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Comments on disabled regen in the cold

Discussion in 'Roadster' started by cinergi, Dec 7, 2010.

  1. cinergi

    cinergi Active Member

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    The night before last, it got down to about 22F (-6C) and when I left for work it was 29F (-2C). The battery was still warm from charging, so regen was working. The car sat at work for 12 hours, most of which was just above freezing. When I left, it was 29F (-2C) and regen was off. This was my first experience with it off.
    Indeed, the car really rolls! It didn't take long to adjust to it, but 1) half the reason I enjoy driving the car is the fact that regen braking is via letting up on the accelerator pedal and 2) the mapping on the pedal isn't adjusted for when regen is off. For example, when traveling at 40 MPH and you've moved to the brakes and back, you must significantly depress the pedal before *anything* happens. You sort of have to search for the point where you'll get the acceleration you need.
    When regen is off, you get a yellow light in the instrument cluster. Unfortunately my eyes are being trained to ignore yellow lights in that area due to the constant "hey it's cold outside" light.
    Based on what I read here, I expected the battery to warm up quickly and regen to turn on. It didn't. Even at a few degrees below freezing, it took 13 out of my 16 mile commute for it to turn on. Really disappointing to drive the car without regen on for so long -- I think I'll need to get back to pressing my place of work for a plug.
    I wonder if it's practical for the car to allow me to say "keep the pack warm enough for regen for X hours" -- that would make me feel better about the upcoming winter. Without a place to plug in, there'll be a LOT of commutes without regen and I find that very displeasing.
    Today I left after the car had been parked for 9 hours and I was able to mash the pedal more than I was yesterday. Regen kicked back in after 2 miles...
    I'm not sure how the average consumer will take to this. Regen shutting off and thus changing the driving behavior of the car seems like a pretty negative point for cold environments.
    Thoughts / comments?
     
  2. Doug_G

    Doug_G Lead Moderator

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    Ditto on all counts.
     
  3. doug

    doug Administrator / Head Moderator

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    Maybe they could shunt some of that would be regen power to a resistive cabin heater.
     
  4. Alan

    Alan Member

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    And / or the battery pack heater as well
     
  5. edo

    edo Member

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    I wonder if they could do at least a little bit of regen to warm the battery up faster...
     
  6. Doug_G

    Doug_G Lead Moderator

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    The battery chemistry used doesn't like to be charged below freezing.
     
  7. scott451

    scott451 KWH-PWR#1349Sprt,S Sig#96

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    #7 scott451, Dec 8, 2010
    Last edited: Dec 8, 2010
    Disclaimer: The following may not be the best for the long term health of the battery. YMMV
    Quick answer: drive at lower states of charge in performance mode when it's really cold.


    Here's a couple of things to try...
    • Use a scheduled charge, starting 3-4 hours before you go to work, in performance mode. (note: when you open the charge door,it will revert back to standard)
    • Do not fully charge. Stop around 60-75% SOC. This keeps the pump from running and I've noticed that the battery heats up faster at lower states of charge.
    • Drive in performance mode. This draws more current and will heat up the battery faster.
    • Drive heavy on the accelerator.:cool: Heat up the thermal mass of the motor & PEM.
      Use the cabin heat to heat the car as much as possible. Without external power, ultimately, it's a thermal mass problem. So get everything around the battery as warm as possible.
    • Get a long extension cord and charge at [email protected] a few hours before you leave (don't ask first, they'll think it's an engine block heater). No one is going to complain if you plug your "cool" electric car into a nearby electric oulet.
    • Use the log parser tool to extract the "Sleep" record and plot the battery temperature vs time. Then see how long it takes for the battery to cool down to the "no-regen" point (or send me a log and I'll do it for you)
     
  8. TEG

    TEG TMC Moderator

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    Scott, being that you live in San Diego, I am surprised that you know so much about cold weather behavior!
     
  9. TEG

    TEG TMC Moderator

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    The resistive regen shunt would be nice for full pack situations too. Better to have consistent regen and the ability to save the friction brakes all the time.
     
  10. mpt

    mpt Electrics are back

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    I don't think regen is on/off, it's something like 1A to 80Amps charge current.

    Here's my theroy: When the car goes into overrun, the motor creates a back voltage - charge potential. That voltage is managed by the PEM to be enough over the pack voltage to push the right current into the pack, hence charging the pack and creating a load on the motor (ne. generator) to slow the car. When it's very cold the voltage is managed down to prevent the flow of current into the pack, hence little or no resistance from the motor. However, I think that there is still a tiny increase over the pack voltage that has the effect of pushing a little into the pack (I never see '0' on the dash display) but also, preventing the pack from being discharged by the cars accessories such as the heater. In fact, as the voltage from the motor, now the cars electrical power source, is higher than the pack it'll be powering all of the electrical systems of the car as you slow down.

    I'm basing this on some understanding of circuit design and because I've noticed that regen never fully goes, not 100%. If I switch to neutral the car coasts but back in drive, even with regen locked out there is a little more drag... hence my belief that the heater was getting powered by regen.

    As the pack warms up I've noticed that regen returns progressively, not off to on. The light goes out at about 90% IMO.
     
  11. Doug_G

    Doug_G Lead Moderator

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    One could put a heater into the coolant lines, but I don't think that would be a good approach. 25 kW is a lot of power, so there would be issues of weight, complexity, fire risk, etc...

    A better approach would be to include resistors throughout the battery pack, which could be switched on when needed to directly heat the pack. Both the losses in the batteries and the resistors themselves would heat the pack directly, so it would be 100% efficient. The only concerns would be avoiding hot spots and of course switching the power to all those resistors.
     
  12. mpt

    mpt Electrics are back

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    There is a heater in the coolant lines, it's used on utility power to heat the pack prior to charging.
     
  13. Doug_G

    Doug_G Lead Moderator

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    Oh. In that case maybe there's something that could be done in firmware. Like an option to automatically preheat the pack at a prescribed time, so it can be toasty and ready to go at the end of your workday.

    A similar feature for pack cooling would be great too, for those hot summer days.
     
  14. mpt

    mpt Electrics are back

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    These features are, I think intended to be driven by utility power, probably for no reason other than this is how they did it.

    Whilst we're on pack warming; one recommendation that Tesla makes is to plug the car in and start a range mode change half an hour before setting off on a really hot day. Why? Because the first thing it does is cool the pack down using the HVAC leaving more HVAC capacity available to the driver when you set off. Otherwise you're always sharing HVAC between you and the pack.
     
  15. doug

    doug Administrator / Head Moderator

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    I was being slightly tongue-in-cheek with my comment. 25kW is a lot of power, but that's not continuous unless you're coasting down a long hill. Average power I'm sure is much less.
     
  16. doug

    doug Administrator / Head Moderator

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    Of course this is something that it would be nice to be able to control remotely, as well as cabin temp.
     
  17. mpt

    mpt Electrics are back

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    +1. We'll soon be the only EV on the block without remote programmable heat and our own iPhone app.
     
  18. cinergi

    cinergi Active Member

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    interesting. Not at all the behavior I have. Is all off or all on. I didn't check to see if my amperage is 0 vs 8 while coasting with the heat on. I'll try to do that next chance I get.

    Btw I'd assume the motor won't create voltage for regen when it's off since the pem wouldn't be inducing a magnetic wave in the rotor
     
  19. cinergi

    cinergi Active Member

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    Yup - Appears to be 2kw as it draws 8 amps at 240. When I arrived home 3 miles after regen turned on and plugged in, it went into a preheat cycle first.
     

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