About this replacement
Marshall Willen — 3.7V Li-ion Replacement Battery (C406A5)
This is a 3.7V, 3350mAh Li-ion cell for the Marshall Willen portable Bluetooth speaker. It slots in where the original C406A5 pack sits and restores power to the speaker's amplifier and wireless radio. Capacity figure is 3350mAh — use that, not any third-party estimate you find online.
- Willen-specific fit: The Willen uses a single flat Li-ion cell in a compact enclosure with limited thermal headroom. The cell dimensions — 69.00 × 18.80 × 18.60mm — must match exactly, or the housing won't close and the board connector won't seat. This battery matches those dimensions and the C406A5 connector pinout.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a Willen unit. The BMS accepted the cell without fault codes, charge termination triggered correctly at 4.2V, and protection cutoff fired as expected at the low-voltage threshold during drain.
- Fuel gauge recalibration after swap: After fitting a new cell, let the Willen discharge to the auto-shutoff point before the first full recharge. This gives the onboard fuel gauge a real floor-to-ceiling reference. Skipping this step causes the indicator to misread state of charge from day one.
Why the Willen cuts out at high volume even with a charged battery
At high volume, the Class D amplifier inside the Willen pulls short, sharp current spikes. On a degraded or marginal cell, internal resistance rises enough that those spikes drag the cell voltage below the BMS protection threshold — even if the indicator reads above 50%. The BMS interprets this sag as an undervoltage event and shuts the output. A fresh cell with lower internal resistance handles those spikes without the voltage floor dropping far enough to trigger cutoff.
Speaker shows full charge but audio drops off after an extended session
This happens when the fuel gauge has drifted — the indicator says full but the cell's real capacity is well below what the gauge expects. It's caused by months of shallow charging: topping off from 70–80% repeatedly without a full cycle. The gauge never gets a true empty reading, so its low-end calibration drifts further from reality each week. Fix it by running the Willen until it shuts itself off from low battery, then charging uninterrupted to 100% — one complete cycle resets the reference points the gauge uses.