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Must Read: The best battery management when cold.

Discussion in 'Model S: Battery & Charging' started by ElectricAvenue, Dec 15, 2013.

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  1. ElectricAvenue

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    Now that winter is here, I am trying to figure out the best battery management during the cold winter months.
    My aim is to reduce the wear and tear on the battery when charing and discharging and therefore resulting in longer life all while minimizing energy use.
    I am not an expert in battery technology but understand the concepts very well, so please correct me where needed.

    Assumptions:
    It is not good for the battery to be charged or drained when temps are cold.
    The colder the battery the worse the wear when charging/draining.

    Observations:
    When you start charging the battery when cold, you can see how many miles/hr the car is charing which is very close to the number of miles/hr when warm, therfore the battery does not have a great amount of heating while charing (remember amperage is always constant).
    Goal:
    Always try to charge the battery when warm and drive when the battery is warm.

    Recommended Charging Process:
    Here is my recommended process on how to charge in cold temps for someone who has a regular 9-5 Monday to Friday work week:
    When you arrive home at the end of the work day the battery is probably warm, so this would be the best time to start charging. Before you leave for work in the morning you will also want the battery to be warm so turn on the climate control to 65 degrees prior to driving. This is the minimum temp you can set to heat the cabin but the battery will also be warmed. The electricity for this heat will not be drawn from the battery but will be drawn from shore power.
    Using this process will charge the battery when warm and heat it without drawing power from the batteries when cold.

    Non-Recommended Charging Processes:
    1) Schedule the charge so it comes on early morning and finishes when you are about to drive, leaving the battery warm for driving.
    The problem here is when the scheduled charging starts the battery will be cold which is not good.
    2) Reduce the amperage and charge all evening and all night so the battery is warm both at the start of charge and start of your drive.
    The problem here is now you are wasting energy keeping the battery warm all night.

    Questions:
    To understand a little more about the battery technology I would like help in answering the following questions and maybe change my charging recommendation:
    1) What is the optimal battery temperature for minimal wear when charging.
    2) What is the optimal battery temperature for minimal wear when driving.
    3) I assume when driving, the use of the battery creates warmth because of inefficiencies. Below what outside temperature will the battery need extra heat to keep it warm?
    4) Why does the Miles/Hr charging rate not go down when cold, I would assume some of the energy would need to be for heating the battery and would reflect in a lower Miles/Hr rate?
    5) There are 2 ways to heat the battery; heat the cabin or start charging. I assume that heating the cabin will heat the battery faster. Is this really true? (Even if it isn't, it does not affect my charging process above).
     
  2. ToddRLockwood

    ToddRLockwood Active Member

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    Without going into a great deal of detail, most of your concerns are taken care of by the car's firmware. Limits on regen and consumption are automatically applied when the battery is cold. When you plug in your Model S, the car determines the best way to manage the charge cycle. If the battery is very cold or very warm, the battery coolant loop will be activated prior to charging. All of this is done for you. There's no reason to hesitate to plug in when the battery is cold.

    The main reason to pre-warm your battery before heading out is a practical one: the battery heater draws about 10kW. If the preheating is done while still plugged in, that energy will come from the outlet, not from the battery. It's mostly an issue when you're about to drive a long distance in the winter. For a normal commute, you shouldn't be too concerned about leaving your house with a cold battery.
     
  3. jerry33

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

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    My thought is that if you are not Supercharging, there's so little stress on the battery when cold that using the scheduled time does no measurable harm to the battery. This would explain why the rate does not go down--it's just not necessary. I suppose that if the battery is very cold (below -25) then there could be some degradation if using the timer.
     
  4. ElectricAvenue

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    Todd,
    I understand the firmware imposes limits, but isn't this just to limit extream wear on the battery. If cold is not good for the battery, then even with limits it is still not good (just at a lower extent?). So would pre-heating always be the best bet?
    Also, when the battery coolant loop is activated, it must take some time to bring it to optimal temps and by that time you have already driven a number of miles and caused some 'cold' wear?

    - - - Updated - - -

    Jerry,
    According to this site, the charging temp range for Lion is 0F to 113F, but the note below the chart recommends 50F - 86F
    http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures

    0 degrees F is not too uncommon in the north east, even charging close to it would make me think some wear is occuring.
     
  5. ToddRLockwood

    ToddRLockwood Active Member

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    As Jerry pointed out, Supercharging a very cold battery might cause some stress, I don't know. If the car's firmware is doing its job, I doubt that it is a problem. Otherwise, I wouldn't be too concerned about preheating, except when you're going on a trip. I don't believe that driving while the battery is warming up causes any undue "cold wear." Charging your battery to 100% every day would be more of a concern.
     
  6. stevezzzz

    stevezzzz R;SigS;P85D;SigX

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    To the OP: you're over-thinking something that should be the essence of simplicity, thanks to Tesla's sophisticated battery management protocols. If you like that sort of thing, there's a chance you could make a difference to your battery pack's health over the life of the car; but for most people it comes down to a few simple rules of thumb.

    Meat and potatoes:
    1. Plug in when you can.
    2. Don't do a Trip charge when you don't need the range.
    3. Don't be afraid to do a Trip charge when you do need the range.
    4. Don't leave the car at a very high SOC or a very low SOC any longer than necessary.

    The rest is gravy.
     
  7. stopcrazypp

    stopcrazypp Well-Known Member

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    You actually got the numbers wrong, it's 32F to 113F (0C to 45C). It also notes that it should never be charged below freezing (0C) and that below 5C, charging rates should be lowered. The BMS itself will take into account of those limits, so I don't see the need to worry about this.
     
  8. Enadler

    Enadler Member

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    +1 stevezzz the only thing i would add is to prewarm the pack prior to driving in cold weather either by topping off the charge or preheating the cabin.
     
  9. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    +2 to Stevezzz.

    In terms of preheating before a range challenged trip, I did some experiments today as my MS sat in its 50˚F. garage. The following graph is from my TED 5000 that monitors the HPWC feeding the MS only.

    Preheating.png

    The blue line is power with the scale on the left, and the red line is one phase Voltage with the scale on the right. This is power used by the MS after a normal 90% charge the day before.

    At about 1am the car took a 20 kW drink to feed the vampire. At a little before 10 am, I turn on climate control with the temp set to 65˚F., and you see the small draw. I turned that cycle off and then started my usual, before a long trip, pre-warm at 79˚F. The MS automatically ends any climate control remote cycle after 30 minutes, and sometimes a little sooner if its pretty warm. I was out breaking snow-shoe trails and checking on a few remote parts of my property, so as I took the occasional break, I turned the climate control back on at the 79˚ set point. If you look at these data, it looks like it takes multiple half hour cycles to get the battery warm; also, note that it seems the waste heat from the battery is heating the cabin. By the later cycles, the interior was showing temps of 90˚F, with the set point still at 79˚. This tells me that if I want to really pre-warm the MS, I should plan on at least 2-3 half hour cycles at a reasonably warm temperature before that long trip. :wink:
     
  10. Mario Kadastik

    Mario Kadastik Active Member

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    As some have mentioned forget about it. The car knows best and does it all for you. Besides charging is the only true way to warm the battery in cold weather and that is because the car will first warm the battery and then start charging. When I didn't do scheduled charging at start and turned on charging myself with the car fully soaked below -1C the car would draw full 3x12A that I charge with, but showed 0km/h for the first 5+ minutes. It then starts to creep up and goes to the normal 47km/h when the battery is warm in ca 30 or so minutes. So I don't know where you took the statement that the car starts to charge at full speed when cold. It definitely first starts the warming process. Then again, maybe that's because I've only ever driven with firmware 5.8.
     
  11. ElectricAvenue

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    Crazy, thanks for the correction.
    What does BMS mean?

    - - - Updated - - -

    Cotton, Thanks for the data, this is great information.
    The only question I have is how you could tell from the data when the battery became warm?
     
  12. Enadler

    Enadler Member

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    BMS is Battery Management System
     
  13. ElectricAvenue

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    Mario,
    I have never seen the charge start with 0 Miles/hr and the coldest I have tried so far has been about 20F. So thanks for this info because it does show that 20F or above does not activate the auto preheat.
    You said that "Besides charging is the only true way to warm the battery in cold weather" what about turning on the cabin heat, I was fairly sure this also heated the battery?
     
  14. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    It is informed supposition combined with looking at the output of many control systems in my career. If you look at the combination of the 5 half-hour cycles, you start to see what looks like an exponential approach to a final value. What would heat the cabin to 90˚, but a warm battery? If the heater control circuit is using a simple proportional control loop to heat the large mass of the battery, then that will give you an exponential approach to the final value. This looks like an exponential time constant of an hour or so. If that is the case, the temp will get to within 37% (1/e) of the final value in an hour and within 13% (1/e^2) in two hours.

    In my 50˚ garage, it looks like it is approaching a steady value of about 1.25 kW. That is probably what it takes to keep the battery warm in my garage.

    Also note that when the cabin temp was set to 65˚, the power draw was a lot less. If there is battery heating happening (I believe that it is), then it looks like the battery is warmed more with higher cabin set points. Also note that the cabin temp is allowed to go higher than the cabin set point. If I turn on climate control in the summer, then the MS turns on the AC to cool the cabin. In this case in my 50˚ garage, the car appears to make no attempt to cool a 90˚ cabin with the set point at 79˚.
     
  15. Mario Kadastik

    Mario Kadastik Active Member

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    Because the heating seems to use the battery as well it may put some heat into it, but I tried it a few times at start by charging the car the moment I arrived home and put the car to heating in the morning, but it was always strongly limited when I drove away and battery was heating the whole trip. Now when I schedule it so that it either finishes right when I leave or I stop the last few % and drive away the car is unlimited (no regen limit etc) so the battery is warm. It's also somewhat logical as I doubt the cabin heating takes more than a few kW of power while charging pushes into the battery ~10kW.

    Oh and was your car really at 20F and it started charging at full speed or was that the outside temp, but maybe the car was still above that enough to be "warm"? Because 20F is -7C and even at -3C with interior temp at 0C the car started off with 0km/h charge rate, but full power draw. I guess if you feed the car more than 36A (3x12A that I use), then it might be able to heat the battery faster and crank up the charge speed faster, but still.... Also, what version are you on, maybe the pre-heating is something from recent fw's.
     
  16. 100thMonkey

    100thMonkey Member

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    I've been noodling over this issue for a couple winters now with the Leaf and now with the S as well. Cold is your friend in terms of long term life of the battery but hard on the short term capacity (reduces floating voltage/slows ionic flow across the cells). I leave the car plugged in whenever in the garage with the slider typically set to 50%. based on what I've read and my experience with quite a few other batteries, the combination of a 50% charge and cold should help extend the long term life of the battery. If I need to maximize range I time the topping up charge to end just prior to leaving, to warm the battery internally. I let the battery management do the rest. your battery will age and loose some range gradually no matter what you do, don't worry about it too much.
     
  17. BetterThanEdison

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    I would like to add one tip from experience:

    * you should always preheat the car if you plan on driving more than 5 miles in the cold (~30F).

    If you don't preheat, the car will automatically heat up the battery while driving anyway, and while the battery is cold, the car is less efficient (regen is limited). So you may as well use a couple kWh to stay warm and be efficient the whole time instead of being cold and inefficient. You'll end up using pretty close to the same kWh in the end anyway.
     
  18. montgom626

    montgom626 Active Member

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    Your comments are spot on! Just plug it in.
     
  19. scaesare

    scaesare Active Member

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    To the OP:

    While I commend your starting a dialog on a given subject, I think to title your thread that you authored, "Must Read: The best...", especially when it contained a large amount of speculation and some questionable permises is a bit misleading.

    If this were directly form an authoritative source (such as Tesla Engineering, etc...), or were backed with logs of actual experimental data from which you could extract facts and conclusions, then that might be one thing... but not knowing what a battery management system is or that it takes care of a number of your issues doesn't make this a "must read"...

    In the future, using a title such as "Thoughts on battery management..." might be a bit more descriptive.

    My $0.02 of course.
     
  20. ElectricAvenue

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    Scaesare,
    Sorry you are right. It wasn't meant to me misleading. I just wanted to get as much input from others to made sure I was making good judgment. Hence the questions at the end.
     

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