Regenerative braking is the technique of energy recovery which slows a vehicle, in our case a road car, by converting its kinetic energy into an energy form that can either be used immediately, or stored until needed. An electric generator uses the vehicle’s momentum to recover that energy when slowing down, which would otherwise be irrecoverably dispersed at the brake discs, and the tyres as heat. Electric cars, not uncommon early in the last century, have used regenerative braking since that dawn of motoring. The early recovery systems were often complex affairs, where the driver had to flip switches between various operational modes, and the Baker Electric Runabout, of around 1910, was an early example, which used many switches and modes controlled by a ìblack boxî and a ìdrum switchî as part of its electrical system. They could also only practically be used on longer downhill portions of a trip, and they had to be manually engaged.

Today, many conventional cars employ their alternators to initiate a degree of battery regeneration, when engine drive power is removed, and this simply saves a little fuel, and ensures that the battery always stays fully charged. In contrast, the process is critical to the efficiency and economics of hybrid cars and plug-in electric vehicles. The drive motors of such cars are engineered to work in reverse as generators, converting mechanical energy fed from the wheels to charge a storage unit, usually the primary traction battery. Other systems, using flywheels, compressed air, and electric capacitors can also be used to store such recovered energy, although battery storage is the usual practice. A sizeable electric capacitor, charged by using an AC/DC rectifier can also store regenerated energy, and has some advantages, like more rapid peak storage of energy. Many forecast that such ìsupercapacitorsî will eventually replace batteries as the primary storage unit in electric vehicles, as they offer longer life, do not require the exotic raw materials used in lithium-ion batteries, and present no recycling problems. Peugeot/CitroÎn’s proven Hybrid Air system, using compressed air as a power storage medium, and developed for hybrid cars, has never made it into production, apparently on account of its unpopularity with governments, and thus a lack of purchase grants like those offered for plug-in hybrids and electric vehicles.

All today’s hybrid and electric vehicles use regeneration technology to extend the range of the battery pack, be it a small battery in a hybrid, a plug-in hybrid, or a large one in a pure EV. They may use parallel and/or series systems, the former where friction braking and regenerative deceleration both operate together, with the relative proportions often changing with pedal pressure. Series regeneration systems, as used for instance in Tesla cars, use normal friction braking alone for mild deceleration, and only bring regenerative braking into effect when, as Tesla say, more useful amounts of energy are recoverable ñ at higher speeds, and with higher deceleration rates. Most hybrids and EVs offer drivers a choice of how much braking energy they wish to recover. It’s a choice that can significantly change the feel of the braking, most significantly the level of deceleration attained when you lift off the accelerator, equivalent to the experience of engine braking in a conventional car. 

More significant energy recovery problems occur as speeds rise, when the aerodynamic and rolling resistance energy losses are irrecoverable. They both act to slow the car when drive power is cut, but the energy dissipated is not recoverable, nor is the kinetic energy of the car’s mass and speed. The losses of energy conversion are also significant. Battery-to-wheel energy conversion efficiency is around 80 per cent, but when the kinetic energy gets recovered, and thus converted twice, the efficiency may only be 64 per cent. Hence the attraction of the Musk Hyperloop transport system, where evacuation of the transportation tube largely eliminates the aerodynamic losses, even at the envisaged 750mph, and electromagnetic levitation eliminates rolling resistance.