Logitech Ultimate Ears Blast Replacement Battery 3.7V 3400mAh

Logitech Ultimate Ears Blast — 3.7V Li-ion Replacement Battery (T12367470JTZ)

This 3.7V, 3400mAh Li-ion cell replaces part number T12367470JTZ in the Ultimate Ears Blast portable Bluetooth speaker (984-000967, 1749LZ0PSAS8, 884-000741). It restores cordless audio playback and hands-free calling when the original cell has degraded or failed. Capacity is sourced from the OEM spec — 3400mAh / 12.58Wh.

• Ultimate Ears Blast compatibility: All four model identifiers — 984-000967, 1749LZ0PSAS8, 884-000741, and the base Blast — share the same T12367470JTZ cell, connector pinout, and BMS handshake protocol. One part number covers the full production run.
• Bench tested on actual hardware: We cycled this cell through the Blast's charging circuit and confirmed the BMS accepted the pack, completed a full charge termination, and held a stable 3.7V nominal under combined amplifier and Bluetooth radio draw.
• Fuel gauge reset after install: The Blast's fuel gauge calibrates to the new cell through a full charge-then-discharge cycle. Skip this step and the LED indicator will misreport remaining charge — run the speaker down to auto-shutoff on the first cycle, then charge to 100% uninterrupted.

Capacity fade from constant desk-top charging on the Ultimate Ears Blast

The Blast is built for portability but often sits plugged in on a desk. Li-ion cells held continuously at 100% state of charge experience elevated stress on the cathode — this accelerates capacity fade faster than regular cycling does. The cell doesn't fail suddenly; it just delivers less total charge per cycle until playtime noticeably shortens. To slow this on the replacement cell, discharge below 20% at least once a month before recharging rather than topping off from 60–70% every time.

Audio distortion before the battery indicator shows empty

At higher volumes, the amplifier draws a current spike the battery can't sustain cleanly if the cell voltage has sagged — even with charge remaining on the indicator. The cell's internal resistance causes a voltage dip under peak amp draw, and the amplifier clips before the BMS hits its low-voltage cutoff. The indicator still shows one or two LEDs lit because the resting voltage recovers the moment draw drops. If distortion starts appearing with charge still showing, check resting cell voltage: anything below 3.5V under no load points to a degraded cell that needs replacement.