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Loss of regen on a full charge

Discussion in 'Roadster: Technical' started by Norbert, Oct 25, 2009.

  1. Norbert

    Norbert TSLA will win

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    #1 Norbert, Oct 25, 2009
    Last edited by a moderator: Oct 27, 2009
    I guess I would prefer regen (or rather, the braking effect of the motor) to continue working even when the battery is full, if that is technically possible.
     
  2. TEG

    TEG TMC Moderator

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    I would think it would be entirely possible to provide some mechanism to "waste power" when regen is helpful but the battery is already full. At the most simple it could just be a big resistor+heatsink that the extra regen power is dumped into.
     
  3. Tdave

    Tdave Member

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    If only there were some system on the car that could reduce the speed of the car through waste heat, like with friction or something, maybe with a couple pistons pressing a pad against a metal rotor. Something like that.
     
  4. TEG

    TEG TMC Moderator

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    Yeah, yeah - but the nice thing about using the regen is you don't have to heat the brake rotors or eat away at the pads. It saves pad life and keeps the friction brakes from fading.
     
  5. Serge

    Serge Member

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    Hmm, perhaps turning on the cabin heater or AC either manually by driver or automatically by car when battery is "full" and regen braking is in place? On the other hand, short of very specific situations like Roadster being towed (and on) or driving downhill immediately after charging full, such confluence of conditions seems atypical.
     
  6. TEG

    TEG TMC Moderator

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    Yes, well, some people do live at the top of a hill and charge fully before starting their daily drive. In their case it would be a typical condition.
     
  7. tomsax

    tomsax Member

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    Our house is about 0.4 miles up a hill with a very steep 500-foot driveway. It is a momentary surprise on those rare occasions when I'm charged all the way up in range mode and start down the driveway and quickly realize I need to apply the friction brakes. It happens so fast, I don't even realize what the issue is until it's solved.

    You only lose regen if you charge all the way up in range or performance mode. No one should be charging like that on a daily basis. I've only needed to charge in range mode four times in five months and 5,100 miles. Even then, regen starts coming back after just a couple of miles and seems fully back to normal within 10 or 15 miles.
     
  8. bolosky

    bolosky Member

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    I don't think this would help much. The regen can draw as much as 40kW, while my best guess at the cabin heater is ~3kW. The AC is even less.

    Given how quickly the cabin heats up with the 3kW heating, I'm not sure I'd want to have the full 40 dumped on me, even for 10 seconds or so.
     
  9. bobw

    bobw Tesla Reader

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    You're talking about dynamic brakes.
     
  10. Serge

    Serge Member

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    Is there an ability to adjust percentage of charge in performance or range modes?
     
  11. tomsax

    tomsax Member

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    No, or 'sort of' depending on what you want to accomplish. The whole thing is pretty goofy. The charge modes set not only the portion of the battery pack to be used, but also the power limit.

    Standard mode means: use only the middle 80% of the battery and put a bit of limit on power use. In other words, charge to 90% and hold the bottom 10% in reserve. That typically results in 194 ideal miles after a complete charge, with about 25 more in reserve. The charge gauge will show 100% and around 194 ideal miles when fully charge in standard, and 0% means you're at the end of standard mode, but there's still 10% of the full battery pack left, or about 25 ideal miles.

    Range mode means: charge all the way to the top and show the full battery range, i.e., nothing in reserve. Also, limit power a little more than standard. 100% on the charge gauge in range mode means 100% of the usable battery pack range, 0% means the end of the usable battery pack, so the ideal miles shown at a full charge are about 244.

    Performance mode means: charge all the way to the top, but hold the bottom 10% of the battery in reserve, and allow full power use. I've never charged to the top in performance mode, but I'd expect it to show about 220 ideal miles, with 25 or so held in reserve. I've also heard that charging in performance mode will let the battery pack get warmer than in range mode, which may or may not be better for drag racing (I've heard it both ways).

    You can mix and match modes between charging and driving. If you want full acceleration, but don't need to put extra wear on the battery by charging up through the top 10% of the battery, charge in standard mode, then switch to performance mode after charging is complete. If you're out driving after a standard mode charge and find yourself needing a bit more range, switch over to range mode to get the standard mode reserve to show up on the charge gauge.
     
  12. Serge

    Serge Member

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    Well, for scenario like yours instead of dumping excess regen energy I thought it would be possible to adjust standard mode to charge up to say 79.5%. This way extra .5% of battery pack would be available to absorb regen energy when starting downhill after unplugging the car.

    From your description of different modes it seems that a similar effect will be achieved when charging up in normal mode, then switching to range or performance mode before starting to drive downhill. On the other hand, adding ability to adjust percentages via VDS and setting as preferences seems more convenient than cycling between modes.
     
  13. donauker

    donauker Member

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    There is no need to do any adjustment in standard mode charge as far as regen in concerned. When fully charged in standard mode there is still room for plenty of regen without switching modes. It may be that you could max out the limit if you had an immediate decent of several thousand ft. after charging but I have no way of testing that where I live.
     
  14. bolosky

    bolosky Member

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    You'd need a pretty big descent. The extra 10% of the battery that you have to fill is about 5.3kWh. The mass of the car is a little over 1200kg, so the gravitational potential energy is about 12,000 J/m (since g is about 10 on Earth and gravitational enegry is mgh). One kWh is 3.6MJ so if you were perfectly efficient in transferring the potential energy into the battery it would take 5.3 * 3.6MJ/ 12,000 J/m = 1590m, or more than 5000 feet.
     
  15. tomsax

    tomsax Member

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    Indeed! Driving down from the Sunrise visitor center on Mt. Rainier, I descended somewhere around 4,000 feet in 14 miles and only gained two ideal miles on the charge. Since you've got 10 to 15 ideal miles worth of full regen on top of a full standard mode charge before regen starts to fade out, that would take a serious descent.

    The descent from the observatories at the peak of Mauna Kea (13,803 feet) down to Saddle Road (6,600 feet) is about 14 miles, but that is a very nasty gravel road above the visitor center at 9,000 feet.

    Is there a higher and steeper paved descent with electricity at the top anywhere on the planet?
     
  16. BBHighway

    BBHighway Member

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    I'm not the ultimate expert on AC induction motors, but as I understand it, there are two ways to generate a reverse torque. One is the conventional regen method, in which the motor becomes a generator and recharges the batteries, and a second method in which the motor would consume power, much as it does when powering the car in either forward or reverse.

    It would be possible to use the second method to "fake" regen when the battery is full. There are two problems with doing that. The first is that it would be very difficult to smoothly transition to normal regen as the battery level drops, and even more important is that range mode is intended to provide - maximum range - and using up power to slow down goes against the whole idea.

    I think the temporary loss of regen is a minor quirk in the Roadster that is easy enough to live with.
     
  17. DrComputer

    DrComputer Member

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    Actually yes, there is. If you took your Tesla to the top of Haleakala volcano in Maui (10,023 feet) you could coast all the way back to sea level in about 30 miles. If you don't have your Tesla it is also a fun thing to do on a bike (I did it last month).
     
  18. doug

    doug Administrator / Head Moderator

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    I drove up there to watch the sun rise and froze my butt off. Well worth it. On the way back down I had to dodge a bunch of people on bikes. ;)
     
  19. tomsax

    tomsax Member

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    Yeah, Cathy and I have done that sunrise bicycle tour, but it's twice the distance and not twice the elevation gain as Mauna Kea, but it is a LOT better road.
     
  20. Serge

    Serge Member

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    I don't recall seeing any outlets on top of Haleakala, so you wouldn't start with a full pack anyway :)
     

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