Techniphone ISI 2 Replacement Battery 3.7V 1200mAh

Techniphone ISI 2 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 1200mAh Li-ion battery for the Techniphone ISI 2 smartphone. It replaces the original cell when the phone no longer holds a charge, shuts down unexpectedly, or fails to power on. Capacity is rated at 4.44Wh and matches the original cell's dimensions at 53.20 × 34.00 × 5.70mm.

• ISI 2 cell compatibility: The ISI 2 uses a fixed-voltage 3.7V single-cell Li-ion architecture. The connector pinout and physical footprint match the original cell exactly — no modification needed to fit the battery compartment or reconnect the flex tab.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the ISI 2 platform. The BMS accepted charge on the first connection, completed a full cycle without cutoff errors, and held voltage within expected range under screen and call load.
• Fuel gauge recalibration on first use: On the first full cycle after fitting, disable fast charging if your charger supports it. This lets the ISI 2's fuel gauge IC measure the new cell's discharge curve accurately before high-current charging runs against an uncalibrated baseline — preventing false percentage readings early on.

Why the ISI 2 reports wrong battery percentage after a cell swap

The ISI 2 stores discharge curve data for the original cell inside the fuel gauge IC. When a new cell goes in, that stored data no longer matches the actual cell chemistry. The gauge reads voltage and applies the old curve, producing percentage figures that are off by 10–20%. One full discharge down to auto-shutoff, followed by an uninterrupted charge to 100%, forces the IC to rewrite its reference data against the new cell.

Sudden shutdown at 20–30% on the ISI 2

This happens when the cell voltage drops sharply under modem or display load — a voltage cliff the fuel gauge did not predict from its stored curve. The phone reads 25% but the cell is already below the BMS cutoff threshold under that load spike. It is not a faulty battery. Run one full calibration cycle first, and confirm the charger is delivering a stable 5V — fluctuating input voltage accelerates the apparent cliff effect.