ion cell replacement for the Casio GzOne Rock C731 and C751 rugged phones. It matches the OEM part numbers BTR731B, BTR751B, and BTR751. Capacity figure is taken from the product data: 1000mAh / 3.7Wh.
- C731 and C751 platform fit: Both models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — that is why a single cell covers both. The 44.60 × 38.00 × 6.64mm form factor seats flush in either chassis without modification.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the C731 platform. The BMS accepted the charge IC handshake on the first cycle, held voltage across screen-on and call-active load states, and triggered the low-voltage cutoff at the correct threshold without premature shutoff.
- Fuel gauge recalibration on first use: After installing this cell, run one full discharge to auto-off, then charge uninterrupted to 100% before normal use. The C731 fuel gauge IC is calibrated to the old cell's discharge curve — skipping this step causes the percentage counter to read inaccurately for the first several cycles.
Sudden shutdown at 20–30% on the GzOne Rock C731
The C731 modem and screen together draw enough current to cause a voltage cliff in a degraded or deeply discharged cell — the battery voltage collapses faster than the fuel gauge IC can track, and the phone cuts off well above 0%. A new cell with full capacity holds voltage under this combined load long enough for the gauge to catch up. On the bench, this replacement held above the 3.2V cutoff threshold through simultaneous screen-on and active-call load at room temperature. If shutdowns continue after fitting this cell, run a full recalibration cycle first.
Phone won't power on after sitting in storage
Li-ion cells self-discharge during storage — if the C731 sat unused for several months, the cell voltage may have dropped below 2.5V per cell, triggering a BMS lockout that blocks normal power-on. Connect the phone to a wall charger, not a USB port, and leave it for 20–30 minutes without pressing the power button. The charge IC needs sustained input voltage to nudge the cell above the BMS re-enable threshold. Once the charging indicator appears on screen, the BMS has unlocked and normal charging can continue — target at least 3.6V before attempting to boot.