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Ignition On mode - does 12V battery charge, and how fast?

Discussion in 'Gen 4 Prius Technical Discussion' started by The Professor, Sep 19, 2018.

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  1. The Professor

    The Professor Senior Member

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    EDIT: I meant On Mode, not Accessory Mode - can't edit title :D

    Obviously in a traditional ICE-only car, the 12V battery will not charge without the ICE running.

    However, the Prius isn't one of those cars...

    My question is: Does the Prius Gen4 use the traction battery to charge the 12V battery when the ignition is in Accessory On Mode? Or does it have to be in Ready mode?

    The reason I ask, is that my wife and I were enjoying the heated seats today, and they were on with the Ignition in Accessory On Mode. I noted on my previous car that the heated seats would draw around 20 Amps when both were in use. That would flatten the tiny 12V battery in the Prius fairly quickly.

    On the same topic - does anyone know how much current the charger delivers to the 12V battery? It's not like it's a hefty alternator that can kick out 100 Amps. I've seen numbers floating around like "4 Amps" (but without any reliable source cited) so I'm also worrying if it can keep up during prolonged use while driving?

    Any thoughts?
     
    #1 The Professor, Sep 19, 2018
    Last edited: Sep 19, 2018
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  2. alanclarkeau

    alanclarkeau Senior Member

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    No, sorry. But I wonder if he's covered it in this video - I haven't watched it through completely?
     
    #2 alanclarkeau, Sep 19, 2018
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  3. The Professor

    The Professor Senior Member

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    Thanks for that video.

    I learnt at 11:40 that when running diagnostics on the battery, you're looking for 10 Amps or less going into the battery when in Ready mode. That tells us that the 12V battery charger can only provide up to 10 Amps.

    At around 12:40 the vehicle is in Ready Mode, and the DC-DC converter is outputting 23 Amps (10 of which are going to the battery to charge it). The car obviously draws a lot of current just power up all the stuff it needs to get ready for driving. Obviously this will include air con, computers, instrument cluster, pumps for brake pressure, and so on. It doesn't seem excessive.

    At 14:50, after he turns on just about everything he could turn on, the DC-DC converter was pushing out 70 Amps. It's rated at 100 Amps, so still has plenty to give. It's also worth noting that by this time the 12V battery is being charged at 5 Amps - it seems to settle at around this current.

    So the above answers part of my question. While in Ready Mode (i.e. driving), the 12V system can supply a constant current of around 100 Amps thanks to the DC-DC converter which will be running. It can supply shorter bursts of more than that by drawing current from the 12V battery too. So running the heated seats won't be a problem in that scenario.

    To answer the other part of my questions, later on (there are 70MPH winds outside right now), I'll stick my multimeter on the battery and see if it jumps up from 12-13V (give or take) closer to 14V when I turn the ignition to On Mode. That will tell me if it's charging in On Mode or not. If it's charging, that means the DC-DC converter is providing current, and as above it can provide up to 100 Amps. However, that makes me wonder what action the car will take as the traction battery starts to get low. If the 12V battery isn't charging, that means the heated seats will flatten the 12V battery. Both assuming that the ICE will never start (to charge the traction battery) when not in Ready Mode.

    My previous car flashed up a huge warning on the Sat Nav screen saying "BATTERY LOW!!! START THE ENGINE TO PREVENT DAMAGE" if you sat there with the engine off for too long and the ignition On. I would assume the Prius does something similar?
     
    #3 The Professor, Sep 19, 2018
    Last edited: Sep 19, 2018
  4. bisco

    bisco cookie crumbler

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    i think not. doesn't it shut off acc mode after so many minutes with a warning?
     
    #4 bisco, Sep 19, 2018
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  5. The Professor

    The Professor Senior Member

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    Sorry, I meant On Mode, not Accessory. I can't edit the title :D
     
    #5 The Professor, Sep 19, 2018
  6. bisco

    bisco cookie crumbler

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    gotcha.:)

    still, if the engine can't start, i think toyota is loathe to allow much battery drain.
     
    #6 bisco, Sep 19, 2018
  7. alanclarkeau

    alanclarkeau Senior Member

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    You can request the title be modified - click - and explain what you wanted it changed to:
    upload_2018-9-19_23-17-53.png
     
    #7 alanclarkeau, Sep 19, 2018
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  8. alanclarkeau

    alanclarkeau Senior Member

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    I imagine - like if I lock my mother (her choiceo_O) in the car with the A/C on (and POWERED ON) - occasionally the ICE will kick in and replenish the Traction Battery - presumably keep the 12v replenished too.

    I don't have heated seats so can't test them. BUT - I thought, but could be wrong, that the A/C doesn't run 12v, but runs from the Traction Battery direct. Any idea if that's correct?
     
    #8 alanclarkeau, Sep 19, 2018
  9. bisco

    bisco cookie crumbler

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    correct. the a/c is high voltage, seat heaters are 12v. don't thing they'd want you sitting on high voltage.:eek:

    are you saying ig-on, and not 'ready'?
     
    #9 bisco, Sep 19, 2018
  10. The Professor

    The Professor Senior Member

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    That's right. Pressing the Start button twice, without pressing the brake pedal. Ignition On (not Ready).
     
    #10 The Professor, Sep 19, 2018
  11. bisco

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    thanks, i was wondering if that was what alan was alluding to.
     
    #11 bisco, Sep 19, 2018
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  12. GadgetVirtuoso

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    I haven't gone to the drive-in theater with the new Prius but my 2012 Prius C would turn off if left in this mode for an extended period. When we went I would just leave the car in Ready the whole time so we could use the radio and such. the ICE would come on a few times during the movie to keep the battery charged but since we were only using the radio you're not using a lot of power. If you were using the heated seats I would expect the ICE to come on quite a bit more to compensate for the extra power drain.
     
    #12 GadgetVirtuoso, Sep 19, 2018
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  13. bisco

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    gen 3: one push without foot on brake = acc mode. in acc you get radio and a few other small items.

    2 pushes without foot on brake = ignition on mode. in this mode you get acc items plus instrument cluster, hvac fan, power windows, lights and etc.

    foot on brake, one push = ready mode. you get everything plus engine start, a/c, heat and etc.

    is gen 4 different? i only doubt it because they are always trying to protect the batteries, and unfortunately, gen 4 12v has proved to be as weak as its predecessor.
     
    #13 bisco, Sep 19, 2018
  14. The Professor

    The Professor Senior Member

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    Same for Gen 4, and in fact all 5 cars I've owned from 3 manufacturers with Start buttons instead of a key.
     
    #14 The Professor, Sep 19, 2018
  15. kithmo

    kithmo Couch Potato

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    The answer is no, it doesn't charge the 12v when in "ON" mode, I just checked on mine with the voltmeter on my 12v power socket adapter and it reads 11.9v in "ACC" mode and 11.9v in "ON" mode then goes up to 14v in "READY" mode.
     
    #15 kithmo, Sep 19, 2018
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  16. jerrymildred

    jerrymildred Senior Member

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    No need to guess and wonder now. A definitive test. (y) Now the Professor won't have to do that after the wind dies down at his location. ;)
     
    #16 jerrymildred, Sep 19, 2018
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  17. The Professor

    The Professor Senior Member

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    Thank you!
     
    #17 The Professor, Sep 19, 2018
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  18. kithmo

    kithmo Couch Potato

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    I got this about a month ago on the recommendation of another member.

    Very useful ;)
     
    #18 kithmo, Sep 20, 2018
  19. alanclarkeau

    alanclarkeau Senior Member

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    Oh - I thought that's Accessory mode - but the manual indicates there is a 4th mode 1) OFF; 2) Accessory; 3) ON; 4) READY.

    It indicates that it'll turn off both 2) or 3) after 20 and 60 minutes respectively. It also warns that both 2) and 3) can cause the 12V to go flat.

    Why wouldn't you just put it in READY mode under those circumstances? It's be safer.
     
    #19 alanclarkeau, Sep 20, 2018
  20. The Professor

    The Professor Senior Member

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    Yeah without pressing the brake pedal, starting from Off, it goes Off -> Accessory -> On -> Off.

    If you press the brake pedal at any point in the above while pressing the start button it goes straight to Ready, so you can also do:

    Off -> (Press Brake + Start) Ready
    Off -> (Press Start) Accessory -> (Press Brake + Start) Ready.
    Off -> (Press Start) Accessory -> (Press Start) On -> (Press Brake + Start) Ready.

    Thank you - I couldn't find that in the manual. But even 60 minutes with 20 Amps being drawn is going to drop the voltage of the 45Ah battery down to a voltage where Prii are known to be quirky.

    It might be safer for the car to go to Ready, but in this particular instance my Prius was inside my Garage (i.e. enclosed), so not safer for us humans :D
     
    #20 The Professor, Sep 20, 2018
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