FiiO X1 ii Compatible Battery 3.7V 1800mAh Li-Polymer

Fiio X1 ii / FX1221 — 3.7V Li-Polymer Replacement Battery (AEC404677)

This is a 3.7V, 1800mAh lithium-polymer cell replacing part number AEC404677 in the Fiio X1 ii and FX1221 portable digital audio players. It matches the original cell's dimensions at 80.10 × 46.50 × 4.30mm, so it fits the battery bay without modification. Swap it when the original cell no longer holds charge across a full listening session.

• X1 ii and FX1221 fit: Both models share the same PCB layout, battery connector, and BMS communication protocol. The AEC404677 part number covers the full X1 second-generation line — same footprint, same four-pin connector orientation, same charge termination voltage at 4.2V.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the X1 ii platform. The BMS accepted the cell cleanly, reached full charge without tripping protection, and the player's battery indicator tracked accurately across the discharge curve.
• First-cycle calibration on the X1 ii: Run the player at 50% volume for the first full charge cycle. Full-volume operation during the first cycle draws peak current from an uncalibrated cell and can cause the BMS to trip before the cell reaches its rated 1800mAh delivery capacity.

X1 ii shutting off before the battery indicator reaches empty

The X1 ii's amplifier stage requires a minimum rail voltage to keep audio output stable — typically around 3.5V at the cell terminals. The battery percentage display, however, is calibrated to a different voltage curve and can still show 15–20% remaining when the cell drops below that threshold under load. The result is an unexpected shutdown that looks like a firmware or hardware fault but is actually a voltage floor cutoff. A new, fully calibrated cell resolves this: after one full charge-discharge cycle, the BMS recalibrates and the indicator aligns with actual cell state.

Audio distorting or clipping at moderate volume after battery replacement

Clipping on a fresh cell almost always points to a BMS that hasn't completed its first calibration cycle rather than a faulty battery. When the cell is new, internal resistance readings are conservative, and the BMS may limit peak current delivery to protect the cell — this creates momentary voltage sag when the amplifier draws a burst of current for a loud transient. Run one complete charge and discharge cycle at moderate volume first. If clipping persists after calibration, check that the cell connector is fully seated — a partially engaged connector raises contact resistance and produces the same sag symptom.