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Why doesn't preheating while plugged in completely warm the battery?

Discussion in 'Model X: Battery & Charging' started by carteriii, Jan 30, 2017.

  1. carteriii

    carteriii Member

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    I believe I've read every post on cold weather driving, including and especially the great one by @Doug_G : Cold Weather Driving

    My question is based on my personal experience and this statement made by @Doug_G : "The second thing you'll notice is that you have regenerative braking at about half power instead of zero. That's because your battery pack is a lot warmer."

    Note that Doug_G mentions "half power". I agree with Doug_G as I've personally never experienced full/100% regenerative breaking or completely "normal" energy use even after preheating.

    Why can't or why doesn't the preheating completely warm the battery sufficient to provide full regenerative braking and more importantly to allow reduced energy usage immediately when driving? I notice that even if/when I preheat for 20+ minutes (while plugged in & NOT using range mode), my Model X still limits regenerative breaking and uses abnormally high energy, presumably until the battery pack heats up even more. Only after driving for a while do the yellow marks on the energy usage chart (indicating the reduced regenerative breaking) then go away and I notice a significant drop in the energy usage for the same given speed & conditions that I may have been driving just a few minutes earlier.

    My curiosity is based on the assumption that the battery pack does not need seem to need ANY additional heating when the ambient temperature is ~70 degrees F outside. When it's only 30-40F outside, it seems reasonable that a plugged-in vehicle could on its own get the battery up to at least 70F. Afterall, the battery gets to that level once we're driving for a bit, so why can't preheating while plugged in do the same?

    I realize none of us may know the real answer, so some speculation is welcome as long as it's (please) not crazy talk. For example, I can guess that the battery pack "heater" can only raise the temperature to a max temperature that is less than 70F (independent of ambient air temp), and the rest MUST come from the heat of the motors, etc. That's purely speculation on my part that the battery pack heater just can't do enough on its own. Does anyone know better than I?
     
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  2. jlund

    jlund Member

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    I really hope that the upcoming ios app overhaul adds support to 'battery preheating' within climate controls.

    I've heard the trick is to charge your car to say 75%-80% overnight and then when you start preheating the cabin, you also start charging your battery so that it charges to 80-85%. Charging is a great way to condition and warm up the cells so regen braking can be used right away.
     
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  3. AB4EJ

    AB4EJ Member

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    Relevant to this post, can someone explain the "battery pack heater" ? Is there actually an electric heater that warms the battery coolant when ambient temperature is low? I had been thinking (rightly or wrongly) that any heating of the pack is solely provided by using waste heat from the motor... I would like to understand how it is really working...
     
  4. Andyw2100

    Andyw2100 Supporting Member

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    I think the simple answer is that the battery heater is not powerful enough. Somewhere on TMC people have posted the specs of the battery heater. I just don't know them off the top of my head.

    I have a couple of threads here involving letters to Tesla requesting battery pre-heating not tied to cabin pre-heating. We're still hoping!



    Yes, there is an actual heater. Please see limited details above.
     
  5. tbleakne

    tbleakne Member

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    There was a diagram posted several years ago. The cooling/heating system is quite sophisticated with lots of valves. There is a coolant loop that circulates through battery, inverter, motor, cabin, external radiator, heater, AC. The system will use waste heat from the motor and inverter, but supplement that with electrical heat if necessary. Valves control which of above are actually in loop.

    I believe charging to 75% or so overnight, at slow rate to keep battery warm all night from charging itself, then complete charge in morning with cabin heat on is effective.

    It would be nice if Tesla used a heat pump rather than resistance heating, but I guess the only time you really need the heat to protect the battery is when it is too cold out for a heat pump to be effective. When you are driving in mild cold weather, the heat from the motor is enough to avoid wasting power from the battery.

    It would also be nice if Tesla displayed how much energy was being consumed by the heat/AC. The LEAF would do that.
     
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  6. FarmerDave

    FarmerDave Member

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    I have all the same questions. It would be great if J.B. Straubel would make a video or create a technical document that explained all this for the technically inclined.
     
  7. K-MTG

    K-MTG Sunshade Captain of TMC

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    Controlling range mode with the app would be great, but also I am curious on why the on board 12V electronics can't run off the DC to DC converter directly while the car is plugged in? It would extend the life of the 12V battery and it would mitigate vampire drain.
     
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  8. RDoc

    RDoc S85D

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    My understanding is that the same software team responsible for adding battery pack pre-heating is responsible for the Media System UI. Once they get that straightened out, they'll jump right onto it.
     
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  9. Saghost

    Saghost Active Member

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    Hmm. 1300 pound battery pack, 40F temperature change. (~580 kg, 22C) .

    I think the majority of the pack structure is graphite and nickel? If you assume a specific heat that's halfway between the .720 J/gC of carbon and the .440 of Nickel - .58 J/gC or so - then a 580 kg pack heated 22C requires about 7400 kJ of energy.

    I believe the Tesla battery coolant heater is capable of 6kW maximum - which implies that if the pack is capable of safely absorbing all that energy at that rate (sizing of coolant channels, safe thermal gradients in the cells,) the heater should take around 1230 seconds to bring the pack to temperature if there are no thermal losses to the environment - a little over twenty minutes.

    Which as you say does not square with my experience with preheating - do we know if the car uses the heater at full power while preheating?

    Anyone want to take a stab ay heat flow/thermal losses in the pack? Mostly the flat aluminum bottom surface and convection I'd think, presumably some conduction into the car's frame.

    It seems like it takes around half an hour in freezing weather for the power limiting to stop and efficiency to rise while driving...
     
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  10. andrewket

    andrewket Well-Known Member

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    For starters, just because the car is plugged in doesn't mean it's charging or drawing current. People with TOU plans would be unhappy if the car was always drawing current.
     
  11. scaesare

    scaesare Active Member

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    I agree is simply the sheer thermal mass of the pack along with an extremely large conductive surface area (flat and thin) that is the issue... my rough calculations suggest the pack has ~45 ft^2 of exposed bottom surface area.

    With around-freezing temps the 6KW heaters simply have an uphill battle to raise the pack temp very quickly. Wind shill effect really affects this in my experience.

    I've thought that an insulator on the bottom of the pack would be an interesting experiment. The pack has C-channel-like rails that run it's length. They stand ~3/8" tall off the pack. I wonder what rigid-foam insulating panels of that thickness adhered in between each set of rails would do with cold temps. To help protect it, a strong thin poly sheet could cover the entire bottom, perhaps with anchors along it's length that take advantage of the C-rails by sliding in to them.
     
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  12. K-MTG

    K-MTG Sunshade Captain of TMC

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    But I would assume the charge would me super minimal?
     
  13. Solarman004

    Solarman004 Member

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    Agree!! I use the Energy Screen on my Leaf quite often, especially in cold temperatures.
     
  14. andrewket

    andrewket Well-Known Member

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    I might have this wrong, but my recollection is the battery heater can consume up to 6kW. My cost per kWh outside of the "ultra low" times is 4-5x the normal rate. As it is, if I remotely turn on cabin heating/cooling it does use shore power, which is what most people would want, but not me. For this reason I rarely use the feature when my car is plugged in at home. I use it all the time when away from home.
     
  15. Saghost

    Saghost Active Member

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    Unless I'm confused, the context of this sub thread was vampire drain and carrying the 12V aside from the charger directly, not anything to do with heating.

    Folks on time of use rates still have the big difference you mentioned, but you're talking about a kWh per day or less.

    Obviously the rate difference will eat up any cost savings from improved efficiency in your case, and the benefit of less battery wear maybe be offset by the risk of being connected to the grid and potentially subject to surges for much longer periods.
     
  16. K-MTG

    K-MTG Sunshade Captain of TMC

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    The battery isn't heated with the 12V, that can be an option to heat the battery. I just merely want the 12V electronics powered from the grid. Perhaps 1 kilowatt per day at most
     

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