Regen is a case of a percieved free lunch. Efficiency losses in the motor work in your favor (at least in slowing the car down, if not recapturing the energy) here if you can dissipate the pulse of heat in the motor and controller. And sometimes regen efficiency is not as good as tractive efficiency depending on how the motor controller is programmed. So your loss is your gain, sort of.
That being said, regen set to the same power level as traction can be violent if they aren't easily and finely controlled. And the batteries need to be able to charge at the same rate as discharge, which may be difficult with a high voltage pack if you want to respect battery and controller voltage limits. For example, 70kW (peak) into a 336V pack (28 modules) at less than 420V (15 volts per module and a high voltage for a controller with 600V IGBTs) is 167A DC. Sometimes the kinetic energy is absorbed before the voltage roof is hit, sometimes it isn't. Battery chemistry, temperature, Ah capacity and SOC all are factors. Which is another argument for regen being supplementary, not the primary retarder. Seth
