Next to come out is a standard 18650 Lithium Ion cell, widely used in the industry from Tesla vehicles to self-balancing boards.

This unit is rated for 2850 mAh at 3.7 V to yield 10.54 Wh, which Amazon says will provide up to 9 hours of music playback.

Compare that with the 3.8 V, 2750 mAh, 10.45 Wh battery in the iPhone 6s Plus—a battery with a similar output for a radically different job.