Kyocera KX2 Compatible Battery TXBAT10040 3.7V 750mAh

Kyocera KX2 / KOI — 3.7V Li-ion Replacement Battery (TXBAT10040)

This is a 3.7V, 750mAh Li-ion replacement battery for the Kyocera KX2 and KOI smartphones. It replaces OEM part number TXBAT10040 and restores power to phones where the original cell has degraded or failed to hold a charge. Voltage and physical form factor match the factory specification.

• KX2 and KOI compatibility: Both models share the same battery bay geometry, connector pin-out, and 3.7V nominal voltage rail. The BMS handshake protocol is identical across both, so one cell fits either device without modification.
• Bench tested on actual hardware: We cycled this cell through a full discharge and charge sequence on KX2 hardware. The BMS accepted the cell without fault flags, and charge termination triggered correctly at 4.2V with no thermal events recorded.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging and run one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before the charge controller starts pushing higher current into an uncalibrated cell.

Sudden shutdown at 20–30% on the KX2 after a cell swap

The KX2 fuel gauge IC retains the discharge curve from the old, degraded cell. When the new cell is installed, the IC's internal model no longer reflects actual cell voltage under load. At peak demand — modem transmit bursts or screen brightness spikes — the cell voltage drops below the shutdown threshold even though the reported percentage appears healthy. One full discharge-to-empty and charge-to-full cycle forces the coulomb counter to reset and re-learn the new cell's curve. After that cycle, the percentage readout tracks accurately and premature shutdowns stop.

Phone warm near the battery compartment on the first charge after replacement

A new cell arrives from storage with elevated internal impedance compared to a conditioned cell. The charge IC on the KX2 applies its standard charge current regardless, and higher impedance means more energy is dissipated as heat during the first charge cycle. This is normal and typically resolves after two or three full cycles as the cell's impedance drops. If warmth persists past the third cycle or the case becomes hot to the touch, check that the battery connector is fully seated — a partial connection increases contact resistance and amplifies heating.