The energy density of comressed air is greater than that of batteries. But when you add the container you loose that advantage. And the conversion from potential to actual energy is much less efficient. You start out with high pressure that drops to zero as you use it. Bottom line: I doubt that compressed air storage is lighter than batteries for the same useable energy. And it certainly takes up more space.

The advantages of compressed air in industry are:
1. Air energy can be transmitted a long ways in useable form with low loss. A 1000 foot long air line is not uncommon. Compare that with the length of an electrical extension cord.
2. Air is non-flammable. Gasoline or electricity in the wrong place can cause a lot of injury and damage.
3. Air motors, although inefficient, are cheap and light weight.
4. In underground mining, the air line is also a source of breathing air. Try to think of any other power source whose exhaust can be used to sustain life.

Mike T.