First thing is that KERS is not 100% efficient. Only about 90% (it can vary but is in that ball park) end up in the battery for reuse. When that energy is reused to accelerate the car, there is a similar 90% loss so the actual amount of energy that is recovered and reused is 81% for the store and use route. However that 90% is only the 90% of braking energy from regen.
Low levels of regen generate charge currents around that of an AC charger. High levels can easily reach that of a DC DC charger! This also affects how much of the energy can actually be stored in the battery.
A 2800 kg car at 100km/h is equivalent to 0.3 kW over a one hour period. The car is braking over 4 seconds so that works out at 0.3*60*60/4 = 270 kWh over that 4 seconds. It is like plugging into a DC charger that immediately goes to a 270kW charging rate. Not good. The car cannot accept that level of charge instantly being applied so the regen is reduced and the discs and pads slow the car by converting it to heat. This reduces the recovery efficiency to something like 10 to 20%.
The biggest regen current I have seen on my MG5 is 200A which at 400V is 80kW and that was fleeting. It probably averages around 20kW or less over the four seconds. That means from that 4 second 100km to 0 brake, only around 10% of the total energy is recovered. And using the foot brake at the same time. The conclusion is that regen is not that efficient in such cases because it is not solely responsible for the braking. It is surprising the number of drivers that then assume that regen captures 90+% plus of energy when it doesn't. Last minute braking is an inefficient in an EV as in an ICE.
When EV manufacturers quote 90% energy recovery through regen when braking, that does not mean that regen recovers 90% of all the available energy but that when regen is solely used, it can recover 90% of the energy into the battery. It ignores the 90% loss to use it again. It also ignores the losses caused by using the foot brake.
Big difference.
So the key is making sure that braking is kept to an absolute minimum. This requires driver anticipation and that is more important than what KERS setting is etc. Given that a stop is anticipated:
- Coasting is most efficient i.e. current is zero which may require a little throttle to overide the KERS setting.
- Next is regen at the appropriate level to give just the required amount. i.e lift off.
- Finally foot braking.
PS the braking method makes no difference to tyre wear.
PPS Cruise control uses the mechanical brakes to adjust the car speed so can be less efficient.
