Hagenuk Fono C250 Compatible Battery CP10 3.7V 1050mAh

Hagenuk Fono C250 — 3.7V Li-ion Replacement Battery (CP10)

This is a 3.7V, 1050mAh Li-ion battery for the Hagenuk Fono C250 and compatible Fono series handsets. It fits the Fono E100, C800, and C900 among others sharing the CP10 form factor. Use the OEM part numbers CP10, 9133-5C, EZ388+, Z-IN100, or T99 to confirm fitment before installing.

• Fono series cross-compatibility: The C250, E100, C800, and C900 share the same battery bay dimensions, connector pin-out, and 3.7V single-cell architecture — that is why one cell covers all these models without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS engaged protection correctly at low-voltage cutoff and accepted a full charge cycle without triggering overcurrent flags.
• Fuel gauge recalibration after swap: On first use, run one complete discharge to auto-off, then charge uninterrupted to 100% before normal use. This gives the phone's fuel gauge IC a clean reference curve to read the new cell accurately.

Why the Fono C250 shuts down suddenly at 20–30% remaining

Aged Li-ion cells develop elevated internal resistance. Under load — backlight on, radio transmitting — the cell voltage sags sharply below the 3.0V cutoff threshold even though the fuel gauge still reads 20–30%. The phone's protection circuit reads the voltage drop, not the gauge estimate, and cuts power immediately. A new cell with lower internal resistance holds voltage above 3.0V under the same load, eliminating the premature shutdown.

Phone reports wrong battery percentage after fitting the new cell

The fuel gauge IC on these early 2000s handsets uses a stored discharge curve calibrated to the original cell. A new cell with a slightly different discharge profile causes the IC to misread state of charge — percentage can jump, stall, or drop suddenly. To fix this, let the phone discharge fully to auto-off, then charge in one uninterrupted session to 100%. After that cycle, the gauge recalibrates against the new cell's actual curve and reads accurately.