Siemens X2 Replacement Battery 3.7V 650mAh Li-ion

Siemens X2 / S670 — 3.7V Li-ion Replacement Battery

This 3.7V 650mAh Li-ion battery replaces the original cell in the Siemens X2 and S670 mobile phones. Both handsets use a removable battery format with the same voltage rail and physical form factor. Capacity is 650mAh (2.41Wh) — matching the original specification.

• X2 and S670 compatibility: Both models share the same battery bay dimensions, 3.7V nominal voltage, and connector pinout. A single cell covers both handsets without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the X2 platform. The BMS accepted charging current normally and held voltage above 3.6V through the bulk of the discharge curve before dropping off near depletion.
• Fuel gauge recalibration on first cycle: After fitting a new cell, run one complete discharge down to automatic shutdown, then charge uninterrupted to 100% before normal use. The phone's fuel gauge IC is calibrated to the old cell's discharge curve — this full cycle resets it to the new cell's actual capacity profile.

Sudden shutdown at 20–30% on the Siemens X2

The X2's fuel gauge IC tracks remaining capacity using a charge curve mapped to the original cell. A new cell with a slightly different internal impedance will cause the gauge to misread the voltage cliff — the point where cell voltage drops sharply under load. When the modem transmits or the backlight fires, current draw spikes and the cell voltage falls below the BMS cutoff threshold before the gauge expects it. One full discharge-charge cycle corrects this by letting the gauge relearn where the real voltage cliff sits on the new cell.

Phone warm near the battery compartment during first charge

A new Li-ion cell typically has higher internal impedance than a well-used one. During the first charge cycle, the charge IC on the X2 pushes current into that higher-impedance cell, which converts more energy to heat than usual. This is normal for the first one or two cycles and settles as the cell forms. If the handset stays hot beyond the second full charge, check that the battery contacts are clean and making firm contact — poor contact increases resistance and sustains elevated temperature at the connector pins.