Sagem 710 Replacement Battery 3.6V 750mAh Ni-MH

Sagem 710 / 712 / 715 / 725 — 3.6V Ni-MH Replacement Battery

This is a 3.6V, 750mAh Ni-MH replacement battery for the Sagem MC710 series candybar phones. It fits the 710, 712, 715, and 725 models. The original cells in these phones are now over two decades old — most have lost meaningful capacity or fail to hold a charge at all.

• 710 / 712 / 715 / 725 compatibility: These four models share the same battery bay dimensions, connector pin layout, and 3.6V nominal voltage rail. The charge circuit in each handset accepts the same charge profile, so one cell covers all four without modification.
• Bench tested on actual hardware: We ran charge and discharge cycles through the MC710 charge IC. The BMS accepted the cell without triggering an over-voltage fault, and the handset powered through a full cycle without thermal cutoff or false-low shutdowns.
• Fuel gauge recalibration on first use: After fitting this cell, run one complete discharge to auto-off, then charge uninterrupted to 100% before normal use. The MC710's fuel gauge IC was calibrated against the original cell's discharge curve — one full cycle re-anchors the percentage readout to the new cell.

Why the MC710 reports wrong battery percentage after a cell swap

The MC710 uses a simple coulomb-counting fuel gauge that stores a learned discharge curve from the original cell. A new Ni-MH cell has a different internal resistance profile and a slightly different voltage slope across its discharge range. Until the gauge recalibrates, it reads percentage against the old curve — so the displayed figure drifts away from actual remaining capacity. One full discharge-to-cutoff followed by an uninterrupted charge resets the learned curve to match the new cell.

Sudden shutdown at 20–30% on the replacement cell

Ni-MH cells have a sharper voltage cliff near end-of-discharge than the fuel gauge anticipates during the first few cycles. Under load — an active call, backlight, or keypad polling — the cell voltage drops below the cutoff threshold faster than the displayed percentage predicts. The phone interprets this as a hard undervoltage event and shuts down before the gauge reaches zero. After two or three full discharge-charge cycles, the fuel gauge IC maps the real voltage cliff and the premature shutdowns stop. If they continue past three cycles, check that the battery contacts are clean and making firm connection — corroded pins add resistance and accelerate the voltage drop under load.