About this replacement
Marshall Stockwell II — 7.4V Li-ion Replacement Battery (C406A3)
This 7.4V 2600mAh Li-ion battery replaces the original C406A3 / C406A3-1 cell in the Marshall Stockwell II portable Bluetooth speaker. It fits the Stockwell II directly — same connector, same BMS handshake, same form factor at 69 × 38 × 21mm. Capacity is rated at 2600mAh (19.24Wh), matching the original specification.
- Stockwell II cell compatibility: The Stockwell II uses a single Li-ion pack at 7.4V to power both the Class D amplifier and the Bluetooth radio simultaneously. Any replacement must satisfy the BMS handshake on the speaker's charge board — this cell carries the correct protection circuit and cell chemistry to do that.
- Bench tested on actual hardware: We ran this pack through charge cycles on the Stockwell II board. The BMS accepted the cell, balance charging completed without error flags, and the protection circuit tripped correctly at the low-voltage cutoff threshold rather than letting the cell sag into damage territory.
- Monthly discharge cycle for Stockwell II users: If the Stockwell II sits on a desk and gets topped off constantly, let it drop below 20% at least once a month before plugging it back in. Constant shallow cycling causes fuel gauge drift over time — the speaker reports full charge while actual capacity quietly shrinks.
Audio distorting before the battery indicator reaches empty
This happens when a degraded cell can no longer hold voltage under the combined current draw of the amplifier and Bluetooth radio. As the pack loses capacity, internal resistance rises. At higher volumes, the amplifier pulls more current and the cell voltage sags below the threshold the amp needs — the speaker clips the audio signal rather than shutting down cleanly. The battery indicator still shows charge because the BMS reads resting voltage, not load voltage. A fresh cell with lower internal resistance eliminates the sag and restores clean output at full volume.
Stockwell II won't wake up from USB-C after sitting unused for weeks
A deeply discharged Li-ion cell can drop below the minimum voltage that USB-C PD negotiation requires — the charger and speaker simply don't complete a handshake. This is a cell protection state, not a port fault. Connect the speaker to a 5V USB-A charger first for 10–15 minutes to trickle current into the cell past the 2.5V recovery threshold, then switch back to USB-C. If the cell won't recover above 3.0V per cell (6.0V for this 2S pack) after 20 minutes on trickle, the original cell is too far gone and replacement is the correct next step.