Originally Posted by Dave H

Chrysler experimented a fair bit with a flywheel storage system, and there has been other research. Also some have experimented with super capacitors. But as far as I know all current production electric/hybrid cars just recharge the battery. Going back at least to the GM EV-1/Impact from the mid '90s or so.

In the future I suspect we will see suspension energy converted to the batteries. Shock absorbers burn off a noticeable amount of energy.

There is some construction equipment, primarily excavators I think, that use hydraulic accumulators to store energy. Since they already have significant hydraulic and related control systems I've heard it works quite well.