Kyocera TXBAT10121 KX21 Replacement Battery 3.7V 850mAh

Kyocera KX21 / K612 / STROBE — 3.7V Li-ion Replacement Battery (TXBAT10121)

This is a 3.7V, 850mAh Li-ion cell built to the TXBAT10121 specification. It fits the Kyocera KX21, K612, K612B, and STROBE handsets. Voltage and connector geometry match the originals — no modifications needed to seat the cell.

• KX21, K612, K612B, and STROBE compatibility: These four models share the same battery bay dimensions, 3.7V nominal rail, and contact alignment. The BMS handshake on each accepts the TXBAT10121 cell without flag errors or charge suppression.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the K612 platform. The BMS accepted the cell on first insertion, charge current ramped normally, and the protection circuit triggered correctly at low-voltage cutoff without locking out the device.
• Fuel gauge recalibration on first use: On first installation, disable fast charging if your handset supports it and run one full discharge-to-charge cycle before normal use. This lets the fuel gauge IC map its coulomb counter to the new cell's actual discharge curve — skipping this step is the main reason percentage readings jump erratically in the first few days.

Why the KX21 and STROBE report wrong battery percentage after a cell swap

The fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer reflects the actual charge state of the replacement. The phone reads voltage and interpolates percentage against the old curve — so it can show 40% while the cell is already near its low-voltage floor. One complete discharge from 100% to automatic shutoff, followed by a full uninterrupted charge, forces the IC to rewrite its reference curve against the new cell. After that cycle, percentage reporting stabilises.

Sudden shutdown at 20–30% remaining on the replacement cell

This is a voltage cliff issue, not a capacity defect. Under modem transmission or screen-on load, the cell must sustain voltage above the BMS cutoff threshold — typically around 3.0V per cell. If the fuel gauge IC is still calibrated to the old cell, it allows the device to draw into a state-of-charge band the new cell cannot support under load, and the BMS cuts power instantly rather than allowing an orderly shutdown. Run one full calibration cycle first. If the shutdowns continue after calibration, check the resting voltage with a meter — a healthy cell at 20% charge should read approximately 3.6V at rest.