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Current (Amps) bouncing while charging at 100% capacity: is this battery balancing?

Discussion in 'Model S: Battery & Charging' started by MarcG, Jun 22, 2015.

  1. MarcG

    MarcG Active Member

    Oct 29, 2014
    San Francisco
    #1 MarcG, Jun 22, 2015
    Last edited by a moderator: Mar 13, 2016
    I supercharged at Vacaville, CA yesterday evening and set the limit to 100% since I was going to have dinner while charging -- and before you ask, I did go out there about 40 min later (at 90%) to check that there were other spots available, just to make sure I wasn't preventing others from charging. There were 4 (half the SpC stalls), so I proceeded to 100%.

    Upon getting back in the car, it was still around 99% and 249 rated miles, so I waited a few minutes to observe what would happen. After 2 more miles were added to the rated range, it looked like battery capacity hit 100%, and the time remaining changed from 5 minutes to "Calculating time remaining..." indefinitely.

    The interesting part is that, while the voltage stayed around 400V, the current then started bouncing around quickly between 11 and 22 Amps -- something I had never observed before. I took a short video to demonstrate how fast this was occurring (see below).

    Is this battery balancing at work? If so, why does the current change so dynamically?

  2. islandbayy

    islandbayy Active Member

    Feb 25, 2013
    Greendale, Wisconsin
    #2 islandbayy, Jun 22, 2015
    Last edited by a moderator: Mar 13, 2016
    Yes, it was balancing. As the bleed off resistors bled away power as needed from the cells that were "over full", it would pump a bit more in to bring all the cells back up.
  3. AmpedRealtor

    AmpedRealtor Well-Known Member

    Jun 30, 2013
    Buckeye, AZ
    My car gradually ramps down the Amperage and only in one direction - downward. I've never seen my car behave this way when charging to 100%.
  4. KyP85D

    KyP85D Member

    Feb 25, 2015
    I saw my car do this same thing for the first time last week. P85D with 12.5k miles on it.
  5. pgiralt

    pgiralt Supporting Member

    Jun 16, 2013
    Cary, NC
    I'm more inclined to think that this might be the DC-DC converter switching on and off or something like that. I know when AC charging, the amps include the power used to power HVAC/Heating and other systems in the car as well, in addition to the current being used to charge the battery. I suspect the same is true when DC charging and there's just something else using power. Next time it happens, try exiting the car and letting it shut down, then observe the current on the dash through the window.

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