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Recharging the 12v battery

Discussion in 'Model 3: Battery & Charging' started by xpitxbullx, Jul 24, 2018.

  1. xpitxbullx

    xpitxbullx Member

    Joined:
    Jun 15, 2018
    Messages:
    210
    Location:
    Las Vegas
    I know it can vary depending on the situation but here’s my question.

    While driving, what percentage of the traction battery’s power is used to keep the 12v battery charged?

    While parked, if you use solar panels to keep the 12v topped off, will this eliminate 99% of the vampire drain on the traction battery?
     
  2. BestHand

    BestHand Member

    Joined:
    Jul 18, 2018
    Messages:
    109
    Location:
    Staten Island, NY
    As I understand there are two causes of vampire drain: real drain which is actually energy used to support some functions like bluetooth, WiFi connection and so on and not a real drain, but loosing some capacity over the time. Even if you completely disconnect the battery, it still will lose some charge over the time (which we cannot call a drain). Also dc2dc converter from 400V to 12V works constantly, does not matter car is in a sleeping mode or not. That converter also has some losses.
    When you drive day time charging the 12V battery does not take much higher energy, probably most of it goes to the screen and some pumps. I guess AC takes energy from high voltage battery directly.
    So, back to your question. I thought about it. I think that power drained by 12V battery is probably a biggest part of all "vampire drain". If it is, yes, connecting your 12V battery to some charger (solar panels) will help. But from another point, charging 12V battery is still not free generally. Also, dc2dc converter will still connected, and have some loses even with no load. If car left for long time, probably it worth to try and see what happened.
    I think in a future Tesla will get rid of 12V battery completely and will use more than one dc2dc converters.
     
    • Like x 1
  3. ChrisPDX

    ChrisPDX Member

    Joined:
    Mar 19, 2013
    Messages:
    215
    Location:
    Hillsboro, OR
    I would be very concerned about connecting any type of trickle charger to the 12v battery. As discovered in the dashcam thread, the battery is monitored very closely by the car. Running a 2-3 watt continuous load from a dashcam will trigger an alert on the car because it thinks the battery is going bad due to loosing power too quickly. Adding power to it that's not managed by the car would probably cause some other alert or worse, overcharge the battery.

    So while a nice idea to add a solar panel to keep the vampire drain at a minimum, I don't think it will work on the 3. The power system is just too much of a closed system. It might work on a S/X though. But even then, Tesla did some design improvements such as adding a smaller DC2DC converter to keep the 12v system powered off the main HV battery more efficiently. This greatly reduced the vampire drain over the early Model S.
     
  4. BestHand

    BestHand Member

    Joined:
    Jul 18, 2018
    Messages:
    109
    Location:
    Staten Island, NY
    Correctly working trickle charger will not overcharge and will not charge constantly. It just should support battery voltage on a some level. The only problem can be if system will "think" that something wrong with the 12V battery because it does do some load on a dc2dc converter (if system actually checks that and see as something wrong). But again, that charger will consume energy, and only plus is that it will not allow 12V battery to discharge.
     

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