Sagem my721x Replacement Battery 3.7V 700mAh Li-ion

Sagem my721x / my721z — 3.7V Li-ion Replacement Battery (287196843)

This 3.7V 700mAh (2.59Wh) lithium-ion battery replaces the original cell in the Sagem my721x and my721z mobile phones. It slots into the same battery bay using OEM part references 287196843 and 287196831. If your my721x no longer holds a charge or fails to power on, this is the direct cell swap.

• my721x and my721z compatibility: Both models share the same battery bay dimensions (40.00 × 34.10 × 5.60mm), connector pinout, and 3.7V charge cut-off. One cell fits both without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the my721x platform. The BMS accepted the charge handshake cleanly, held voltage through the flat discharge curve, and triggered low-voltage cutoff at the correct threshold — no false trips.
• Fuel gauge recalibration on first cycle: After fitting this cell, run one full discharge down to automatic shutdown, then charge uninterrupted to 100% before normal use. The my721x fuel gauge IC was calibrated against the original cell's discharge curve — skipping this step causes the percentage counter to read inaccurately from the start.

Sudden shutdown at 20–30% on the Sagem my721x after a cell swap

The my721x shuts down abruptly at 20–30% because the fuel gauge IC is still reading a voltage-versus-capacity curve mapped to the old, degraded cell. A new cell has a steeper voltage cliff near the bottom of its discharge range. When load from the modem or backlight pulls current, the cell voltage briefly dips below the BMS cutoff threshold — even though the fuel gauge still shows charge remaining. Running one complete discharge-charge cycle forces the coulomb counter to re-anchor its curve to the new cell. After that cycle, the percentage display and the actual cutoff point realign.

my721x shows 100% immediately after fitting the replacement cell

If the phone reads full charge the moment the new battery is inserted — without any charging — the fuel gauge IC has latched onto a stale state-of-charge estimate from the old cell's last reading. This is a calibration artefact, not a fault with the new battery. Connect the phone to a charger and let it charge to the point the charge indicator stops, then discharge fully to automatic shutdown. That single full cycle resets the fuel gauge IC to 0% at cutoff and 100% at float voltage — approximately 4.2V for this cell chemistry.