Logitech 984-001405 Replacement Battery 3.7V 2600mAh

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

This 3.7V, 2600mAh Li-ion battery replaces the original pack in the Logitech Ultimate Ears Boom 3, 984-001405, and S-00170 portable Bluetooth speakers. It restores cordless audio playback and Bluetooth connectivity when the original cell has degraded. Capacity figures come from the product data — 9.62Wh total stored energy at 3.7V nominal.

• Boom 3, 984-001405, and S-00170 compatibility: All three share the same cell format, connector pinout, and BMS handshake voltage thresholds. Swapping between these models works because the protection circuit reads the same charge termination and low-cutoff signals from each device's charging board.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Boom 3 platform. The BMS accepted charge correctly, held the low-voltage cutoff at the expected threshold, and the protection circuit tripped cleanly on overcurrent without latching the pack into a fault state.
• Monthly discharge cycle for the Boom 3: The Boom 3 typically sits on a desk or shelf between uses, charging opportunistically throughout the day. Let the speaker run down past 20% at least once a month before recharging. Constant top-off charging without a deeper cycle causes fuel gauge drift and speeds up capacity fade on this cell size.

Why the Boom 3 cuts out at high volume even on a charged battery

At maximum volume, the amplifier and Bluetooth radio draw current simultaneously. A 2600mAh cell with degraded internal resistance cannot sustain that combined spike without the terminal voltage sagging below the BMS low-cutoff threshold — even when the fuel gauge still reads above 50%. The BMS interprets the voltage sag as a critically low cell and shuts the output rail to protect the cell. Replacing a cell whose internal resistance has climbed above roughly 200–300mΩ resolves this even though the old pack appeared to charge normally.

Speaker won't wake from USB charging after sitting discharged for weeks

If the Boom 3 sits unused for several weeks and the cell self-discharges below the USB-C PD minimum acceptance voltage — typically around 2.5V — the charger cannot negotiate a handshake and the speaker appears completely dead. The BMS has entered a deep-discharge lockout state to prevent cell damage. Connect the speaker to a USB port and leave it for 20–30 minutes without pressing any button — the BMS requires a trickle pre-charge phase to bring the cell back above the handshake threshold before normal charging resumes. If the speaker still shows no activity after 30 minutes, the original cell has likely dropped below recoverable voltage and this replacement cell is the correct next step.