About this replacement
Samsung SGH-X670 / SGH-X678 — 3.7V Li-ion Replacement Battery
This is a 3.7V, 850mAh (3.15Wh) Li-ion replacement battery for the Samsung SGH-X670 and SGH-X678 flip phones. Both models share the same battery bay dimensions, connector pinout, and charge management circuit, so one cell covers both. Install this when the original cell no longer sustains voltage under screen and modem load.
- SGH-X670 and SGH-X678 compatibility: These two models use the same physical housing, connector, and charge IC handshake. Samsung sourced the battery identically across both SKUs, so the cell, contact spacing, and BMS parameters are interchangeable between them.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles, monitoring BMS cutoff at the low-voltage threshold and verifying the charge IC accepted the cell without fault flags. Charge termination and over-discharge protection both triggered at expected thresholds.
- Fuel gauge recalibration on first use: On first installation, run the phone down to auto-shutdown, then charge uninterrupted to 100% before resuming normal use. The SGH-X670's fuel gauge IC calibrates its coulomb counter against a full discharge-to-charge sweep — skipping this step causes the percentage display to drift early in the cell's life.
Sudden shutdown at 20–30% on the SGH-X670 after a cell swap
The SGH-X670's fuel gauge IC retains the discharge curve of the original cell in its internal registers. When a new cell is installed, the IC applies the old curve to the new cell's actual voltage output — the two curves diverge sharply below 3.6V. At that voltage point, the phone reads roughly 20–30% remaining, but the fuel gauge triggers a low-voltage shutdown because the real cell voltage has already dropped below the safe floor under modem transmit load. One full discharge-charge cycle rewrites the IC's learned parameters to match the new cell.
Phone warm near the battery compartment on the first few charges
A new Li-ion cell arrives from storage with elevated internal impedance compared to a well-cycled cell. The SGH-X670's charge IC pushes its standard constant-current rate into that higher-impedance cell, which dissipates more energy as heat than usual during the first two or three cycles. This is not a fault — impedance normalises as the cell's electrolyte wets the electrode surfaces fully. If warmth persists past the third full charge, check that the battery contacts are clean and seated flush, as a loose connection raises resistance further.