Sony SRS-X55 Replacement Battery 7.4V 2600mAh ST-04

Sony SRS-X55 / SRS-X77 / SRS-BTX300 — 7.4V Li-ion Replacement Battery (ST-04)

This is a 7.4V 2600mAh Li-ion replacement battery for the Sony SRS-X55, SRS-X77, and SRS-BTX300 portable Bluetooth speakers. It matches the OEM part number ST-04 and fits directly into all three models. Capacity is 2600mAh (19.24Wh) — identical to the original Sony specification.

• SRS-X55, SRS-X77, and SRS-BTX300 compatibility: All three speakers share the same 7.4V two-cell Li-ion configuration, the same ST-04 footprint, and the same BMS communication protocol — which is why Sony used one battery across the range. The connector orientation and locking tab are identical on all three.
• Bench tested on actual hardware: We ran this cell in an SRS-X55 and cycled it through charge and playback. The BMS accepted the pack without fault flags, the charge circuit brought it to 8.4V at full capacity, and audio output held steady across the full discharge curve without amplifier sag tripping the protection circuit.
• Discharge cycling for the SRS-X55 fuel gauge: These speakers are typically left on a USB charge cable between uses — rarely dropping below 60% before topping off again. That shallow cycling causes fuel gauge drift. Let the battery run down below 20% at least once a month before recharging to keep the gauge accurate and slow capacity fade on the Li-ion cells.

Why the SRS-X55 reads full charge but audio drops after extended play

The SRS-X55 fuel gauge relies on coulomb counting — it tracks charge in and out rather than measuring actual cell voltage continuously. After months of shallow top-off charging, the counter drifts away from the cell's true state of charge. The speaker reports 80% or higher while the cell voltage is already sagging under amplifier load. That sag triggers the BMS undervoltage cutoff, and audio drops even though the indicator still shows charge remaining. A full discharge-to-empty followed by a full charge resets the counter baseline.

Audio distorting before the battery indicator reaches empty

This is voltage sag under amplifier load, not a speaker driver fault. When the Li-ion cell voltage drops toward 6.8–7.0V under heavy draw — loud playback at high volume — the amplifier stage clips because it loses the voltage headroom it needs to swing the output signal cleanly. The BMS has not yet cut off, so the speaker stays on, but the audio distorts. Reducing playback volume by 20–30% reduces current draw enough to stop the sag. If distortion appears at moderate volumes on a new battery, check that the cell is fully charged to 8.4V before dismissing it as a speaker fault.