TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker and becoming a Supporting Member. For more info: Support TMC

What charges the 12v battery and when?

Discussion in 'Model X' started by fwgmills, Feb 23, 2017.

  1. fwgmills

    fwgmills Supporting Member

    Joined:
    Jun 30, 2015
    Messages:
    135
    Location:
    Roanoke, TX
    We know that most of the car's regular systems (screens, radio, amps, lights, etc.) run off of the 12v electrical system and that Tesla has had issues with reliability on the 12v battery due to it being undersized and/or defective, but has Tesla ever stated under what conditions the 12v battery is "maintained" or charged? For an ICE the battery is only charged when the engine is running because that is the sole source of energy for recharging it. In a Tesla there is a DC to DC converter that can charge the 12v system from the high voltage battery but when does this happen? I would think it would/could charge the 12v system under any one or a combination of the following:

    1) When the car is "on"
    2) When the car is plugged in
    3) When the "accessory" is active (screen/climate/radio is on)
    4) Anytime the vehicle system sees fit to charge it

    It seems important to know under what conditions the 12v system is maintained because the high voltage battery can charge the 12v battery, but the 12v battery is required to close the contactors that allow the high voltage pack to do it's thing. Seems like a sort of catch-22 from an engineering standpoint. I would assume it to be #4 since the vehicle can be left for a long time plugged in but off and still be functional after months.
     
  2. Tam

    Tam Active Member

    Joined:
    Nov 25, 2012
    Messages:
    2,363
    Location:
    Visalia, CA
    Lead Acid battery does not like low voltage so there is no advantage to deplete it before charging.

    I would choose 4) Anytime the vehicle system sees fit to charge it.

    That is part of the reasons of why you got vampire drain even when you are not using your car.

    I heard ICE modern cars use 12 V battery more intensively so they don't last as long as in the good old days either (I don't have references.)
     
  3. David99

    David99 Active Member

    Joined:
    Jan 31, 2014
    Messages:
    2,569
    Location:
    Brea, Orange County
    Any time the 12 Volt battery needs a top off, the car will do so. When the main battery is critically low, it will disconnect itself from any power draw from the car and as a result the 12 Volt battery will also not be charged.
     
  4. Lbkmxp100d

    Lbkmxp100d Member

    Joined:
    Dec 7, 2016
    Messages:
    191
    Location:
    Lubbock, TX
    Fwgmills, I'm glad you asked the question as I have often wondered this as well.
     
  5. Lbkmxp100d

    Lbkmxp100d Member

    Joined:
    Dec 7, 2016
    Messages:
    191
    Location:
    Lubbock, TX
    How does it convert DC to AC and is there a certain percentage at when the vehicle charges the 12V battery? Ex: when 12V battery drops down to 10V the vehicle charges the battery.
     
  6. ecarfan

    ecarfan Well-Known Member

    Joined:
    Sep 21, 2013
    Messages:
    12,547
    Location:
    San Mateo, CA
    Both the 12V battery and the main battery are DC. All batteries are DC.
     
  7. Lysol

    Lysol Member

    Joined:
    Oct 22, 2016
    Messages:
    98
    Location:
    Virginia
    What you would have is a DC (high voltage) to DC (12v) charger.

    A good way to test when it's charging would be to measure the current going into the battery when the car is sitting in various configuration over a period of time.
     
  8. strider

    strider Active Member

    Joined:
    Oct 20, 2010
    Messages:
    3,117
    Location:
    NE Oklahoma
    • Informative x 1
    • Like x 1
  9. David99

    David99 Active Member

    Joined:
    Jan 31, 2014
    Messages:
    2,569
    Location:
    Brea, Orange County
    There is a powerful DC to DC converter in the car, that provides a lot of power to the entire 12 Volt system, not just the battery. The 12 Volt battery is just used as a small buffer so the DC to DC converter doesn't need to run all the time. The 12 Volt runs everything except the traction motors and HVAC.

     
    • Informative x 1

Share This Page