With a listed capacity of .19 Whr, this small 3.7V lithium-ion battery pumps out 15 hours of audio playback.

To calculate the capacity in amp-hours, we know P(power) = I(amps)*V(volts), so I = P/V. That yields a capacity of about .051 amp-hours, or 51 mAh. To say the least, that's diminutive in the world of batteries.

It's remarkable how long a battery lasts when it doesn't have to power a backlight.