Logitech UE Boom 3.7V Replacement Battery 3400mAh Li-ion

Logitech Ultimate Ears Boom 3 — 3.7V Li-ion Replacement Battery (T123682016VK)

This 3.7V, 3400mAh Li-ion cell replaces the factory battery in the Logitech UE Boom 3 portable Bluetooth speaker (model 984-001405 / S-00170). It restores the full charge capacity the original cell had when new. Capacity figure is from the product specification, not estimated from use.

• UE Boom 3 compatibility (984-001405, S-00170): All three model references share the same battery bay dimensions, 3.7V single-cell architecture, and BMS handshake protocol. The connector pinout and thermistor line match across the production run, so the cell seats and communicates correctly with the onboard fuel gauge.
• Bench tested on actual hardware: We cycled this cell through charge, full playback draw, and USB wake-up on a UE Boom 3 unit. The BMS accepted the cell without error flags, the fuel gauge tracked accurately across discharge, and audio output held stable under combined amplifier and Bluetooth radio draw.
• Monthly full-discharge cycle for UE Boom 3 users: Let the speaker drain below 20% at least once a month before plugging in. The UE Boom 3 is frequently used as a desk speaker and topped off constantly — this shallow-cycling pattern causes fuel gauge drift and accelerates capacity fade on the Li-ion cell faster than normal use would.

Why the UE Boom 3 shows full charge but audio drops after an hour

This happens when the original cell has degraded but the fuel gauge still reads from a calibration built on a healthy cell. The reported state of charge no longer matches actual usable capacity. The speaker cuts out because the real cell voltage drops below the BMS protection threshold while the display still shows bars. Replacing the cell and running two full discharge-recharge cycles re-anchors the gauge to the new cell's actual capacity curve.

Audio distorting before the battery indicator reaches empty

At high volume, the amplifier pulls sharp current spikes that the aging cell can no longer sustain without voltage sag. That sag causes the amplifier to clip before the battery indicator triggers low-battery warnings. The distortion is amplifier starvation, not a speaker driver fault. A fresh 3400mAh cell at full charge holds voltage above 3.6V under peak draw, which is enough headroom to eliminate clipping at normal listening levels.