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Where is the power coming from when plugged in but not charging?

Discussion in 'Model S: Battery & Charging' started by DirectAndCurrent, Jun 17, 2014.

  1. DirectAndCurrent

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    Technical question that I wasn't able to find a definitive answer to on the forum search:

    If my Model S is plugged in but NOT charging, perhaps because the charge is delayed or because it is complete, is the 12V battery still effectively bypassed while using the car's electronics/HVAC? Or does it require the high-voltage battery to be charging to net out whatever energy consumption I'm doing while plugged in ?
     
  2. Zextraterrestrial

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    should be using 'shore' power when plugged in, even if charge timer is set for later

    If you turn on climate it will pull shore power instead of drawing from the HV battery when plugged in.
    seems like it could be configurable to not pull power if charge timer is set, but not so
     
  3. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    From what I've observed, it's a "net-out" scenario. You'll note a slight lag between the power draw (e.g., turning on the climate control) and the time that the HPWC/UMC clicks on and ramps up.
     
  4. Todd Burch

    Todd Burch Electron Pilot

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    To answer your question more precisely:

    When plugged in and using HVAC, the car draws power from the wall to compensate for the power consumed by the HVAC system. However, as mentioned there's a slight lag on first startup. It's not DIRECTLY using shore power--it's pulling off the battery but recharging the battery at approximately the same rate.

    As for other systems like the display, sound system, pano roof, lights, etc--they are powered directly off the 12V auxiliary battery. When the car sees the state of charge of the aux battery drop below a certain threshold, the DC to DC converter between the main high voltage pack and the 12V battery kicks on, replenishing the aux battery from the main pack.

    I have not observed this 12V replenishment ever draw shore power...primarily because the draw is so small that it's not worth sucking the little bit of additional power from the wall.
     
  5. tezco

    tezco Sig P85

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    My experience is the same as Todd's. Use of the electronics draws power from the 12V, which is recharged from the main pack as needed. Power is only drawn from the wall for battery charging and/or when the HVAC is turned on while the car is still plugged in.
     
  6. DirectAndCurrent

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    Thanks for the tidbits everyone! Appreciate the clarification.
     
  7. HankLloydRight

    HankLloydRight Fluxing

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    Side question: Assuming one has a good charge in the traction pack, is it safe to run the electronics "indefinitely" and not drain/damage the 12v battery?

    IOW, does the 12v DC-DC charger kick in whenever the 12V battery voltage drops to a certain point, or say, only when driving when the main inverter is running? Or, is the 12v battery only in danger of running down when the traction pack is empty?

    I just got back from a week long trip via SCs, and had a bit of "downtime' showing the car to people -- I was curious when and how the 12V battery maintains its charge.
     
  8. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    The car actively manages the 12V system's health. If the 12V battery becomes depleted enough, the car will engage the contactor to recharge it through the DC-DC converter.
     
  9. tezco

    tezco Sig P85

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    Well, you will slowly damage the 12V battery because there are only a finite number of charge/discharge cycles for any battery. I'm not paranoid about running the electronics when parked (what little damage occurs is just a cost of enjoying the car). On the other hand, I'm not planning to tap into the 12V power to leave something running in the car 24/7 (unless the power draw is known to be minute).
     
  10. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Yes and no on the degradation. If the battery management allows a lot if voltage drop on the 12V before letting it sip from the big pack then yes you'll have cycling, but if the BMS let's the big pack charge yhe 12V as soon as voltage starts dropping you could think if it as electrons flowing from the big pack through the 12V with little charge/discharge happening. The exact workings of the 12V BMS is not really known and has probably evolved (there were a lot of 12V failures early on).

    Besides, a new 12V battery is cheap.
     
  11. tezco

    tezco Sig P85

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    Unfortunately the 12V is fairly difficult to replace in this car. I've had mine replaced twice so far, and that's without any user added loads attached, so my suspicion is that there is quite a bit of cycling still going on. For my last replacement, a deep cycle battery was installed, which also indicates that the 12V is under a lot of stress, even in the absence of any added electron sucking gadgets in my car.
     

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