Cingular 3125 STAR160 Replacement Battery 3.7V 850mAh

Cingular 3125 — 3.7V Li-ion Replacement Battery (STAR160)

This is a 3.7V 850mAh Li-ion cell built to the STAR160 spec for the Cingular 3125 candybar smartphone. It fits the 3125 directly, matching the original connector and form factor at 44.97 × 37.16 × 5.44mm. Capacity is 850mAh (3.15Wh) — identical to the factory spec.

• Cingular 3125 fit: The 3125 uses a slim single-cell Li-ion pack at 3.7V nominal. This cell matches that voltage rail and the physical bay dimensions exactly — no adapter or modification needed.
• Bench tested on actual hardware: We cycled this cell through a full charge and discharge sequence, monitoring BMS cutoff at both ends. The protection circuit triggered correctly at low-voltage cutoff and held charge acceptance within expected bounds at 4.2V top-of-charge.
• Fuel gauge recalibration on first use: On first use after fitting this cell, run one complete discharge down to auto-shutoff, then charge uninterrupted to 100%. The 3125's fuel gauge IC maps its coulomb counter to the new cell's discharge curve during that first full cycle — skipping it causes erratic percentage readings from the start.

Why the Cingular 3125 reports wrong battery percentage after a cell swap

The 3125's fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The gauge reads voltage and maps it to the old curve, so percentages appear higher or lower than reality. One full discharge-to-shutoff cycle followed by a full charge lets the IC re-anchor its coulomb counter to the new cell's actual curve.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under load — a voltage cliff — before the fuel gauge reaches 0%. During a call or data session, the modem draws a current spike the cell can't sustain at low state-of-charge, and voltage collapses below the BMS cutoff threshold. The gauge still shows 20–30% because it's tracking coulombs, not live voltage sag. Run the first cycle as a slow discharge with no calls until shutoff — this exposes the real low-voltage floor and lets the gauge IC set an accurate empty threshold at approximately 3.0V under load.