Sagem 818 Compatible Battery 3.7V 1200mAh Li-ion

Sagem 818 / 820 / 815 / 835 Series — 3.7V Li-ion 1200mAh Replacement Battery

This is a 3.7V 1200mAh Li-ion cell built to replace the original battery in the Sagem 818, 820, 815, and 835 handsets, along with nine additional compatible models in the same series. The original cell degrades after repeated charge cycles, losing capacity and struggling to hold voltage under screen or call load. This replacement restores full charge capacity to the handset.

• 818 / 820 / 815 / 835 platform fit: These models share a common battery bay geometry, connector pinout, and BMS communication protocol — which is why one cell covers the full range. Swapping between models in this cluster does not require any hardware modification.
• Bench tested on actual hardware: We cycled this cell in a Sagem 818 chassis. The BMS accepted the cell without error flags, charge termination triggered correctly at 4.2V, and the protection circuit tripped as expected under simulated short-circuit load.
• Fuel gauge recalibration on first use: On first installation, run one full discharge to automatic shutoff, then charge uninterrupted to 100% without interruption. This lets the fuel gauge IC map the new cell's discharge curve before the phone begins reporting percentage readings.

Why the Sagem 818 reports wrong battery percentage after a cell swap

The Sagem 818 uses a fuel gauge IC that stores a learned discharge curve from the old cell. When a new cell goes in, the IC keeps referencing its old calibration data. The result is percentage readings that drift — showing 40% when the cell is nearly flat, or jumping suddenly from 30% to 1%. One complete discharge-charge cycle forces the IC to re-learn the curve against the new cell's actual voltage profile. After that cycle, percentage reporting stabilises.

Sudden shutdown at 20–25% on the Sagem 818

This happens because the cell hits a voltage cliff under load — typically during a call or screen-on burst — before the fuel gauge expects it. The phone reads 20–25% but the cell voltage drops below the 3.2V cutoff threshold the moment current draw spikes. A degraded original cell does this chronically; a new replacement cell can do it on the first few cycles before calibration. Run the full discharge-charge calibration cycle first. If shutdowns persist on a new cell, check that the battery connector is fully seated — a loose pin causes intermittent voltage sag that mimics cell failure.