Startup Idea : A mechanical Battery.

There is power everywhere, you just need to have eyes. Also there are techniques already found to convert this power to something storable. But the problem i see is storage. I see that no battery last longer than a few months. So my idea is to make a mechanical battery which does not loses its power with time and whenever needed can produce power in form of electrical power. Mechanical things have their own problem like bulk etc, but then there is a whole lot of advantage i see here.