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Understanding the battery heater

Discussion in 'Model S: Battery & Charging' started by David99, Jan 4, 2016.

  1. Kalud

    Kalud Active Member

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    P85D / P90D have "Set max battery power" option to heat the battery (more power available) but also useful to get rid of regen limit.

    For other cars, you can toggle the use of the battery heater when regen is limited by using Range Mode OFF. On Range Mode ON it doesn't use the heater and takes a serious long drive to gain regen from self battery heating from discharge. Having a heavy foot here helps get it done faster. On range mode OFF, the battery heaters starts once you drive (or of you pre-heat the cabin), and uses 5kW max.
     
  2. David99

    David99 Active Member

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    Yes you see a higher energy consumption when starting with a cold battery vs starting with a warmer battery. What you see is a high consumption, but this doesn't mean the battery is less efficient. The reason the car uses more energy is because its heating the battery as you are driving. When you start with a warm battery the car doesn't need to do that, or at least much less. There is an extra power draw that contributes to the overall consumption. Unlike Nissan, Tesla doesn't show you what part of the car uses how much. The battery itself is not less efficient when it's cold. Well maybe a little bit, but it's not anywhere near the 10% you are seeing in your tests.

    Of course it comes down to the same thing. When you start with a cold battery you have less total range because the car uses some energy to heat the battery up. It's not an efficiency loss, though. My initial point was not to say, battery heating isn't required. It certainly becomes important once the temperatures drop to a level where it limits both power draw and regen. That's exactly why Tesla heats the battery. But once the battery has reached the point where the cold temperature limits are within normal driving needs, there is no point to run the battery pack heater more. The extra energy needed to bring up the temperature more would not result in more range, thus it's a waste of energy. If the battery is seriously cold to a point where it actually can only deliver a significantly smaller amount of energy (the blue section on the battery gauge shows up), then heating it up has a positive effect. You gain access to more energy compared to what you need to put in to heat it. But once you heat beyond that point, the net energy turns negative.

    Again, once the battery has good enough temperature, the normal losses in the drive train seem to be enough to keep the battery at temperature. Since those losses happen anyways, this energy is basically free. That's why you can drive with a pretty reasonable energy consumption in cold weather.
     
  3. GlmnAlyAirCar

    GlmnAlyAirCar Active Member

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    That's fine if you're just moving heat. Heat conducts from high-energy (hot) to low energy (cold). Using a mechanical device, such as a heat pump, can accelerate or slow that process. That's great if the exterior of the vehicle is hotter than the interior. But if the exterior is colder, you are reversing the natural direction of conduction and that requires energy. Some of the energy goes into moving heat (increasing the energy differential between the interior and exterior). The rest is wasted as heat. If that heat is wasted on the hot side, that helps. But it's still less than 100%.

    I don't believe the fundamental laws of thermodynamics have changed just because HVAC industry decided it wanted to sell its air conditioners all year round.
     
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  4. AWDtsla

    AWDtsla Active Member

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    As said above, the purpose of a heat pump is to achieve efficiencies greater than 100. I didn't suggest the MS had one, maybe pay attention to what I quoted.
     
  5. Kalud

    Kalud Active Member

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    Did you ever study the refrigeration cycle ? You should...
     
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  6. AWDtsla

    AWDtsla Active Member

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    You're misunderstanding a whole bunch of stuff. Maybe start with why absolute zero is called absolute zero and that it's -460F. Heat pumps do not violate the laws of thermodynamics.
     
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  7. ohmman

    ohmman Maximum Plaid Member

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    A small caveat is that the exterior of the vehicle can be colder if the medium has a higher specific heat capacity. In our case, they'd both be air, so you're absolutely correct. But geothermal heat pumps take advantage of the higher specific heat and thermal mass of the earth to obtain higher efficiencies (COPs of 3 to 6).
     
  8. Lloyd

    Lloyd Well-Known Member

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    They have the ability to do this now as the Rav 4 EV with the Tesla drive train does this already. I have mine programmed to finish at 6:00 am every morning. Incorporating this into the existing software might be more difficult with the S/X, however. I would prefer to charge this way daily, and it would be better for the utilities. The way we currently charge everyone starts charging at the moment that the rates decrease. If enough Teslas start charging at the same time it could create an issue with the demand. By having the charge programmed to the finish time, everyones start time would be different, thus relieving some of the strain on the utilities.
     
  9. AWDtsla

    AWDtsla Active Member

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    Can't be done. Kirchoff's law and all.

    There is approximately 11-12kW of resistive draw available when cold. Resistive heating elements are both light and cheap, it would be trivial to double that draw.

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    No he is not right. It doesn't matter the source is colder than the output. They make air-source heat pumps that have guaranteed heating down to -13F now. I know, I heat my house with one.
     
  10. ohmman

    ohmman Maximum Plaid Member

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    Fair enough. I suppose I was assuming a small closed system with equal volumes of air. Since the outside air has a nearly unlimited source of replenishment, that makes sense. I used to heat my house with a swimming pool sourced heat pump. We could draw a ton of heat out of a 45F degree pool, because it was constantly in contact with the ground surrounding it. That said, the drop across the coil was about 10 degrees, so once we hit 42F there were freezing concerns.
     
  11. AWDtsla

    AWDtsla Active Member

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    Yeah, defrost cycle is a downside with anything but geo source. But even so you can get BTU/$ lower than natural gas. I would think Tesla would be looking at improved cold weather performance in the Model 3, but given what I've seen so far I'm not so sure.
     
  12. Andyw2100

    Andyw2100 Well-Known Member

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    We're driving with range mode on, so I believe the only heating of the pack should be via the drive-train, right? There should be no energy directed towards pack heating with range mode on, which is why it takes forever for regen limits to go away when driving with range mode on.
     
  13. GlmnAlyAirCar

    GlmnAlyAirCar Active Member

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    Agreed. But we're not talking geothermal heat here.

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    Did you? You appear to have some fundamental misunderstandings.

    Please examine this link: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html, especially the section on refrigerators and heat pumps.

    It's dumbed down sufficiently.
     
  14. David99

    David99 Active Member

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    That is what people experience. With Range Mode ON the pack heater doesn't turn on. Or maybe it only turns on at a lower temperature.
     
  15. Andyw2100

    Andyw2100 Well-Known Member

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    Right. So then what do you think explains the fairly significant efficiency differences across the three days that you initially attributed to the pack being heated during the drive? (In fairness, that was before I noted that we drove with range mode on, and suggested that couldn't be it.)

    Thanks.
     
  16. RDoc

    RDoc S85D

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    Before people get too upset about heat pump efficiency it would be good to think about what >100% efficiency means. The comparison is between using electric power to heat the interior using resistance, vs using some scheme with compressors, etc. to cool the environment, thereby extracting energy from it, then dumping that energy into the interior.

    Think about a closed system containing the interior of the car, heated to 20C with atmospheric air and electric power as inputs, and atmospheric air as the output.

    Using resistance heating, 0C atmospheric air and electric power go in, warmer air, say 5C, exits after being heated by conduction from the interior of the car. 100% of the power goes to heating the exiting atmospheric air with the interior heating as an intermediate step.

    Using a heat pump, the same 0C atmospheric air and the same amount of electric power go in, however, much colder air, say -10C exits. What's going on is that the heat pump creates a local spot of say -20C which is used to extract energy from the 0C air, cooling it to -18C. That energy can then be put into the car's interior along with most of the waste heat from the heat pump. Of course the interior and some waste heat from the heat pump then heats the exiting air up to -10C, but from the outside of the box, you see 0C air going in and -10C air coming out while using the same amount of electricity.

    That's how the efficiency can be said to be over 100% compared to straight resistance heating, the rest of the energy came from the cooled air exiting the box.
     
  17. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Warm air and fluids rise, heat radiates in all directions. That said, heating the air under the car sounds very inefficient compared to heating the car directly with shore power.

    - - - Updated - - -

    That would depend on the temperature of the battery. The battery won't be allowed to go below a critical threshold, in which case it will use the battery heater. I park outside and even using range mode it appears as if the battery heater is used sometimes (based on energy used).
     
  18. AWDtsla

    AWDtsla Active Member

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    Right. This stuff is easily available on various physics websites along with Wikipedia. But if someone can reference those very articles, interpret them incorrectly and insist they are correct no amount of arguing is going to convince that person, so I bowed out.
     
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  19. David99

    David99 Active Member

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    There is a simple way to figure this out. Turn Range Mode on. Get in your car when it's really cold and the battery has not been charged and pre-conditioned in any way. Basically when you know the battery must be very cold. Turn on the heat to what you want. Without driving, see if the power meter shows a visible power consumption. If yes, something is using power that will contribute to the overall energy consumption. My guess is when the car is overall really cold, the resistive heater will kick in to get the cabin warm since the hear pump has no way of working right away. Once the motor and inverter warms up, the heat pump can work and make heating more efficient and then the overall consumption goes down.

    Maybe the pack heater still kicks in even in range mode when the battery is too cold. I would actually assume that the pack heater will turn on no matter what once the battery reaches a critically low temperature and you try to drive.
     
  20. GlmnAlyAirCar

    GlmnAlyAirCar Active Member

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    Ok, I'm with you here. I suspect in those earlier exchanges people (including myself) were talking past each other. >100% efficiency claims invoke an emotional response. Thank you for laying this out in objective and clear terms.
     
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