DaveinOlyWA wrote:well, in all things its still relative. if you can power up to 80kW and regen up to 30 kW that would be regen at 37.5% right?
ah, if only it were that easy!! but lets face it. power used is power used. there is no free lunch...BUT there is "wasted" lunch because the power that could be regenerated was equal to 35 kW, then we are wasting that 5 kW in heat from brakes.
I think you may know this, Dave, but the way you have stated things here is rather misleading. What we have in our universe is a law of conservation of energy; we do not have a law of conservation of power. Regen deals not with recovering power (kW), but with recovering energy (kWh).
There are two kinds of energy that regen can be useful in recovering: kinetic energy (based on velocity) and potential energy (based on altitude). It may well be that the efficiency of regen varies with speed, which takes us into calculus if we are trying to understand regen of kinetic energy. 50+ years ago I majored in math, but these days calculus makes my
head spin, so let's keep things simple and look at regen of potential energy instead.
Here's a simple experiment that someone might be willing to do, though it is of questionable safety. You would need to live close to a freeway with a significant elevation change, such as close to the base of a thousand foot or more climb up to a mountain pass, and you would need to be able to measure charging energy at the wall:
- Charge to 100% at home and drive the freeway to the pass at a steady speed, say 65 mph.
- Come back down the hill at the same speed, using neutral whenever possible, and never using regen. Apply the brake in neutral as needed to keep your speed constant.
- Charge to 100% at home again, and repeat the test.
- This time come back down the hill in ECO mode, again holding the same constant speed, but never using neutral.
- Charge to 100% at home a third time.
The difference in kWh between the second and third charge is the amount of regen energy you recovered. Since the speeds were held constant and the same at all times, that was all recovered from the potential energy you had at the top of the hill. That potential energy can be calculated easily based on the elevation difference and the weight of the car (including driver).
I'm sure someone is going to post and say this is crazy, that for a fair test you need to allow the car to speed up going downhill. That is true if you are trying to compare the efficiency of regen with the efficiency of coasting, and certainly speeding up will give better numbers than holding your speed and using the brake. That could be a useful test for a third run, to see whether regen or freewheeling is more efficient at a particular starting speed. But the answer will vary based on the speed, and it will not tell you the efficiency of the regen itself.
My definition of the efficiency of regen, for my proposed test, is (energy recovered) / (potential energy difference at the pass). I am personally guessing that number will be somewhere between 50% and 70%, but that is only a guess.