Also known as "Regen"

What is it? A regenerative brake is an energy recovery mechanism that reduces vehicle speed by converting some of its kinetic energy into a useful form of energy instead of dissipating it as heat as in a conventional brake. The converted kinetic energy is stored for future use or fed back into a power system for use by other vehicles. -I got this definition from Wikipedia

In Electric Vehicle applications, the most common approach is to use electronics to temporarily use the motor as a generator, which creates resistance to both slow the vehicle and recharge the battery pack simultaneously.

It goes without saying, that an endurance road course where drivers must decelerate for corners or road hazards (more quickly than coasting allows for) would have a potential benefit from using regenerative braking instead of (or along with) conventional brakes. 

My first experience was several years ago when I installed a 200A DC regen controller at Oregon's "Portland International Raceway" event held on Memorial Day.  In test runs, blasting down the long straightaway between 60-65mph and instead of braking before turn 1, applying full re-gen. I read above 60 amps (charging) at times.  However, those tests were more aggressive than I would drive in a race. The problem with my set-up at the time was an undersized regen controller. Its continuous power rating couldn't handle the 24v, 35-40 Amps average load without overheating at about half way through the race and turned itself off until it cooled down - which took several minutes.

In later tests (pictured below), I was able to simulate the load and found that the controller shuts down consistently, after about 35 minutes with the loads I tested at.  An added heat sinc and computer fan helped it complete the 60 minute duration without over heating to the point of shutting down. However, with the controller generating that much heat, it's not efficient anyway!

There are other means of regenerating power such as shocks that generate electricity as they absorb the energy from bumps over a long drive. But that is a far less refined product at this time. Or the idea that a wind mill on top of a moving vehicle somehow generating significant power. This doesn't go over well because the energy it takes to move the vehicle forward (to turn the wind mill blades) will exceed the energy recovered from the windmill since generation systems are somewhere less than 100% efficient. I'm not ignoring that a side wind could be taken advantage of, however it's not a practical solution because streets are not at predictably optimal angles to the side wind and can't be traversed by cars like the open ocean can be by sail boats.