Marshall MID Replacement Battery 3.7V 650mAh Li-Polymer

Marshall MID / Major II / Major III — 3.7V Li-Polymer Replacement Battery (VDL603040)

This is a 3.7V 650mAh lithium-polymer cell that replaces the internal battery in the Marshall MID, Major II, Major II Bluetooth, and Major III wireless headphones. It matches the original VDL603040 pack in voltage, chemistry, and physical dimensions. Fit this when the original cell no longer holds a usable charge through a listening session.

• MID and Major series compatibility: These models share the same 3.7V Li-Polymer cell format and BMS handshake protocol. The 41.50 × 30.00 × 5.20mm footprint and VDL603040 designation appear across all four headphone variants, so one SKU covers the full group without modification.
• Bench tested on actual hardware: We ran this cell against the original BMS at full charge and discharge cycles. The protection circuit responded correctly to over-voltage and under-voltage cutoffs, and the cell settled at 3.7V nominal under steady audio load without triggering a false low-battery interrupt.
• First-cycle charging on Marshall headphones: Charge the headphones to full before the first use. Marshall's firmware reads the cell state on the first full cycle and uses that baseline to calibrate the battery indicator — skip this and the charge display will read inaccurately for several cycles.

Why the Marshall MID cuts out mid-track on a new battery

The MID draws simultaneously from the audio amplifier and the Bluetooth radio. On a cell that has been sitting at storage voltage, that combined draw can pull the terminal voltage below the BMS cutoff threshold for a fraction of a second — enough to trigger a shutdown. This is most common in the first few charge cycles before the cell reaches its rated capacity. Running two or three full charge-discharge cycles closes the gap and the cutouts stop.

Battery indicator shows full but headphones cut off after a short session

A new cell shipped at storage voltage — typically around 3.6V — has not yet been read by Marshall's firmware. The indicator defaults to "full" because no discharge history exists, but the cell is not at a true full charge. One complete charge cycle from flat to 4.2V corrects this. After that cycle, the indicator tracks accurately against actual cell capacity.