Once the battery is warm most braking is already regenerative unless you are driving quite aggressively and need to reduce speed very rapidly. At least that's

Once the battery is warm most braking is already regenerative unless you are driving quite aggressively and need to reduce speed very rapidly. At least that's how it seems to be in my 2015 S 70D. So ultra capacitors would have the most impact where people have short commutes in cold climates. Would be handy for me and might make a significant difference in the winter as the nearest thing I do to commuting is a 22 km journey into town and the same coming back after the car has been sitting idle for hours. It takes between 5 and 10 km for the battery to be warm enough for regen to be effective. But after that I can get 50 kW of regen which is plenty enough to slow the car in normal circumstances.