12V Lead Acid Battery ... why?

Discussion in 'General' started by brulaz, Sep 12, 2019.

  1. brulaz

    brulaz Active Member

    Have been reading about issues with the 12V Lead Acid battery installed in Bolts and Konas.
    It needs to be kept charged during storage or the car won't start and so on. According to the Bolt manual the Li-Ion battery (with very low self-discharge) can be left for up to a year with a ~50% SOC, but the 12V lead acid battery needs to be disconnected and put on a trickle charger. (Presumably by disconnecting the 12V battery you remove all the parasitic loads from the Li-Ion battery as well.)

    Why is that 12V battery necessary? Why not just a high-voltage DC to 12VDC converter instead of a 12VDC battery charging circuit? A separate battery seems like an unnecessary complication to me. (And just add a switch to disconnect all the little parasitic loads for long-term storage.)

    Do all EVs have a separate 12V lead acid battery?
    Do the Teslas?
  2. NeilBlanchard

    NeilBlanchard Active Member

    I have also wondered about this. As far as I know, every EV has a 12v lead acid battery.
  3. bwilson4web

    bwilson4web Well-Known Member Subscriber

    Usually a small 12V battery that might be used in a lawn tractor. However, there are 12V powered devices like key fob receivers and cell-based data links (aka., Tesla and BMW.)

    Personally I think a better answer is a solar 12V tickle charger embedded in the upper dash, sun screen, or conformal with the hood, roof, or trunk lid.

    Hyundai did a clever trick with their traction battery by putting a DC-to-DC converter in the pack to generate 12V. This saves the weight of a lead block and gives the car a substantial 'turned off' period.

    Bob Wilson
    NeilBlanchard likes this.
  4. brulaz

    brulaz Active Member

    That's exactly what makes sense to me.
    But the Hyundai Kona EV has a regular 12V battery in addition to the Li Ion (as does the Bolt and apparently the Teslas). And if the 12V battery is too low in charge, you cannot operate the car, even if the main Li Ion battery has a good charge.
  5. bwilson4web

    bwilson4web Well-Known Member Subscriber

    No doubts from me.

    I believe it was the IONIQ hybrid that had the traction battery DC-to-DC converter and no lead acid 12 V battery.

    Bob Wilson
  6. gooki

    gooki Active Member

    Tesla have expressed a desire to move away from 12v batteries.

    We might see it in their next gen vehicles.

    My 2 cents says 12 v is having around because many OEM parts are still 12v. And having a battery on board means you can get away with a lower cost DC to DC converter as you only have to deal with average loads will the battery takes care of peak loads.
  7. brulaz

    brulaz Active Member

    Makes sense.
    But if you don't have an ICE's starter motor, wonder what sort of short-term, peak loads there are.
    Lights, computers and resistance heating (would that be 12V?) are rather continuous.
    Maybe the audio's heavy bass beat? :confused:

    Anyway, like you say, maybe the next generation of EVs will lose the 12V battery. :)
  8. clarks03

    clarks03 New Member

    I had the same issue a long time ago.
  9. You would want to boot up the electronics off 12V to assess the condition of the traction battery before connecting to it to draw power. It's just conservative engineering. Also, for 12V accessories the battery provides a buffer so the charger ('DC-DC') does not have to run continuously when you just want to listen to the radio, close a window, have the courtesy light come on or have the car recognise that a charger plug has been inserted ... all after turning off the power button. It could be done with a smaller Li-po, or perhaps a supercapacitor, but a lead-acid battery is the low-cost solution.
  10. brulaz

    brulaz Active Member

    You want to isolate the main battery behind a single connection; and use an auxillary battery to fire up the electronics and test the main battery before connecting it. OK, that gives you max protection of the main battery; makes sense.

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