TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

Regen numbers--how much does it really help?

Discussion in 'Roadster: Technical' started by ChadS, Jul 25, 2010.

  1. ChadS

    ChadS Petroleum is for sissies

    Joined:
    Jul 16, 2009
    Messages:
    2,399
    I recently saw a blog posting http://jackrickard.blogspot.com/2010/07/more-regenerative-braking-results-it.html where the writer said that he'd tested regen on a couple of EVs, and according to his tests more energy was used when regen was on.

    That didn't seem right to me. I assumed he was driving differently with regen, trying to use it often as if it was a magic way to add range. Of course it can't add to your theoretical range; it can only add to your observed range by keeping you closer to your theoretical range. It only helps if you regen instead of wasting energy by braking. Regenerating when you don't need to slow down is a waste, so not braking at all is best if you can swing it.

    I tried my own simple test to make sure that regen really helps on our Roadsters. This was a short drive, and I only did it twice, so this is not authoritative, but I feel better that the results were as expected.

    I took two trips; in the first I drove normally (using regen on the accelerator whenever I wanted to slow down); in the second I tried to drive exactly the same, except for when I started to slow down--then I would pop it into Neutral and use the brake pedal. I took exactly the same path each time; traffic was not identical but it seemed there were a roughly equal number of slowdowns on each trip; when possible I used cruise control to keep at the speed limit; I tried to accelerate at 25kw each time.

    I drove in a triangle:

    1. On the 1.65 mile 35mph 300' uphill leg, I used 747wh with regen, and 824
    without. That's 10% more energy without regen. I had to stop about 5
    times, so that's where regen was useful.

    2. On the 2.05 mile 60mph 300' downhill leg, I used 183wh with regen, and
    376 without. That's 105% more energy without regen. Note that much of this
    downhill was at freeway speed, so I was still using energy (no regen)
    despite going downhill. It wasn't until the stoplight at the bottom that I
    used regen. That was the only stop on this leg (there was another light,
    but I hit it green both times).

    3. On the 1.8 mile 35mph flat leg, I used 400wh with regen, and 470
    without. That's 17.5% more energy without regen. There were 3 stops and a
    couple of slowdowns on this portion.

    For the overall trip, I used 25% more energy when not using regen. I guess
    a better way to say it is that I didn't reclaim 20% wasted brake energy when
    I didn't regen. The overall difference after all the ups-and-downs is, no
    surprise, roughly similar to the difference when driving on the flat and
    making occassional stops. Going downhill obviously gets you the biggest
    gains. Going uphill gets you the least; even if you make stops, gravity is
    doing some of the work rather than regen. Everything is pretty much as I
    expected it to be, so I feel better now.
     
  2. Tdave

    Tdave Member

    Joined:
    May 16, 2008
    Messages:
    721
    Location:
    Columbia, MD
    You don't have to go through all that empirical comparison testing. The Roadster trip computer tells you exactly how much energy regen generated compared with energy consumed.
     
  3. ChadS

    ChadS Petroleum is for sissies

    Joined:
    Jul 16, 2009
    Messages:
    2,399
    Yes. But Jack's claim isn't that regen doesn't put any energy in to the battery. He is claiming that when regen is available, you slow down more than you have to, so you consume more. He doesn't want the regen vs energy consumed; he wants to see total energy consumed with regen on and off.

    He may still argue that I didn't really have it off; but I did my best to pop the car in to neutral at any time that the car tried to do some regen, and to make my speeding up and slowing down the same on both drives.
     
  4. ChargeIt!

    ChargeIt! Member

    Joined:
    Jul 24, 2008
    Messages:
    619
    Thanks for doing this, Chad. (Jack's article irked me too.)
    How about charging full in Range Mode, doing a run (hardly any regen due to full battery). Then repeat when full in STD Mode ?
     
  5. qwk

    qwk Model S P2681

    Joined:
    Dec 19, 2008
    Messages:
    2,817
    Regen works. How well probably depends on your driving style. Of course there are examples of home conversions that aren't anywhere near what the roadster regenerates, which seems to be the case in the link you posted.

    You can't assume all regen doesn't work because a half assed home conversion.
     
  6. qwk

    qwk Model S P2681

    Joined:
    Dec 19, 2008
    Messages:
    2,817
    Regen works. How well probably depends on your driving style. Of course there are examples of home conversions that aren't anywhere near what the roadster regenerates, which seems to be the case in the link you posted.

    You can't assume all regen doesn't work because of a half assed home conversion.
     
  7. Doug_G

    Doug_G Lead Moderator

    Joined:
    Apr 2, 2010
    Messages:
    15,848
    Location:
    Ottawa, Canada
    Just going by the computer, I'm getting about 20% of my energy back with the regen - right in line with your empirical results.

    Sounds like more FUD.
     
  8. PaulM

    PaulM Member

    Joined:
    Sep 8, 2008
    Messages:
    107
    Location:
    Ottawa, Canada
    I also saw that article and think there must be a problem with his testing methodology. Correct me if I’m wrong but you can’t only keep track of the energy being removed from the batteries, you must subtract the energy being put back into the batteries. Perhaps this is what was done incorrectly.

    Logically, there is no way using regen will make your car LESS efficient. Regen simply puts some of your kinetic energy back into the batteries instead of transforming it to heat via your break pads.
     
  9. ChadS

    ChadS Petroleum is for sissies

    Joined:
    Jul 16, 2009
    Messages:
    2,399
    Jack didn't disagree that turning necessary brake pad heat into battery energy is good. His claim seemed to be that when you have regen on, you slow down more often than you have to. Because regen is not 100% efficient, if you slow down (and spend energy speeding back up), it is possible that regen could have a net cost.

    He's basically saying that people drive different when they have regen than when they don't. I couldn't imagine driving that way, but he appeared to have data, so that's why I wanted to check it out just to make sure I wasn't missing anything. It does look likely that he's done something wrong, but I am not sure what. One possibility is that he initially thought regen was a good thing on its own (not just better than braking), so he regen'd more on purpose. I have spoken to some people that seemed to think this was the case, and in fact a few seem to think you can power the car entirely by regen. Jack seems to have way too much experience to have done that, though. Maybe the extra regen was accidentally, because they were not used to the controls? Maybe (as suggested above) it was a net metering error? Who knows.
     

Share This Page