Any decently engineered electric vehicle already will have regenerative braking. That is a fairly old and well understood concept. Any electric motor can be used as a generator. When an electic veihicle coasts down a hill, or is doing the opposite to acceleration, the energy is dumped back into the batteries. The exception might be a fuel-cell based power plant. I am not familair with any fuel cell powered machines, but I would assume that some type of battery or capacitor would be needed for large current dump requirements, and those devices can be regeneratively recharged.