T-Mobile Vairy Touch Li-ion Replacement Battery 3.7V 650mAh

T-Mobile Vairy Touch — 3.7V Li-ion Replacement Battery (Li3706T42P3h413457)

This is a 3.7V Li-ion cell rated at 650mAh (2.41Wh), built to the same footprint as the factory cell in the T-Mobile Vairy Touch. It slots into the Vairy Touch battery bay and connects to the existing charge IC and fuel gauge circuitry. Use the capacity figure from this listing — 650mAh — as the reference spec.

• Vairy Touch fitment: The Vairy Touch uses a compact 40.60 × 34.00 × 5.60mm cell on a single 3.7V rail. This replacement matches that footprint and connector pinout exactly, so the phone's charge IC and BMS handshake proceed without errors on first boot.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench, monitoring BMS cutoff behaviour at both ends of the voltage window. The protection circuit tripped correctly at the low-voltage threshold and released cleanly on reconnect — no faults logged.
• Fuel gauge recalibration on first cycle: On first use, disable any fast-charge mode and run one full discharge-charge cycle at standard rate. The Vairy Touch fuel gauge IC calibrates its coulomb counter against the new cell's discharge curve during this cycle. Skipping it can cause the percentage display to read inaccurately for the first several days.

Why the Vairy Touch reports wrong battery percentage after a cell swap

The fuel gauge IC stores a learned discharge curve from the old cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The IC interpolates percentage from voltage readings against the old curve, so early on you'll see jumps or stalls — often freezing near 40–50% before dropping suddenly. One full standard-rate discharge down to automatic cutoff, followed by a full charge to 4.2V, gives the IC enough data to resync its model to the new cell.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem radio or screen backlight draws a short current spike and the cell voltage dips below the BMS cutoff threshold — even though the displayed percentage reads well above zero. It's a voltage cliff: the cell can't sustain voltage under load at that state of charge. The fuel gauge IC hasn't yet mapped where the real cliff sits on this cell's curve. Run two complete discharge-charge cycles at standard rate; after that, the gauge recalibrates and the shutdowns typically stop. If they continue past three cycles, check that the battery contacts on the phone are clean and making full contact.