Sony SRS-XB21 ST-05 Replacement Battery 3.78V 3000mAh

Sony SRS-XB21 / XB22 / XB23 — 3.78V Li-ion Replacement Battery (ST-05)

This 3.78V 3000mAh Li-ion battery replaces the ST-05 (also sold as ST-05S, ID1056) in the Sony SRS-XB21, SRS-XB22, and SRS-XB23 portable Bluetooth speakers. It restores wireless playback on speakers that no longer hold a charge or fail to power on away from a USB source. Capacity is 3000mAh (11.34Wh), matching the original cell specification.

• SRS-XB21 / XB22 / XB23 platform fit: All three models share the same ST-05 cell format, connector pinout, and BMS handshake protocol. Swapping between models doesn't change the battery — Sony used the same internal pack across this speaker generation.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the SRS-XB21 chassis. The BMS accepted the pack without fault codes, completed a full charge cycle to 4.2V, and cut off cleanly at the low-voltage threshold without audio dropout during the transition.
• Discharge cycling for fuel gauge accuracy: If the speaker is charged from a desk or power bank before dropping below 30%, the fuel gauge drifts over weeks. Run the speaker down below 20% at least once a month before recharging to keep the charge indicator reading accurately.

Why the SRS-XB21 shows full charge but audio cuts out after extended play

The XB21 amplifier and Bluetooth radio draw current simultaneously. At full volume, that combined load can pull the cell voltage down faster than the fuel gauge updates. The BMS reads an apparent "safe" state-of-charge but the real cell voltage has already sagged toward the cutoff threshold. When the BMS trips the low-voltage protection, the speaker shuts down even though the LED indicator hadn't flagged low battery. A new cell with full capacity recovers faster between transient load spikes and delays that premature cutoff.

Audio distorting before the battery indicator reaches empty

This happens when the cell's internal resistance has risen — common in a battery that's been shallow-cycled for months. As voltage sags under amplifier load, the amp clips the audio signal before the BMS cuts power entirely. The distortion isn't a speaker driver fault; it's the amplifier running out of headroom as supply voltage drops below its minimum operating point. Replace the battery and confirm the new cell charges to 4.2V — distortion at high volume should clear immediately.