To make quick regenerative braking efficient, you need something other than batteries to store the energy -- they can't absorb that much power so quickly. They'

To make quick regenerative braking efficient, you need something other than batteries to store the energy -- they can't absorb that much power so quickly. They're working on ultracapacitors, which seem like an expensive solution to me. Our strategy, which is also used by so called hydraulic hybrids (in garbage trucks, most notably, which are constantly stopping and starting), is to compress air during braking: this can be done quite quickly and efficiently, and avoids conversion losses. We were expecting a cycle efficiency of between 70% and 80%. We're not working on vehicle technology at present, though.