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Tesla, we need auto adjusting charge amplitude

Discussion in 'Model S: Battery & Charging' started by kishdude, Apr 10, 2013.

  1. kishdude

    kishdude P #130

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    With version 4.3 we got a needed feature with charging in that we can now set when we want charging to start. What we need now is a feature that allows us to set when we want charging to end. For example, if I set my charging to end at 7AM (the time when I leave for work) the software would automatically figure out what amperage to charge at (many times a lower amperage than the 40 amp max) so that the charge will compete exactly at 7AM. This would have multiple advantages:

    1) This will allow for charging at the lowest amperage that will still provide a full charge thereby increasing battery life. Currently I purposefully set my charge amplitude to 15 amps.
    2) No more loss from vampire load after charging. As it is now, if your charge completes at midnight, then from midnight to 7AM you are slowly losing range.
    3) No need for battery to warm up in colder climates. At present, if your charge completes at midnight, then the battery is no longer being warmed from midnight to 7AM and you have a cold battery when you leave for work.
    4) 12V battery will not be draining. As we have all seen the 12V is a critical component of the car. While the main battery is charging, the 12V is charging as well. After a charge completes, the 12V slowly drains.
     
  2. strider

    strider Active Member

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    I would prefer auto adjusting start time. So you set the end time and it will wait to start charging until necessary. Once it starts it will charge at max rate. This will have two benefits. 1) you wouldn't have to set both a start and end time (otherwise it might start a 13A charge at 6pm when you come home from work but you have a TOU rate that starts at midnight). 2) if you have a very long time before your car needs to be ready, it would choose a long charge at a low rate which is inefficient as there is a certain amount of overhead to run the charging and temp systems and so at very low rates, a higher percentage of charge is lost as overhead.
     
  3. wraithnot

    wraithnot Model S VIN #5785

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    To add my 2 cents to this, I'd prefer to be able to schedule the start time, the stop time, and the current (4.3 already gives us two out of the three by controlling the start time and the current). From what I've learned about lithium ion batteries on this site, it sounds like shallow cycles, slow charge and discharge, and keeping the battery around a 20% SOC for the majority of the time is the best way to keep it healthy.

    With the added ability to stop the charging at a scheduled time, I could avoid charging the battery more way than needed for my daily commute, have the charging end right when I need the car, and/or stay within the cheap off-peak time-of-use period.

    I'm sure the Tesla engineers already did a fantastic job of protecting the battery while making the car easy to use. But since I'm the only one who knows exactly how far I plan to drive the next day, with a little added control I bet I can do an even better job of keeping the battery in good shape.
     
  4. NielsChr

    NielsChr Member

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    I guess, we all have diffeent view to what is needed.
    end time is deffently a goog Thing, i would also like a custom max SOC for standard mode, right now this is fixed at 80%(I belive) Most of the time 60% would be plenty for me, this could help conserve batteri longtime health.
    Though it is more impotant that Tesla fixes the vampire load/sleep mode.
     
  5. Cattledog

    Cattledog Active Member

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    wraith - I'm not a battery engineer, but Tesla (who I assume has a few hanging around) recommends keeping the car charged at its 90% level.
     
  6. wraithnot

    wraithnot Model S VIN #5785

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    I’m treating this whole Model S ownership experience as a bit of a hobby in that I’m trying to learn as much as I can about the car and all its new technology (or at least technology that is new to me). I found this article to be pretty informative: http://www.nrel.gov/docs/fy12osti/53817.pdf

    The bottom panel of figure 1 shows predictions for the capacity loss of graphite/NCA battery cells and it looks like storage at both high temperature and high states of charge (SOC) accelerates the capacity loss. If I understand things correctly, a range charge stops at about 90% SOC and a standard charge stops at about 80% SOC. From this graph, I can understand why Tesla doesn’t let you charge the battery all the way to 100%, why they suggest range charging sparingly, and why they designed such a complex system to regulate battery temperature.

    But it also shows there are additional smaller gains to be made by keeping the SOC lower than 80% when you don’t think you’ll need it. Of course, an unplanned spontaneous trip can then get you in trouble. So I’ve adopted the “just in time” charging schedule in table V by scheduling the charge to start late at night and dialing the amperage back so that it finishes just before I normally drive the car in the morning. This way I always start each drive with a full standard charge, but the battery spends most of its time at a lower SOC. Is this likely to make a massive difference? Probably not. But it should help at least a bit. And I learned something in the process.
     
  7. smorgasbord

    smorgasbord Active Member

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    Roadster owners have discussed this in more depth a while ago - go search the forum. But here's a few thoughts:

    One reason to charge right away after driving is the charging actively drives battery temperature down. If you drive hard, then park your car for many hours or even days before charging, the batteries could be sitting at 100deg F for quite a while. That's probably worse then them sitting at 80%, but under 70 deg F.

    You don't want to not charge the car. If you drive hard, it's better to do so with the batteries more full than more empty. Roadster had a Storage Mode to keep SOC around 50%, but Tesla abandoned that for Model S, so they must have felt it didn't do much.

    Here's what I think is needed:
    1) A "spend money to preserve my batteries mode." When the car is plugged in but charging is on a timer to start later, this mode will invoke active battery cooling to bring down the battery temps below 80deg F.
    2) A TOU aware charging protocol. Most utilities have pretty simple TOU periods. Enter them into the car.
    3) A "car fully charged by" time program. Tell the car when you'll be using it next. It then looks at the time between now and when you'll be driving to find the cheapest TOU periods and perform charging during those. If those won't fully charge the car, then it does some charging in the next lowest TOU period - and so on.
    4) For Roadster, the sweet spot of charging efficiency was about 40 amps, nicely fitting with UMC capacity. Assuming that's still true for Model S, charge rates by default would be set to 40 amp, then modified based on #3 (time a full charge is needed).
     
  8. stopcrazypp

    stopcrazypp Well-Known Member

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    There's diminishing returns for low charging rates, so I'm not sure it's worthwhile to charge slowly. Slower charging also is less efficient since the charging overhead becomes a larger component of your charging. This may mean you use more electricity for the same amount of charge.

    Even with a HPWC, it takes about 5 hours to charge fully. This charging rate is 1/5hours = 0.2C. NCA laptop cells are charged at 0.3C (3.3 hours to fully charge). There really isn't much to worry about with Level 2 charging.
     
  9. smorgasbord

    smorgasbord Active Member

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    As I said, there is a lot of info in the Roadster section of the forum, including testing which showed that 40amps was about the most efficient, at least for Roadster. It seems unlikely that Model S would be a lot different.
     

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