FiiO Q5 Portable Amplifier Replacement Battery 3.7V 2500mAh

Fiio Q5 — 3.7V Li-Polymer Replacement Battery (AEC874866)

This 3.7V 2500mAh lithium-polymer cell replaces the internal battery in the Fiio Q5 portable headphone amplifier. The Q5 runs a dual DAC and discrete amplifier stage that draws variable current depending on gain setting and output load. Matching the OEM cell spec keeps the BMS calibration accurate across the Q5's full voltage window.

• Q5 platform fit: The Q5 uses a specific cell footprint — 63.50 × 41.00 × 7.60mm — with a BMS handshake tuned to the amplifier's regulated power rail. A cell outside this envelope can cause fit issues or incorrect fuel gauge readings on the Q5's onboard display.
• Bench tested on actual hardware: We cycled this cell through the Q5's charge controller and monitored BMS cutoff under high-gain loads. The cell held stable voltage delivery across both gain settings without triggering a premature low-voltage trip.
• First cycle on the Q5: Run the amplifier at 50% volume for the first full charge cycle. Full-volume operation during an uncalibrated first cycle pulls peak current before the BMS has established its delivery baseline, which can cause an early cutoff that under-reports true capacity on subsequent cycles.

Amp shutting off before the battery indicator reaches empty

The Q5's amplifier stage needs a minimum supply voltage to keep the output rail clean. When a degraded cell sags under load, the amp hits its undervoltage threshold before the fuel gauge reaches zero — so the device cuts off with charge still displayed. A fresh cell with low internal resistance restores the delivery headroom the amp needs. After fitting this replacement, run two full charge-discharge cycles to let the BMS re-anchor its empty point to the cell's actual cutoff voltage.

Audio clipping or distortion at moderate volume on the Q5

Clipping on a portable amp mid-charge-cycle usually points to voltage sag rather than a gain setting issue. As a lithium-polymer cell ages, internal resistance rises — under audio transient loads, voltage briefly drops below the rail the output stage needs, and the signal clips. This happens at moderate volumes because transients, not average power, cause the sag spikes. Fitting a fresh cell with rated internal resistance keeps the voltage rail stable through peaks; confirm the Q5 charges fully to 4.2V before drawing conclusions.