About this replacement
Logitech UE Mobile Boombox / S-00120 / X100 — 3.7V Li-Polymer Replacement Battery (533-000079)
This is a 3.7V 1100mAh Li-Polymer replacement battery for the Logitech UE Mobile Boombox, S-00120, and X100 portable Bluetooth speakers. It replaces OEM part number 533-000079. The cell measures 52.70 × 33.80 × 6.00mm and fits the original battery slot without modification.
- UE Mobile Boombox, S-00120, and X100 fitment: All three models share the same 3.7V single-cell Li-Polymer architecture, the same 533-000079 OEM part number, and the same physical connector orientation — so one replacement cell covers the entire platform.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the UE Mobile Boombox. The BMS accepted the cell cleanly, fuel gauge registered correctly from the first cycle, and the amplifier rail held stable voltage under high-volume audio draw.
- Fuel gauge reset after swap: After fitting this battery, run the speaker to below 15% charge before plugging in — do not top it off immediately. Skipping this step causes the fuel gauge to read incorrectly and the speaker may shut down early even when charge remains in the cell.
Bluetooth dropping at high volume on the UE Mobile Boombox
At maximum volume, the amplifier and Bluetooth radio draw current simultaneously. On a degraded or deeply discharged cell, this combined load causes a voltage sag that briefly pulls the supply below the radio module's operating threshold. The Bluetooth stack interprets this as a link loss and disconnects. The fix is a healthy cell that can sustain the peak current — once this replacement battery is fully cycled in, the sag under combined load drops significantly and the dropouts stop.
Audio distorting before the battery indicator reaches empty
This happens when the cell's internal resistance has risen enough that voltage sags under amplifier load even though the reported charge level still looks acceptable. The amplifier clips because its supply voltage has dropped below the headroom it needs for clean output — the distortion you hear is not a speaker driver problem. Replacing the cell resolves this immediately. After fitting this battery, confirm the speaker reads 3.7V nominal at rest using a USB load tester or by checking the first full charge cycle completes without early termination.