Marshall Willen C406A5 Replacement Battery 3.7V 2600mAh

Marshall Willen — 3.7V Li-ion Replacement Battery (C406A5)

This is a 3.7V, 2600mAh Li-ion replacement battery for the Marshall Willen portable Bluetooth speaker. It swaps in when the original cell can no longer hold enough charge to keep the speaker untethered. Capacity figure is taken directly from product data: 2600mAh / 9.62Wh.

• Willen-specific fit: The Willen runs a single-cell 3.7V Li-ion pack on a compact PCB footprint. This cell matches the original voltage rail and connector pinout, so the onboard BMS communicates with it the same way it did with the factory pack.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Willen's own BMS. The protection circuit responded correctly to overcharge cutoff and over-discharge cutoff thresholds, and the charge indicator tracked state-of-charge without fault flags.
• Fuel gauge maintenance for daily-use speakers: Let the Willen discharge below 20% at least once a month before plugging in. Constant top-off charging without a full discharge cycle causes fuel gauge drift — the speaker reports more charge than the cell actually holds, and perceived playtime shortens faster than the cell itself degrades.

Audio distorting before the battery indicator reaches empty

As the Li-ion cell discharges toward its lower voltage floor, internal resistance rises. Under the combined current draw of the amplifier and Bluetooth radio at high volume, the cell voltage sags below what the amp needs to swing cleanly. The result is audible clipping and distortion even when the indicator still shows one bar remaining. Charging back to 4.2V at the cell clears it immediately — the distortion is a voltage symptom, not a speaker fault.

Willen not waking from USB-C when the pack is deeply discharged

If the Willen sits unused for several weeks, the cell can drop below the minimum acceptance voltage for the USB-C PD handshake — typically under 2.5V at the cell. At that point the charge controller will not negotiate power delivery and the speaker appears completely dead on any cable. Use a USB-A to USB-C cable connected to a standard 5V charger first, which bypasses PD negotiation and trickle-charges the cell back above 2.9V. Once the cell recovers to that threshold, normal USB-C charging resumes.