Why does my battery only charge to 46% in ready mode?

Discussion in 'Gen 2 Prius Main Forum' started by ghostwheel, Jun 15, 2021.

  1. ghostwheel

    ghostwheel Junior Member

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    I'm trying to figure out why my Prius Gen 2, 2009 195k turns the ICE on and off so frequently in Ready mode when parked. I am camping in my Prius. With the A/C on, it turns on every 4 minutes or so for 1 minute.
    I followed what was happening in Torque. It seems the ICE comes on when battery reaches 41%, charges till it reaches 46% and then stops. So it is hovering in the 2 bar area. It seems the on and off limits are not fixed, or maybe depend on something else. When I set my A/C to weaker (i.e. higher temp, 74F instead of 72F), it turned on at 41.5%, off at 43%. When I set it to 70F, again off at 46%, and when I set it to 62F (I think, whatever is lowest but not LO), it charged till 47.5%.
    Why does the ICE keep the battery charged so low? If the battery was allowed to go to 80%, it would probably turn on 8 times more rarely.
    I guess the problem is that I don't know how the car decides when to turn the ICE on and off in Ready mode while standing. The battery temp at the time was ~100F and slowly cooling.

    [​IMG]
     
  2. davecook89t

    davecook89t Senior Member

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    With the possible exception of preserving battery longevity, the car is designed to save gas above all else. I would suspect that charging the battery to a high level becomes an inefficient way of using gas after a certain point. (Something like the law of diminishing marginal returns would probably apply). At the same time, starting the car from 0 rpm to idle speed also uses gas at a faster rate for the brief moment that it takes to spin the engine up to idle speed. The hybrid ECU makes a trade-off between whether to keep the engine running longer and charge the battery less efficiently or stop the engine and start it up again later, with a momentary surge in fuel usage. Apparently this balancing act is affected by the temperature you have chosen for A/C cooling. The lower A/C temperature setting would require more starts within a given period of time, and it is the ECU's judgement that it should run the engine longer to compensate and stretch out the interval between starts.
     
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  3. ghostwheel

    ghostwheel Junior Member

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    This makes sense.
    I am trying to debug why my Prius in Ready mode with A/C on turns ICE on every few minutes, whereas others reported that for them it is much rarer. I thought maybe my HV battery doesn't have enough capacity any more, which is why I started monitoring its charge with Torque. But it seems it isn't that the battery goes from 80% to 40% very quickly, but instead that it only gets charged to 46%.
    You seem to imply that this is normal for a Prius. So, what else could be causing the frequent on/off cycles? Could it be that other Priuses (Prii?) have more capacity between 41% and 46%? That normally the A/C can run quite a long time on 5% of the battery, but mine can only run for 4-5 minutes on 5% of the battery?
     
  4. davecook89t

    davecook89t Senior Member

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    I think that would be part of it. Also, if the outside air is cooler, you could expect to get a longer interval between starts. It might be worth having an A/C mechanic check the efficiency of your A/C system as well (to make sure it has the optimal amount of R134A, etc.) Every 4 to 5 minutes does seem like a short interval, but I'm not really sure what is typical. Hopefully your engine is well tuned, and the motor mounts are not worn, so it can start without disrupting your sleep too much.
     
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  5. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    The computer also wants to keep the gas engine above a certain temperature, because that's a big hammer in reducing overall fuel consumption.

    It is keeping that gas engine ready to leap into high power, high efficiency work at the drop of a hat. And that isn't really what you need for your situation.

    The engine duty cycle is highly optimized for transportation, and less so for stationary hotel loads. Obviously it works, it just isn't optimized for your usage pattern.
     
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  6. ghostwheel

    ghostwheel Junior Member

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    Inside the car I have noise (waterfall) coming from my phone at high volume to overcome all disruptions :)
    In my latest camping trip I was actually mainly worried about noise to other campers.

    I need to measure a bit more precisely, but I think that the draw while at 72F was 4.4A. I assume this is on the same scale that the total capacity of 6.5Ah is. That would mean that in 5 minutes I should use 4.4/6.5/12=5.6% of the capacity, and in 4 minutes 4.5%. Sounds about right....
     
  7. ghostwheel

    ghostwheel Junior Member

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    Maybe it is afraid that someone will come steal the catalytic converter, and wants to be ready to escape. Or maybe it just want to be ready to go home at any moment.
     
    #7 ghostwheel, Jun 15, 2021
    Last edited: Jun 15, 2021
  8. sam spade 2

    sam spade 2 Senior Member

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    The symptoms you describe are often an early warning of a failing HV battery, that is, losing capacity.

    Which should not be too surprising for a 12 year old car with almost 200K miles on it.
     
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  9. ghostwheel

    ghostwheel Junior Member

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    That's what I initially thought. (Though my Toyota repair tested the battery and said it is OK.) When I saw the quick ICE cycles, I was expecting to see the battery going quickly from 80% to 50%, so not holding much charge. But instead it seems that the car only wants to charge 41% to 46%. Are you saying that the onboard computer is somehow detecting that the battery is on its last legs, and only charging it to 46% because of that when not moving? (While driving, the battery does get to green.) Maybe I'll run a Dr Prius battery test.

    Yes...
     
  10. bisco

    bisco cookie crumbler

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    a healthy gen 2 prius runs the engine every few minutes depending on load, and only puts a small charge in the battery. frequent cycling while camping is a common complaint, not a rare one.

    a 12 year old battery won't hold the charge as long as a new one.
     
  11. PriusCamper

    PriusCamper Senior Member

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    Learn about battery reconditioning... When NiMH get old they lose capacity and you can recover that lost capacity by doing deep discharges and recharges.
     
  12. ghostwheel

    ghostwheel Junior Member

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    Thanks! That's how it got to 12 years... But I don't do deep discharges often enough. It is complicated, since this is our only car and really deep discharges take several days.

    I guess not as complicated as a failed battery.
     
  13. sam spade 2

    sam spade 2 Senior Member

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    Yes. That is my "educated" guess.
     
  14. sam spade 2

    sam spade 2 Senior Member

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    Or you could look up REAL electrical and chemical engineering information about how the batteries REALLY work.......and find out that you can NOT "recover" anything significant by doing ANYTHING once they are worn out.

    But some people don't seem to want to hear the truth.
    :rolleyes:
     
  15. sam spade 2

    sam spade 2 Senior Member

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    Well you see.......the BIG problem with that assumption is:
    You don't really KNOW if the "reconditioning" helped any or not.
    Maybe if you had done NOTHING, it would be in even better condition.

    AND.....that is a rather important piece of information to leave out.
    Is there some reason you didn't mention that in your first post ???
     
  16. ghostwheel

    ghostwheel Junior Member

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    Yes. I should have mentioned it. I thought about it, but felt I was just rambling on. Let me fix that...
    Oh, I can't edit once someone replied?
     
  17. ghostwheel

    ghostwheel Junior Member

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    Important to mention: The HV battery is 12 years old, but I have been reconditioning it. I think I did two deep discharges (last one was several years ago - each with 2 or 3 discharge cycles), and a few calibrations. The 12V battery is new. I also have an inverter installed in the car (off while I did the tests), and there was a 12V cooler in the car when I was camping, but not when I did the tests that gave the graph above.
     
  18. PriusCamper

    PriusCamper Senior Member

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    Educate yourself about the truth Sam:
    BU-807: How to Restore Nickel-based Batteries - Battery University
     
  19. PriusCamper

    PriusCamper Senior Member

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    I hear ya... If you only got one battery pack and it's in your car and you depend on your car daily, it's next to impossible to find time to get that work done. At this stage it comes down to what your budget allows. Throw $2K at and be done with it as an issue for a decade or spend way less and buy a second pack and rebuild that and then just swap the packs out when problems arise.
     
  20. PriusCamper

    PriusCamper Senior Member

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    There's always more to learn about Battery Science: Articles - Battery University
     
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