Observations about Discharge Current Limit

Discussion in 'Prime Technical Discussion' started by m8547, Dec 31, 2019.

  1. m8547

    m8547 Active Member

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    I've been watching the Discharge Current* Limit (DCL) and Charge Current* Limit (CCL) on Hybrid Assistant for a while.

    I had my car parked outside at the airport park and ride last week and got back on Sunday, so the battery was cold (right around freezing) and the charge was "stale". My drive home from the park and ride has a big hill that will usually trigger the engine to start under these conditions, but in this case it was after 11pm and no one was on the road (literally no cars behind me for 2 miles), so I decided to try to make it over in EV mode.

    I did make it, but my speed dropped to about 40mph by the top of the hill in my attempt to keep it in EV mode (speed limit is 65). Part of the way up, I noticed the green power bar on the Hybrid System Indicator quickly rise at one point despite constant pressure on the accelerator pedal, which I think indicates the charge current limit sharply dropped. A graph of the drive on Hybrid Reporter shows this sharp drop from about 48kW to about 22kW. I wonder if there is a parameter the car is measuring (like an individual cell temperature or voltage) that causes this sharp drop? Calling for more acceleration than the battery allows will usually trigger the engine to start, so I backed off on the accelerator the rest of the way up the hill.

    This morning on my commute to work it was chilly and I had the heat on. I have to go over the same hill in the other direction, and normally if the car was plugged in (so the battery is warm and fully charged) it will always do it in EV mode. Even on one 9 degree F morning I made it in EV mode. But if I do any errands in the morning (as a result hitting the hill with a lower SOC), or if the battery is colder, the engine will usually start. But this time the engine came on unexpectedly, and frustratingly close to the top of the hill. I didn't have Hybrid Assistant running unfortunately, but I started it after and the battery was relatively warm.

    Anyway, there's no real purpose to this thread. I just want to understand how the car works. And it seems like it would be a good idea to avoid having a frozen engine suddenly start and immediately rev up while driving up hills, if it's possible to predict these "hard starts" and modify my driving behavior to avoid them.

    * Even though these are "current" limits, the value is reported as power in kW, which is more useful anyway.
     
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  2. m8547

    m8547 Active Member

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    Here's a screenshot.
     

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  3. john1701a

    john1701a Prius Guru

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    There really isn't such a thing as a "hard start" with the gas engine, even when it is extremely cold. Watching the RPM, you can easily confirm that. Stress is minimized with electricity to keep it low.

    What you'll want to watch with respect to kW draw is the temperature of the battery-pack. When it drops below freezing, the draw is restricted. The battery-heater helps prevent that, even in the most extreme winter conditions.
     
  4. m8547

    m8547 Active Member

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    The engine behavior seems to be a bit different depending on the circumstances when it starts. For example if it simply runs out of EV mode power and switches to HV mode, the battery keeps driving the wheels for a minute or two until the engine warms up. But if it turns on the engine while still in EV mode (big green EV light goes off), it might spend a lot less time warming up. One example would be in EV Auto where it will turn on the engine to help you accelerate quicker (I have not tested it with a cold engine). And this case where the battery output power is limited, so it turns on the engine to immediately start driving the wheels to help you maintain speed. I haven't watched this kind of start closely on Hybrid Assistant yet since it is somewhat random when it will happen, but maybe I'll try to intentionally cause it one day.
     
  5. vvillovv

    vvillovv Active Member

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    Primes are complex machines and behavior varies a lot sometimes.

    I have noticed with our Prime that the transition from motor to engine is rough when the engine is called for power immediately upon startup across the entire temp range and SOC level. If it's possible to back off the go pedal enough at ICE startup the transition is always barely noticeable for me anyways.

    The other thing more noticeable for me in cold is that if the traction battery is cold, so is most everything else in the car that uses it. The heat pump, pack heaters as well as - preconditioning while plugged in - help somewhat, but the only real way to warm up some of the electric drives parts in low temps is to drive slowly at the beginning and ramp it up.
    Probably very similar when it's hot, but that is a subject for another time.

    I keep searching for a more effective way to get the drivetrain closer to it's comfort zone, but so far that is all I'm been able to come up with.

    Rain and snow offer their own challenges for the Prime too. Things as simple as rolling resistance can have a significant effect on the cars efficiency.

    Than there is the wind, if it shifts to head on and gets stronger in the middle of a typically easy EV trip, one can't always expect to make the full trip in EV.
    as always ymmv
     
    #5 vvillovv, Dec 31, 2019
    Last edited: Dec 31, 2019
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