Please explain 12V system to me--how to keep battery charged while using accessories?

Discussion in 'Gen 4 Prius Technical Discussion' started by dpbsmith4, Sep 5, 2017.

  1. jerrymildred

    jerrymildred Senior Member

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    You cheat as bad as I do! :LOL::ROFLMAO::LOL: Except I was in too big of a hurry to cheat this time.
     
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  2. lextoy

    lextoy Active Member

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    Ya, that manual is ridiculous...
     
  3. JakeCastle

    JakeCastle Junior Member

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    The diagram shows a 100 amp fuse for the DC to DC converter, so at 14.4 volts x 100 amps, you can hook up a 1440 watt inverter?
     
  4. ChapmanF

    ChapmanF Senior Member

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    ... assuming none of the car's own built-in equipment downstream of that fuse was drawing any current at the same time as your inverter.

    ... also assuming the converter's output holds solid at 14.4 volts even at full load. It may droop some.
     
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  5. alanclarkeau

    alanclarkeau Senior Member

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    It's not going to be 14.4 like a normal car - particularly if the ICE isn't running. As for 100 amp, that could be tenuous too - a fuse is always over the rating for the circuit - not an indication of output.

    (2) How to check a Hybrid/EV 12V Charging System - YouTube

    A bit like a speedometer - this is Dad's car in the mid-'60s - with a top speed of 85mph - if you were game. 110 was ... just a figure.

    upload_2021-1-20_9-40-38.png
     
  6. ChapmanF

    ChapmanF Senior Member

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    Whether the ICE is running is one thing that has scarcely any effect on the voltage out of the DC/DC converter (unlike in a conventional car, where the voltage usually goes up some with increased alternator RPM). The innards of the converter are more like those of a regulated computer power supply; in my Gen 3 it is generally 14.7 right on the dot, whether the engine is on or not, with scarcely a wiggle when it starts or stops.

    The voltage can change at times, but for other reasons. It will be dialed back some from 14.7 when the 12 volt battery is thought to be hot or fully charged, and yes, it will droop some near the limit of the converter's capacity.
     
  7. alanclarkeau

    alanclarkeau Senior Member

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    Maybe I'm missing something - but John Kelly on the video I quoted shows:

    upload_2021-1-20_11-42-57.png
     
  8. ChapmanF

    ChapmanF Senior Member

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    That's more a number you might see when the hybrid system is disabled (car is OFF, or in ACC or ON, not in READY).

    If you ever see a voltage like that when the car is READY, your DC/DC converter has failed (or a fuse in that circuit).
     
  9. alanclarkeau

    alanclarkeau Senior Member

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    OK - further on in the video (13:00), he shows it at 14v with it on ON. But he also shows that there is about 13 amps being drawn by the vehicle with "nothing" on.
     
  10. ChapmanF

    ChapmanF Senior Member

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    Prof. Kelly is not prone to making mistakes of that magnitude, and I might not have time right now to find out what in the video might have given you those impressions, but at the 13:00 point the car is clearly in READY, not ON. The voltage reading of 14 is one dead giveaway, and the nonzero amp reading on the leftmost meter (showing the DC/DC converter output) is another. The DC/DC converter only operates in READY mode. It does not operate in ON, or ACC, or OFF. The car entered READY mode at around 12:06.

    I'm not sure what significance you're attaching to that, or how we got from volts to amps. But with the car in READY mode as it is at that point, 13 amps is nothing surprising; there's plenty of gear in the car receiving power when in READY.
     
  11. alanclarkeau

    alanclarkeau Senior Member

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    Post #43 was assuming that because there was a 100amp fuse that he would have 100amps supply available. But if the car, in READY mode is using 13 - that's cutting into the 100 isn't it?
     
  12. ChapmanF

    ChapmanF Senior Member

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    Post #44 was clear that all of the car's built-in equipment has to be accounted for, and that can even be a lot more than 13. Watch the video for a couple more minutes and Prof. Kelly turns on some headlights and HVAC and raises the current draw to over 50 amps.

    There are some subtleties to the wiring diagram; not all of the internal loads come from downstream of that 100 amp fuse. Some do, and some are on the converter side of it. The converter's actual capacity may be somewhat higher than 100 amps, but Toyota doesn't publish it. If you are planning to add a load drawing anywhere near what you think the total capacity is, your best practice is to monitor the converter's IDH signal (indicating when the converter thinks it is being overloaded), and/or watch for a significant droop in the voltage. (You can see in the video that quadrupling the current demand changes the voltage reading by barely a tenth of a volt. That's not a significant drop, but load the thing past capacity and you can see one.)

    Whether the built-in loads cut into the "100 amps" you can draw through the fuse depends on whether they are downstream of it. The loads that are upstream don't contribute to the current the fuse is seeing, but they are still loads the converter has to supply, so you have to allow for them. The way the wiring diagram is divided up like that seems a bit arbitrary and depends on the model; there's no very quick way to sum it up.
     
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  13. jerrymildred

    jerrymildred Senior Member

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    Not because the ICE is off. Because the 12V battery has enough charge that the converter steps down its output. I see those changes on my "cigarette lighter" meter all the time while driving my Prime in EV mode. Starts out over 14V, then down to 12-1/5 or so. Then back up. And so on. Behaves the same in HV and EV.
     
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  14. kithmo

    kithmo Couch Potato

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    I have a similar meter and I find that if it stays at 14v (charging) for any length of time, it's usually an indication that the 12v is low and needs a top up, so I stick the CTek on it for a few hours.
     
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