FiiO BTR3K Replacement Battery 3.7V 280mAh Li-Polymer

Fiio BTR3K — 3.7V Li-Polymer Replacement Battery

This is a 3.7V 280mAh Li-Polymer battery for the Fiio BTR3K portable Bluetooth DAC and amplifier. It fits the BTR3K directly. Use the capacity figure from this listing — 280mAh — not third-party sources, which vary.

• BTR3K platform fit: The BTR3K runs a single-cell Li-Polymer configuration at 3.7V nominal. This cell matches that voltage rail and the physical dimensions (32.00 × 17.40 × 5.70mm) required by the internal battery bay. The BMS in the BTR3K monitors cell voltage directly — a mismatched cell triggers protection cutoff at boot.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the BTR3K platform. The BMS accepted the cell without fault flags, balanced to full charge cleanly, and the protection circuit tripped correctly at low-voltage cutoff — no forced shutdowns during normal audio output.
• Wireless streaming draw on the BTR3K: Bluetooth streaming keeps the radio active continuously, which adds a constant load on top of the DAC and amp circuits. Avoid extended Bluetooth use at maximum gain — the combined draw accelerates cell depletion faster than wired USB DAC mode on the same charge level.

Why the BTR3K won't boot after a battery swap

A freshly installed replacement cell may sit below the BTR3K's minimum boot voltage — typically under 3.0V — after time in storage. The device's BMS interprets this as an unsafe cell and blocks startup entirely. This is not a fault with the hardware or the battery. Plug the BTR3K into USB-C charge for at least 15 minutes before attempting to power it on.

Audio output degrading or distorting at low charge

The BTR3K's DAC and amplifier circuits are sensitive to supply voltage. As cell voltage drops toward 3.2V, the amp stage loses headroom and audible distortion or volume drop can appear — especially at higher gain settings. This is not a hardware fault. Charging the device back above 3.6V restores clean output. If distortion persists at full charge, the original cell has likely degraded past usable capacity and needs replacement.