Sagem 830 Replacement Battery 3.7V 750mAh Li-ion

Sagem 830 / 858 / 840 / 850 Series — 3.7V Li-ion Replacement Battery

This 3.7V, 750mAh Li-ion cell is a direct replacement for the original battery in the Sagem 830, 858, 840, and 850 mobile phones. It restores power to devices where the original cell has degraded, swollen, or stopped holding a usable charge. Voltage and capacity match the OEM specification for this phone family.

• Sagem 830 / 858 / 840 / 850 platform fit: These models share the same battery bay dimensions, connector pinout, and 3.7V single-cell architecture. One cell covers all four variants without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Sagem 830 platform. The BMS accepted the cell without fault codes, and the charge IC completed a full CC-CV cycle to 4.2V as expected.
• Fuel gauge recalibration on first use: On first use after installation, run one complete discharge-charge cycle without interruption. The phone's fuel gauge IC is calibrated to the old cell's discharge curve — a full cycle lets it remap to the new cell before percentage readings become trustworthy.

Sudden shutdown at 20–30% on the replacement cell

This happens because the fuel gauge IC still references the old cell's voltage-to-capacity curve. When the new cell hits a voltage point the IC associates with a deeper discharge than the displayed percentage suggests, the phone cuts power to protect the modem and screen. The cell's voltage drops faster under load than the IC predicts, causing an early cutoff. One full uninterrupted discharge-charge cycle forces the coulomb counter to resync with the new cell's actual curve — after that, shutdowns at 20–30% typically stop.

Phone warm near the battery compartment on first charge

A new Li-ion cell typically has higher internal impedance than a broken-in cell. The charge IC pushes current into higher resistance on the first cycle, which generates more heat than subsequent charges. This is normal for the first one to two cycles and settles as the cell's impedance drops. If the phone stays warm beyond the second full charge, check that the charge IC is not stuck in fast-charge mode — drop to a standard 5V/500mA USB source and confirm the warmth reduces.