BTR781B Casio GzOne Ravine 2 Replacement Battery 3.7V 1250mAh

Casio GzOne Ravine 2 / C781 — 3.7V Li-ion Replacement Battery (BTR781B)

This is a 3.7V, 1250mAh Li-ion cell built to replace the original BTR781B in the Casio GzOne Ravine 2 and C781 rugged flip phone. The stock battery in these devices degrades over charge cycles and loses the ability to hold voltage under load. This replacement restores full electrical capacity to the original spec.

• GzOne Ravine 2 and C781 compatibility: Both models share the same battery bay dimensions, 3.7V nominal voltage rail, and BTR781B connector orientation. No modification needed — the cell slides into the same physical slot and makes contact with the same terminal pads.
• Bench tested on actual hardware: We ran this cell through charge and load cycles on the GzOne platform. The BMS accepted the charge handshake without error, held voltage through simulated call and screen-on load draws, and tripped the protection circuit correctly at the low-voltage cutoff threshold.
• Fuel gauge recalibration on first cycle: After installing this cell, run one full discharge to automatic power-off before charging to 100%. The GzOne's fuel gauge IC was calibrated to the old cell's discharge curve. A complete first cycle gives the coulomb counter a fresh reference point so percentage readings stay accurate.

Why the GzOne Ravine 2 shows wrong battery percentage after a cell swap

The fuel gauge IC in the C781 platform tracks charge state using a coulomb counter that learns the cell's discharge curve over time. When you swap the physical cell, the IC still references the old curve — so it can report 40% when the actual cell voltage is already near the low-voltage cutoff. This mismatch causes early shutdowns that look like a defective battery but are actually a calibration gap. One full discharge-to-cutoff followed by a full charge resets the reference and brings percentage readings back into line.

Sudden shutdown at 20–30% remaining on the Ravine 2

This is a voltage cliff failure, not a capacity problem. Under a real load — active call, backlight on, RF transmitting — the cell voltage drops faster than the fuel gauge predicts. When voltage falls below roughly 3.2V under that load, the BMS cuts the output rail to protect the cell, and the phone powers off regardless of the displayed percentage. A freshly installed cell that hasn't been calibrated is especially prone to this in the first few cycles. Run one full discharge cycle first, then check if shutdowns persist before assuming the replacement cell is faulty.