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Consistent regen performance with load dump resistor - idea

Discussion in 'Technical' started by tom66, Jan 5, 2014.

  1. tom66

    tom66 Member

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    A ~60kW load dump resistor which is mixed into the regen circuit for when the regen is limited due to cold weather or full battery -- this would give consistent performance regardless of battery state or external temperature. Coolant could be pumped through, recycling its heat into HVAC, improving efficiency.

    From another thread -- regen disabled at 5°F even with 75% charged battery: http://www.teslamotorsclub.com/showthread.php/25825-Really-cold-tomorrow?p=542164&viewfull=1#post542164

    If min BEMF of motor is 350V @ 60kW such a resistor would be on the order of 2 ohms. Braking resistors like this exist: http://www.reo-usa.com/docs/drives/BWD-158.pdf Continuous power would probably not exceed a few kW so cooling should not be an issue.

    Doubt will see this on an S/X but hope to see it integrated into GenIII platform of some kind.

    As an aside the VOLT uses a mix between friction brakes and regen braking (to provide consistent braking experience, as they claim), with friction used whenever regen is unavailable, but this has the disadvantage of shortening the very long brake life the S has.
     
  2. abasile

    abasile Independent Software Eng.

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    I live on a mountain and generally sustain 5 - 15 kW in regen in my LEAF over ~14 miles while descending 4900 feet. The trouble is, my car's battery is frequently too cold or too full to accommodate all of that regen, so I am forced to use the friction brakes quite a bit more than I "should".

    Would love to see Tesla implement a solution like you describe.
     
  3. scaesare

    scaesare Active Member

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    An interesting idea. Train locomotives do this. I'm suprised they are as compact as they are (I think the liquid cooling allows this), but they still are some significant bulk. Each 15KW rated unit is 26"x13"x10". That's 2 cubic feet.

    It appears the 60KW version is made up of 4 of the 15KW units ganged together. That doesn't include plumbing, coolant, pumps, and radiator necessary to bleed off heat if needed.

    Not a small system, but then again the Model E might only need a 30KW setup. Still large, but I like the idea...
     
  4. RDoc

    RDoc S85D

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    Why not just apply the brakes to make up the lack of regen?
     
  5. AnOutsider

    AnOutsider S532 # XS27

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    You mean the driver or the car automatically?
     
  6. tom66

    tom66 Member

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    You can apply the brakes but then you lose the benefit of long life brakes and also you don't recover any heat which could be used to warm the cabin. Model S uses a PTC heater for when drivetrain heat is insufficient to warm the cabin (in very cold climates, below 0°C), it's rated around 6.5kW IIRC. If you could recover the regen energy as heat, there would be an efficiency boost so it would improve cold weather performance.

    Looking at the datasheet, it's rated 60kW continuous -- no way that would be in use all the time. I'd say a generous 10kW continuous (30kW peak, for 20 seconds per minute) would do it, and that should be much smaller.

    The way I figure is it would be modulated so it would behave in much the same way as real regen.
     
  7. RDoc

    RDoc S85D

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    I meant have the car automatically apply the brakes to make up for lack of regen due to full charge or cold battery to keep the driver experience constant. Probably only the rear brakes would be used to keep it the same as regen.

    While that wouldn't heat the car, unless you do a lot of regen braking, the amount of heat going into the car would be very small in any event. How often do you use regen when you first start out?

    The amount of additional brake wear would be tiny. It's only when the battery is full or too cold, the rear brakes generally wear much less than the front, and the amount of braking, hence wear, would be relatively small compared to a full brake application.
     
  8. nwdiver

    nwdiver Active Member

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    I like the idea of the car applying the brakes automatically to simulate regen so the car has the same "feel" when REGEN is significantly limited. My car was ~16F this morning so REGEN was disabled... first time that's happened. Not sure why the car couldn't give me at least 10kW of REGEN... I know the cars heaters add up to that much so why not just dump it there? Almost rear ended a car because my "auto-pilot" was expecting the car to slow down when I let my foot off the accelerator. Eventually this is going to cause an accident. This would be another great application of a super capacitor since I don't think they're affected by the cold like batteries are.
     
  9. tom66

    tom66 Member

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    Reason you can't regen is that it's dangerous to charge li-ion cells below about 0°C. If you do so, you can plate metallic lithium on internal parts of the battery, leading to increased risk of short circuit. The battery must be warmed to charge properly. Without this, you will get car fires. Hence why Tesla are extremely cautious in how the battery is charged in cold scenarios, they wait a long time to warm those cells up before charging.

    A super capacitor, to absorb 60kW for 5 seconds, or 30kW for 10 seconds, etc. would need an energy capacity of 300kJ or so, if it is 400V rated, this means a capacitance of 2.2F (C=E/V^2). Which is actually quite doable with modern super capacitors. You'd want to increase it about 5~10x to allow for more regen operations before filling up. So, the energy could be built up into a super cap bank and used for car systems, or discharged into a waste load, if it is getting full. Such a cap bank wouldn't be cheap though. You can buy 2200F 2.5V cells, about $40/pc in reasonable quantities, so a bank of ~72 for 400V (~31F) gives a $2,880 price for that bank, excluding control/management electronics.

    I think any bonus, so long as it offsets cost and weight, would work for cold weather, as it is known range can drop by around half in very cold weather. If you can regen to heat cabin (and you use brakes a lot more in winter) then you can improve efficiency and range.
     
  10. abasile

    abasile Independent Software Eng.

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    In my case, descending a big mountain, the brakes sometimes overheat and I have to stop and let them cool. Remember, there's no engine braking.
     
  11. brianman

    brianman Burrito Founder

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    I think we've talked about this before but I'll ask anyway.

    If regen is limited because the battery is cold, why not just "flash heat" the battery with the "excess" (unstorable) regen energy to immediately bring it up to the desired temperature?
     
  12. wycolo

    wycolo Active Member

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    > descending a big mountain, the brakes sometimes overheat [abasile]

    This is interesting. You certainly are a candidate for a brake upgrade, if indeed one is available for MS. Do you sense overheating by smell or loss of braking force? Around here the biggest drop is 4000+ ft; I'd hate to face that with no regen. But we've already climbed up so our batteries are hot.

    Here's a puzzler: With cold Roadster I climbed 1000 ft (over 11 miles) but still no regen. So had to use brakes all the way down. THEN the regen kicked in. But there was no partial regen going down, pure freewheel. Cloudy day below freezing.
    --
     
  13. tom66

    tom66 Member

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    Where are you getting this flash energy from? How do you maintain battery heat? How do you prevent this damaging the cell when cold?
     
  14. brianman

    brianman Burrito Founder

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    Perhaps my simplistic understanding is too simplistic. Elaborating...

    Suppose the battery is only able to store 20 kW when the battery temperature is X but can store the full 60 kW when the battery temperature is Y. When the battery is in the range [X,Y) spend the extra 40 kW coming from the motor (regen) to "flash heat" the battery to Y. I'm guessing the obstacle is that the battery doesn't like being heated that fast, or that the heater isn't able to handle that much power.
     
  15. nwdiver

    nwdiver Active Member

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    I've seen my power bar sitting at 10kW while not moving so SOMETHING is capable of using that much power besides the motor. It was a VERY cold day and the "battery heating" message was active. I would be happy if I had just 10kW of REGEN... ZERO sucks. Amazing how much I miss something I never used to have...
     
  16. djp

    djp Roadster 2.0 VIN939

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    That's exactly what's being proposed. The "load dump resistor" is a big heater that can bring the battery up to operating temperature faster. The current heater is too small to accept regen power.
     
  17. brianman

    brianman Burrito Founder

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    Got it. I didn't grock that out of the initial post. Thanks for catching me up! :)
     
  18. Doug_G

    Doug_G Lead Moderator

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    This has been proposed before on this forum. One of the issues is that the dump resistor would get very hot, and therefore would have to be physically large and have lots of cooling, which is kinda redundant to the friction brakes. Friction brakes have an advantage of being able to get a lot hotter than would be safe for a resistor.
     
  19. tom66

    tom66 Member

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    You cannot charge a li-ion below freezing (or below about 5°C / ~40°F), at all. Doing so plates lithium on the cells, which damages them and risks fire. The battery must be heated to begin charging, which is what the current battery heater does, it's rated around 7kW IIRC. However, it takes all of its energy from the pack to do this. It's also necessary to heat the pack to improve efficiency in very cold enviroments.

    The idea is to use the regen resistor as additional heating, which means you don't have to use the heater as much and/or you can heat the pack up quicker than you otherwise would.

    That braking resistor linked is rated for up to 10 x overload for 6 seconds every 60, I wonder if this means you can size at around ~5kW rating continuous power, 30kW peak, and make it smaller. But, I don't know how it works. You'd need to monitor heat in the resistor. If it gets too hot, it's going to be necessary to apply friction brakes, perhaps automatically.
     
  20. djp

    djp Roadster 2.0 VIN939

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    Hmm - the battery pack is pretty big. I wonder what it would take to build a heating element into the entire underside of the pack.
     

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