About this replacement
Samsung MX-ST40B Series — 7.4V Li-ion Replacement Battery (AH81-15582A)
This 7.4V 7200mAh (53.28Wh) Li-ion battery replaces the original pack in the Samsung MX-ST40B portable Bluetooth speaker. It fits MX-ST40B, MX-ST40B/XY, MX-ST40B/ZA, and MX-ST4CB/ZA variants. OEM part numbers AH81-15582A and EB-T23C-00 confirm direct compatibility across the model range.
- MX-ST40B series fit: All listed variants share the same 7.4V battery rail, connector pinout, and BMS handshake protocol. Swapping between regional SKUs — /XY, /ZA — does not change the battery specification, so one pack covers the full lineup.
- Bench tested on actual hardware: We cycled this pack through charge and discharge on the MX-ST40B platform and confirmed the BMS accepted full charge without triggering protection cutoff. Cell voltage balanced correctly across both cells at 4.2V per cell at top-of-charge.
- Monthly discharge cycle for MX-ST40B users: This speaker gets charged on a desk before the battery drops below 50% — repeatedly. That shallow cycling causes fuel gauge drift over time. Let the battery run below 20% at least once a month before plugging in to reset the gauge and slow capacity fade on the Li-ion cells.
Bluetooth dropping at high volume on the MX-ST40B
At high volume, the amplifier draws a sharp current spike on top of the radio module's continuous draw. A weakened or ageing battery cannot sustain that combined load without voltage sagging below the radio module's operating threshold. When voltage drops below that point mid-transmission, the Bluetooth stack resets and the connection drops — even if the battery indicator shows charge remaining. Replacing the pack with a full-capacity 7200mAh cell eliminates the sag that triggers the radio cutoff.
Audio distorting before the battery indicator reaches empty
This distortion is amplifier clipping caused by voltage sag — not a speaker or firmware fault. As the Li-ion cells discharge toward their lower cutoff, internal resistance rises and the pack can no longer deliver clean voltage under amplifier load. The amplifier clips the signal before the BMS reaches its hard cutoff, producing audible distortion at normal listening levels. If the distortion clears when you plug in USB-C but the indicator still shows charge, the cell voltage was already below approximately 3.0V per cell under load.