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I am considering a regenerative braking system implemented with a AC permeant magnet motor with an efficiently of ~96%. It is easy to determine via energy equations the kinetic energy of the car and therefore how much energy is possible at max efficiently but regenerative systems also rely on the brakes of the car to slow it down to a stop. Is there a particular rule or reference to how much conventional braking is required for a given scenario? I wonder if it is possible to determine how much the motor would actually decelerate the car and then work out energies from there. Could anyone offer some advice?

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It would depend on how much energy you can store. If you are simply storing in a battery you can calculate the max charging rate and subtract that from the kinetic energy of the moving vehicle. Locomotives use dynamic braking and almost all of them do not attempt to store the energy, they convert it to heat and vent it out the top of the train. A civil Engineer by the name of Travis Painter did a thesis on attempting to store that energy. He selected a flywheel energy storage system due to the fact that it can store a high volume of energy rapidly. Given the other options in his paper that was the best choice but he determined that the losses in generation, storage and retrieval was too high to make it a viable means of regenerative braking.

I think the link below is to a copy of his thesis, if not search for Travis Painter Dynamic Brake Energy.

Travis Painter Thesis

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I had a peugeot 106 electric (battery only, not hybrid) and it was possible to avoid using the brake pedal completely if you read the traffic correctly as it had regenerative braking - the energy went back into the batteries.

Of course if the batteries were already at 100% this won't happen but an absolute max charge is very rare.

As has been mentioned the energy available from slowing the vehicle down is what is available to increase the charge less losses.

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An important energy storage method that is being given very little consideration is gas compression. The kinetic energy of the vehicle can drive an air pump or compressor that would compress air into a tank specially designed for pressures and heat of compressed air. Later the energy stored as compressed air can be released through a regulated nozzle to convert to electrical or mechanical energy.

The air pump would be connected mechanically directly to the drive train or the wheels or axle to have the braking effect. The storage process is not efficient as much heat is generated with the compression. However, the energy can be stored in the compression tank for a very long time with little loss. The capacity for energy storage is only limited by the volume of the storage tank and the maximum design pressure.

Special consideration has to be made for the lubricating oil used for the air pump/compressor. It has to be non-combustible other wise it will ignite with the oxygen in the heated compressed air. This would rule out most hydrocarbon based oils.

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