12V battery question

Discussion in 'Hyundai Kona Electric' started by Jgood, Dec 5, 2019.

  1. Jgood

    Jgood Member

    Since I've not had the pleasure of experiencing a dead 12V battery yet I wanted to ask a question prior to my "first time". I carry one of those small jump-start packs that Costco and others sell in my car which will normally provide power for a ICE car to get started. Does this work for a BEV as well? What I don't understand is the difference between the ICE car — which once started begins recharging the battery via the alternator, and the BEV car — which does, or doesn't do the same thing? I'm not sure how that works. Doesn't the BEV recharge via the traction battery once the car is started? If so, why doesn't the car just use the traction battery to start when the 12V battery is dead? And if it technically does NOT do that, how does the BEV car continue running without the 12V battery being completely recharged? All a bit confusing to me. Thanks for any feedback.
  2. eastpole

    eastpole Member

    My understanding is that the car cannot start up from the traction battery at all, under any circumstance. This is because the big 356 V battery, capable of providing enough current to electrocute a herd of elephants, can't safely be left connected indefinitely. When you shut the car down, the main electrical contacts to this battery are withdrawn into an insulated housing. The car will wait, with only 12 V auxiliary battery available, until you hit the Start button, when the 12 V actuators fire up and insert the contacts to bring the traction battery back into the picture. (I'm pretty sure all the mass-produced EVs work like this.)

    I believe this means that (unless you plan on warming the cabin, seats, and defrosting the window before you start the car) the jump-start pack will work fine. A jump-start process just needs to provide enough current to run the vehicle computers through a safety check and then power the main battery connection procedure.

    I *believe* (less sure about this) that the 12 V auxiliary battery will immediately start charging itself through the DC-DC stepdown converter and you should be off and running. Don't immediately turn the vehicle off again because (as in a combustion car) the battery will take a while to absorb enough charge for the next start-up sequence.

    I hope this was the level of explanation you needed. Please someone let me know if I've stepped off the path of truth here ^^.

    Congrats on your Kona, JGood!
  3. Esprit1st

    Esprit1st Well-Known Member

    Great explanation! Just adding another piece of information: that's what the yellow light in the front Hyundai logo indicates: usage of the high voltage battery without the car driving/driver on board.
    eastpole likes this.
  4. Jgood

    Jgood Member

    Wow. That was a great explanation. Thanks so much. It makes perfect sense once you have some idea of what's taking place. :)
  5. eastpole

    eastpole Member

    Oh right, good point. The auxiliary battery can say "Gosh I'm low!" and try to fix the problem by connecting the traction battery for a short time to run the DC-DC converter.

    I think the "dead 12V battery" thread indicates that it isn't a perfect solution.

    It looks like this:

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