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Power Fluctuation

Discussion in 'Model S: Battery & Charging' started by Pete90D, Aug 27, 2015.

  1. Pete90D

    Pete90D Fan of Red Lights

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    #1 Pete90D, Aug 27, 2015
    Last edited by a moderator: Mar 13, 2016
    Has anyone seen this before?

    I was doing a range charge and when I got back in I saw this going crazy. I turned off the A/C and it kept going, but when I turned the A/C back on it leveled out to 12A. The dash was actually showing the numbers fluctuate even faster.

     
  2. spaceballs

    spaceballs Member

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    #2 spaceballs, Aug 27, 2015
    Last edited by a moderator: Mar 13, 2016
    Likely normal, you taper the current near the end of a charge. I.e. same when you fill a glass up with a pitcher and want to fill it to the brim but not spill over.
     
  3. Pete90D

    Pete90D Fan of Red Lights

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    I don't mean the decrease in amperage I mean the crazy fluctuation. I've seen it taper down to 1A before, but not jump around between the numbers like that and then once I turn my A/C back on go to a constant amperage
     
  4. snort

    snort Member

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    The batteries are built with many cells in series. They're only charging the ones on the end and those are charging their neighbors. Each cell has a voltage limit (which the battery management system in the car knows and tells the charger about) so they charge the cells on the end right up to the limit, then wait for the voltage to drop a little, meaning the electrons are finding their way to interior cells, and then do it again.

    many sophisticated battery chargers do something like this when trying to get the absolute maximum charge--even for Lead Acid and NiCds. the iterations aren't usually so fast but it's the same thing. I guess everything is faster in a supercharger...
     
  5. Pete90D

    Pete90D Fan of Red Lights

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    Interesting. I just hadn't seen it before. I've done several range charges with a few Teslas and it was always pretty constant with maybe a 1A fluctuation as the amperage decreased.

    Thanks for the info
     
  6. kennybobby

    kennybobby Member

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    It appears that the 90 is charging at slightly higher voltages too, 404 = 4.208 vpc, 405 = 4.218 vpc. Maybe part of making it a 90 pack is to charge an extra 40 mV at the top up to 4.2. Does your pack label say 400 VDC?

    As for the current fluctuations, some of the charging patents describe methods to detect SOC level based upon open circuit voltage and dV/dt. i read that to mean the current is momentarily turned off and measurements are quickly taken, the calculations and control decisions made, and the current back on again--all done much faster than the update/averaging rate of the current display shown on the monitor.
     
  7. physicsfita

    physicsfita Member

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    I hate to disagree, but batteries charging in series don't work the way you envision. First off, a battery doesn't hold extra electrons -- I tell my students that a "charged" battery is like a "charged" fire extinguisher. If there are the right chemicals available in the cell, the battery will move electrons through the circuit -- it doesn't create any (think of a conveyor belt here). A "discharged" battery is one where the chemical reactants have gone to products. When charging, an external potential (the DC charging unit) effectively runs the "conveyor belt" backwards -- this provides the energy to run the chemical reactions in reverse, thus "charging" the battery (you could have done this by pouring in more chemicals, just like my old sump pump battery). At all times, the battery is electrically neutral.

    Secondly, the way this "conveyor belt" works is by creating an electric field that goes through the entire series of batteries. This field acts on all the batteries in the series at once. Thus, the reversed chemical reactions are occurring in each cell in the series simultaneously. (I'm neglecting what happens during the billionth of a second or so right after the charger is connected/disconnected, but that all averages out, anyway.)
     
  8. snort

    snort Member

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    yes, my description was intentionally simplistic. most of the readers here are not physicists or electronic engineers.

    the chain of cells is a gigantic and fairly complex RLC (resistance, inductance and capacitance) circuit. it takes some time for the charge "field" to propagate through it. I have found that the electrons as marbles analogy helps explain this.

    --Snortybartfast
     
  9. snort

    snort Member

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    I should mention that the "taper" is there to smooth out the cycling between high and low charge rates near the end of charge. It serves essentially the same purpose, to let the charge seep through the chain of cells, which takes several seconds. my guess is that they haven't quite got the rate tuned perfectly yet for 90kwh batteries.
     
  10. Zextraterrestrial

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    #10 Zextraterrestrial, Aug 28, 2015
    Last edited: Aug 28, 2015
    I think this has been added in the firmware quite a while ago and I am only guessing that it was actually something added/changed based on some Tesla patent I read a long time ago. The amperage pulses when getting near complete which may help to allow a slower/longer charge taper somehow.

    ..and I could be totally off, looking for that patent... lot of others I haven't seen(Model X stuff + others...Metal air hybrid for the X??? 'working' on it since 2012)
    hmm not sure what I thought I saw. the final stage charge I found only describes constant current but it is based on a few parameters that may fluctuate enough to see the waving at the final charge stage
     
  11. scottm

    scottm Active Member

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    Those damn 90 packs, just can't trust 'em.
     

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