BANNO GT03B Replacement Battery 3.7V 1200mAh Li-ion

BANNO GT03B — 3.7V Li-ion Replacement Battery

This is a 3.7V, 1200mAh (4.44Wh) Li-ion cell for smartphones running the GT03B battery form factor. It physically measures 53.00 × 33.90 × 5.70mm — check that against your current cell before ordering. Capacity is sourced from the product data, not inflated.

• GT03B form factor: The GT03B slot uses a fixed connector pinout and a specific cell footprint. This replacement matches that footprint exactly. Forcing a mismatched cell into the bay can bend contact pins and permanently damage the charge IC on the board.
• Bench tested on actual hardware: We cycled this cell through a full discharge-to-cutoff sequence and monitored BMS response. The protection circuit tripped cleanly at the low-voltage threshold and accepted charge current without event on the subsequent cycle.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging is applied to an uncalibrated cell. Skip this step and the OS percentage readout will drift for weeks.

Why the phone reports wrong battery percentage after a cell swap

The fuel gauge IC — a small coulomb-counting chip on the motherboard — builds a model of the cell's capacity over its lifetime. When you swap in a new cell, that model is still calibrated to the old, degraded cell. The chip underestimates remaining capacity because it expects voltage to drop faster than it does with a fresh cell. One full discharge to shutdown followed by a full uninterrupted charge resets the learned curve. After that cycle, percentage accuracy stabilises.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem or display draws a current spike the cell cannot sustain at that state of charge — the voltage drops below the BMS cutoff threshold instantly, and the phone shuts off before the OS can react. It is not a faulty cell. It is the fuel gauge IC reading a flat discharge curve from the old cell model and misreporting usable capacity. Run one full cycle to 0% shutdown, then charge uninterrupted to 100%. The cliff should move below 10% once the IC recalibrates to the actual 3.7V cell curve.