Wintec BT-N3650 Smartphone Compatible Battery 3.7V 1200mAh

Wintec BT-N3650 — 3.7V Li-ion Replacement Battery

The Wintec BT-N3650 is a 3.7V, 1200mAh (4.44Wh) Li-ion cell for smartphones specced to this battery. It measures 53.00 × 33.90 × 5.70mm, so verify your existing cell's dimensions before ordering. Voltage and footprint must match — do not substitute a higher-capacity cell if the chassis does not accommodate the extra thickness.

• Why the BT-N3650 part number matters: Smartphones using this cell rely on a specific connector pitch and BMS handshake tied to the OEM part. A mismatch in either breaks the charge IC communication loop and the phone may refuse to charge or report incorrect state-of-charge entirely.
• Bench tested on actual hardware: We cycled the BT-N3650 through controlled charge and discharge passes. The BMS held cutoff correctly at the low-voltage threshold and accepted CC-CV charging without tripping protection prematurely on the first cycle.
• Fuel gauge recalibration on first use: On first installation, disable fast charging and run one full discharge down to auto-shutoff, then charge uninterrupted to 100%. This gives the fuel gauge IC a clean reference curve against the new cell before high-current charging is applied to an uncalibrated counter.

Sudden shutdown at 20–30% on the replacement cell

A fresh Li-ion cell has a steeper voltage discharge curve than the worn cell the fuel gauge IC was calibrated to. When the modem fires a high-current burst or the screen hits peak brightness, the cell voltage sags below the protection threshold before the gauge reads zero. The phone shuts off at 25% because the gauge is wrong, not because the cell is defective. One full discharge-charge cycle corrects the coulomb counter and the shutdowns stop.

Phone warm near the battery during the first charge

A new cell arrives with higher internal impedance than a broken-in cell. The charge IC pushes current into that higher resistance and the excess energy dissipates as heat. This is normal for the first one or two charge cycles as the cell's internal chemistry stabilises. If the device stays warm beyond the third full charge or exceeds what you can hold comfortably against your hand, remove the battery and check that the connector is fully seated — a partial connection raises resistance further and compounds the heating.