A pool full of special balls

Why should not we use energy developed by children playing in a pool full of balls for producing and storing energy for low-power devices?

Project yourself in 2050.

Due to the last two decades of apocalyptic movies you could think of an arid, unhospital world with few communities striving for survival.
Let's invert this idea.

You are in a high-tech city, with lots of green spots, clean air, low traffic noise, fresh food .
You might have purchased some small electronic devices, such as mediaplayers, which already at the present moment are day-by-day improving the efficiency in energy consumption.
Nevertheless they are not eternally charged and you are out of battery.
And here you have the easy solution: the playground.

You go to the playground, to one of those pools full of balls in which childrens use to play, you take a light, colorful ball, you open it, you put your empty battery in it, you seal it and you throw it in the pool.

A sensor on each ball tells you which ones contain an already charged battery (the all have the same design).
Take it, open it and give power to your favourite music!

Alessio Contini