K-Touch U9 AP18 Replacement Battery 3.8V 1550mAh

K-Touch U9 — 3.8V Li-ion Replacement Battery (AP18)

This is a 3.8V, 1550mAh Li-ion replacement cell for the K-Touch U9 smartphone. It fits the U9 model only and matches the OEM part number AP18. Install it when the original cell no longer holds a usable charge or fails to power the device.

• K-Touch U9 fitment: The AP18 cell uses the same voltage rail, physical footprint (51.40 × 51.00 × 5.50mm), and connector pinout as the factory cell. The BMS communicates with the U9's charge IC on the same protocol — no adapter or modification needed.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a U9 unit and confirmed the BMS handshake, charge termination, and over-discharge cutoff all triggered at expected thresholds. The fuel gauge IC accepted the cell without error flags.
• First-cycle fast charge tip: On first use after installation, disable fast charging for one full discharge-charge cycle. This lets the fuel gauge IC recalibrate its coulomb counter against the new cell's discharge curve before high-current charging begins on an uncalibrated cell.

Sudden shutdown at 20–30% on the K-Touch U9 after a cell swap

The U9's fuel gauge IC stores a learned discharge curve from the original cell. When a new AP18 cell goes in, the IC still references the old curve. At around 20–30% reported charge, the new cell's actual voltage drops below what the modem and display draw can sustain — the phone shuts off even though the gauge reads capacity remaining. One complete slow discharge to 0% followed by a full uninterrupted charge resets the coulomb counter and aligns the gauge to the new cell. After that cycle, the shutdown threshold tracks correctly.

OS battery percentage jumping erratically after replacement

Erratic percentage jumps — such as dropping from 60% to 30% in seconds or bouncing between values — indicate the fuel gauge IC is recalibrating against an uncalibrated cell. This is not a fault with the cell itself. The IC's coulomb counter has no reference data for the AP18's actual capacity and internal resistance yet. Run two full discharge-charge cycles without interruption and without fast charging enabled; the percentage readout stabilises once the IC builds a consistent discharge profile. After the second cycle, charge the phone to 100% before normal use.