Marshall Stockwell II Compatible Battery C406A3 7.4V 3350mAh

Marshall Stockwell II — 7.4V Li-ion Replacement Battery (C406A3)

This is a 7.4V 3350mAh (24.79Wh) Li-ion replacement battery for the Marshall Stockwell II portable Bluetooth speaker. It fits the Stockwell II directly, using OEM part numbers C406A3 and C406A3-1. Swap it in when your original cell no longer holds a charge through a full listening session.

• Stockwell II fit: The Stockwell II runs a 7.4V two-cell Li-ion pack with a BMS that monitors cell balance and communicates charge state to the speaker's fuel gauge IC. This replacement matches that voltage rail and connector, so the fuel gauge reads correctly from the first charge cycle.
• Bench tested on actual hardware: We cycled this pack through charge and discharge on the Stockwell II platform. The BMS reported accurate cell voltage to the speaker's indicator LEDs, and the protection circuit tripped correctly at both the low-voltage cutoff and overcharge threshold.
• Monthly discharge cycle for the Stockwell II: The Stockwell II spends most of its life on a shelf or desk, topped off via USB-C before it ever drops below 50%. Let the battery drain below 20% at least once a month before recharging — constant shallow cycling causes fuel gauge drift, and the speaker starts misreporting remaining charge within a few months.

Why the Stockwell II reads full charge but audio drops after an hour

A degraded or miscalibrated cell still shows 100% on the indicator LEDs because the fuel gauge IC tracks coulombs, not true capacity. Once real capacity has dropped, the gauge hits its lower reference point far faster than the display suggests. The speaker then cuts audio output as the BMS enforces its low-voltage floor. A full discharge-and-recharge cycle on a new cell recalibrates the gauge reference points so the display reflects actual state of charge.

Audio distorting before the battery indicator reaches empty

At high volume the amplifier draws a sharp current spike that pulls cell voltage down momentarily — this is voltage sag under load, not a faulty battery. If the cell's internal resistance has risen through age or deep-discharge damage, that sag crosses the amplifier's minimum operating voltage and causes clipping before the indicator shows low battery. Installing a fresh cell with lower internal resistance eliminates the sag at high-draw moments. Confirm the new pack is charged to at least 7.8V open-circuit before the first playback test.