Since I entered this into the #Coin Cell Challenge, I'd better write something about the battery power. All the prototypes so far have on their back a socket for a LIR2302 battery, and a built-in charging circuit. The later prototypes also have connectors for other batteries.

The 40mAh LIR2302 lets you play for about... 40 minutes. I didn't add any sound to any of my games yet (thought I did test that it works well with wav files) — I expect that using sounds will shorten the play time somewhat.

The charging circuit has been limited to around 40mA, so that it won't damage the tiny batteries.

It is possible to use a different LiPo battery than the coin cells. For example, a battery from a small cellphone (not smartphone) works well, especially since it's about the same size as the device and nicely fits flat on the back. In such case it's recommended to replace the current-setting resistor in the battery charging circuit to a 10kΩ (for 100mAh battery) or even 5kΩ (for 200mAh battery or larger), for faster charging.