Logitech UE Wonderboom Replacement Battery 3.8V 2100mAh

Logitech UE Wonderboom / 984-000839 Series — 3.8V Li-Polymer Replacement Battery (533-000154)

This 3.8V, 2100mAh Li-Polymer cell replaces the original battery in the Logitech UE Wonderboom and UE Mobile Boombox portable Bluetooth speakers, covering fit models 984-000839 and 984-000851. It matches the OEM dimensions of 44.40 × 44.70 × 8.30mm and slots into the same housing without modification. If your speaker drains fast, refuses to charge, or won't power on, this cell restores the pack to factory spec.

• Wonderboom and Mobile Boombox compatibility: Both the 984-000839 and 984-000851 share the same battery bay geometry, connector pinout, and BMS communication protocol — the same 3.8V Li-Polymer cell works across both units without any wiring changes.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the UE Wonderboom platform. The BMS accepted the pack on first connection, fuel gauge calibrated correctly after one full cycle, and the protection circuit tripped at the expected over-discharge threshold.
• Fuel gauge recalibration tip: These speakers use a coulomb-counting fuel gauge. If you top off the speaker before it drops below 30%, the gauge drifts over time and the battery indicator becomes unreliable. Let the speaker discharge fully to automatic shutoff at least once a month, then charge uninterrupted to 100% to reset the count.

Audio distorting before the battery indicator reaches empty

As a Li-Polymer cell ages, its internal resistance rises. Under the combined current draw of the amplifier and Bluetooth radio at high volume, cell voltage sags below what the amplifier needs to operate cleanly — even when the fuel gauge still shows charge remaining. The result is audible clipping and distortion before the speaker shuts down. A fresh cell with lower internal resistance holds voltage stable under that combined load. After fitting the replacement, run one full discharge cycle so the BMS can map the new cell's discharge curve accurately.

Speaker won't wake up from USB charge after sitting unused for weeks

Li-Polymer cells self-discharge at roughly 2–3% per month. A Wonderboom left unused long enough can drop its cell voltage below 2.5V — the minimum the BMS will accept a USB charge handshake at. The speaker appears completely dead and shows no charging indicator. To recover the pack, hold the power button for 10 seconds, then connect USB power and leave it for 30 minutes before attempting a normal power-on — this gives the BMS time to trickle-charge the cell back above 2.8V and re-initialise. If the cell won't recover above 3.0V after an hour on charge, replace the battery.