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Why is it necessary for Supercharging to 'taper off'.

Discussion in 'Model S: Battery & Charging' started by wormhole, Jul 20, 2013.

  1. wormhole

    wormhole Banned

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    just curious. Why can't the SC's pump juice at max input until its 95% and then taper off, versus the slow taper that gets 50% in 20 min, 80% in 40 min and 100% in 75 min?
     
  2. islandbayy

    islandbayy Active Member

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    The simple answer, as the batteries get close to full capacity, they "absorb" the power more slowly. If one continues to supply full current to the batteries faster then they can "absorb" it, battery damage can occur. I do not have time for more detailed answer, sorry.
     
  3. NuclearPowered

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    The battery requires current to taper off in order to maintain the maximum allowed charge voltage.

    Less current = less power = more time to deliver energy
     

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  4. metafor

    metafor Member

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    The voltage is held constant. The intrinsic resistance of the battery grows as it becomes charged. I suppose the SC could increase voltage as the battery becomes full but that may break down the lithium cells faster than desired.

    Current moves because there's a difference in charge between two points; it moves from the place with more charge to that with less. As the battery fills, it gains more charge and thus, has less of a potential difference between it and the voltage source. This inherently slows down the movement of charge as there's less electo-motive force.
     
  5. thehunt

    thehunt Member

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    From the Tesla SuperCharger page under the "How it works" section:
    "Optimal Charging
    The fastest way to replenish your Model S is to charge to 80% state of charge, which is more than enough for travel between Supercharger stations. Charging the final 20% takes approximately the same amount of time as the first 80% due to a necessary decrease in charging current to help top-off cells. It's somewhat like turning down a faucet in order to fill a glass of water to the top without spilling."
     
  6. aaron.s

    aaron.s Member

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    Every time I've supercharged, the voltage has increased gradually to its maximum of 400 volts and amperage has decreased (from 200 amps to below 20 amps near max range) as the charge state of the batteries reaches maximum....

    Aaron
     
  7. brianman

    brianman Burrito Founder

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    When you do a supercharger rally (officially or unofficially... ;) ), you definitely are more time efficient leaving before 100%. That assumes you have enough to make it to the next supercharger, of course.
     
  8. ka9q

    ka9q Member

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    Tapering charge powers

    That's actually an understatement. Exceeding the maximum allowable terminal voltage on a lithium-ion cell is a very good way to set it on fire.

    I don't know if that's true for the newer cathode chemistries now used in most EVs, but it's certainly true for the early LiCoO2 cathodes. (Most publicized Li-ion battery fires have involved LiCoO2 cathodes.) It's so critical that multi-cell li-ion battery packs contain special circuits to monitor the individual cell voltages and disconnect the pack if any of them reaches the safe limit (typically 4.2 V). The circuit is inside the pack itself because the charger sees only the total series string voltage and has no way to detect a weak cell whose voltage is higher than the rest. The same circuits usually also disconnect the pack should any of the battery cells get too discharged.

    The problem gets more acute as more cells are placed in series, and electric vehicles have the longest series strings of any battery application I know.

    - - - Updated - - -

    There's no requirement that the voltage start at maximum. It will do so only if the charger can supply as much power as the battery can then absorb. If a large battery is nearly depleted, and/or the charger is too small, it will begin by limiting the current to some maximum. This is the "bulk charge" phase. The battery's voltage will rise as it fills, and when it reaches the upper limit the charger will hold it there by decreasing the charging current. This is the "absorption" or "acceptance" phase.

    Charging can stop for a number of reasons: when the battery current drops below some specified limit, when the temperature exceeds some limit, or after expiration of a timer. Many chargers implement all three.

    I know of at least some public fast chargers (for the Leaf) that are programmed to limit their maximum power draws to minimize utility commercial customer demand charges based on the maximum average power draw during any 15 minute period in the month. Demand charges can be substantial for a lightly used fast charger; in fact, it can make them completely uneconomical. It's a real problem that hasn't been anticipated by the California PUC, to my knowledge.
     
  9. Chipper

    Chipper Active Member

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    Welcome ka9q! Great first post. You are going to fit right in around here!!
     
  10. andydoty

    andydoty Member

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    I've been reading the posts on this forum in anticipation of parking my very own Model S in my driveway. Test drive on Tuesday!

    Here is Mass, we have a "grant" process for installing solar which I took advantage of. My rebate was $10500. Since then, the rebate has dropped to around $4k. This is largely due to the decrease in panel costs. My panels were $600 each. You can by an equivalent panel today for around $200. Oh well... Such is life!

    I installed a "Grid Interactive" system that requires batteries. So, you become a quick study on how to keep them properly topped off. Lithium ION batteries are pretty cost prohibitive in a home situation so we generally use AGM (absorbed glass mat) batteries if kept inside the home. The grid interactive system acts like a really big UPS in that I had to split the house circuits into "critical load" and "main panel". Grid tied systems are required to "island" themselves when power from the street stops flowing. Mine only islands the grid side.

    So to my point, there are several states to a battery charge. You can't just jam electricity into the battery and expect it to charge. A good reference for this is http://www.batterystuff.com/blog/3-stages-of-smart-chargers.html

    It's a good resource for battery knowledge. And the topic is not light reading by any stretch of the imagination. The solar, like the Model S is certainly a profound investment so I like to educate myself on keeping them around for as long as possible.

    Solar uses slightly less power on the DC side. I'm nowhere near close to the 400 volt of the Tesla battery (48v). What I would be interested in knowing is how people use a full (100%) charge every now and again to condition the batteries. What's the downside?

    IMG_0682.JPG

    I was actually thinking on starting a "Living with solar and a Model S" thread to see how many people and the trials and tribulations of owning both.
    Drew
     
  11. brianman

    brianman Burrito Founder

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    @andydoty
    When I got my panels installed (in WA), I heard something about "some of the incentives don't apply if you don't go grid tied". At least initially, I didn't want to go that route anyway but long-term (when the incentives expire?) I may consider going that route. If you start a thread, I'd be interested in the topic of how to do that kind of conversion, what terminology the installers speak regarding it, and ballpark pricing on what the costs look like.
     
  12. andydoty

    andydoty Member

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    @brianman
    I think the whole incentive for grid tied/interactive had to do with using commercial power to charge the batteries and then selling it back. I really haven't had that problem. We've had a few natural disasters over the past 2 years and every time the system just ran like a top. I also have a heat pump and switch to electric heat at certain points in the winter. I think the Model S would put a little strain on the system at the higher amperage draw, however it would be nice to be able to charge when I don't have commercial power. Even it it's just a little bit. People see solar and they automatically think that you will have power during a power failure. Also, they don't realize that the grid interactive inverters only put out what's required to power the critical equipment. Even if it's not full capacity.

    I think I'll start the solar thread. Seems to be a worth topic.
     
  13. brianman

    brianman Burrito Founder

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    Sounds good. Thanks for the follow-up.
     
  14. andydoty

    andydoty Member

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    FYI, I created a thread. Figured this would be the proper home for it.
     
  15. AmpedRealtor

    AmpedRealtor Active Member

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    I'm sure others have already mentioned this, but imagine filling a glass with water from your bathtub spigot on full blast. You can only do that for a relatively short period of time before you overfill the glass. To prevent overshooting, you gradually close your spigot so that the flow rate allows you to neatly fill the glass without spilling. That's basically how the superchargers work, and why they have to throttle back the charge rate at some point.
     
  16. wcalvin

    wcalvin Member

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    #16 wcalvin, Feb 3, 2014
    Last edited: Feb 8, 2014
    Rate of charging at 120 kW Supercharger

    Supercharging graph.PNG

    Here is a 120 kW supercharging session from a start at 19 miles remaining until, 80 minutes later, 268 rated miles full when I manually stopped charging. Rate of charge is "mph" but it does not agree with observations: mph was reading 5mph at the end, not 160.
     
  17. brianman

    brianman Burrito Founder

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    Supercharger | Tesla Motors
     
  18. JohnQ

    JohnQ Active Member

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    Yes, that's the analogy for us non-technical folks. The one that makes you go "ah, that makes sense," then you realize that it tells you nothing about why the taper is there. I'm not claiming to know, just saying that the Tesla response is a non-answer.
     
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