Inside Electrify America’s plan to simplify electric car charging

Discussion in 'Tech: Batteries, Charging, Alternative Energy' started by EyeOnRivian, May 6, 2019.

  1. Hmp10

    Hmp10 Well-Known Member

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    I am always a bit suspicious of the data my Tesla puts out.

    I have two identical Passport radar/laser detectors installed in my Tesla and in my Honda. They give a speed readout derived from radar readings. The speed indicated on the Passport matches the speedometer of my Honda exactly all the way up the speed range. In the Tesla, however, speed indicated on the speedometer lags behind the speed indicated on the Passport, with the gap growing as speed increases. By 65 mph indicated on the Tesla, I'm seeing 61 mph indicated on the Passport. I have also been able to calibrate speed against the stationary speed readouts that the police often put along the roads here. My Passports always match the speed flashing on the roadside readout, but the Tesla never does.

    Of course, the effect of this is to make you think you're going faster and farther in the Tesla than you actually are. Inadvertent or intentional? Not sure.

    My brother drives a Model 3 and loves it. When I tell him this, it drives him crazy, and he tells me it has to be an anomaly with just my car. (He first tried to tell me I was imagining it until I put him in the car to see for himself. He doesn't have a radar detector and can't check his car.)
     
  2. ajdelange

    ajdelange Well-Known Member

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    #102 ajdelange, Sep 5, 2019
    Last edited: Sep 6, 2019
    From time to time I like to remind readers that I am not either. I've got lots of engineering experience under my belt but none of it automotive and that's why I find this stuff so interesting.


    No, you cannot measure the charge and it not the charge remaining that is of interest - it is the usable charge remaining. What you can measure is the battery open circuit voltage (even when current is being drawn from the battery) and the amount of charge you have put into the battery or drawn out of it. As you put more charge into the battery it's voltage goes up and as you withdraw current the voltage goes down. You pick two voltages, one to represent full and the other to represent empty. You charge of discharge the battery to the empty voltage and from that point add charge recording at each added charge increment the voltage. You do this until you reach the full voltage. The charge added is the capacity of the battery - the amount of charge it to to gets its voltage from empty to full. You record that charge and now divide all the charge numbers is your data table by it. You have a table that runs from Ve to Vf in voltage against charges from 0 to 1. At some time in the future you charge the battery from V1 to V2. You look in your table and find that requires an addition of ∆Q < 1 units of charge and that would be the amount of charge you would have to add to a unit capacity battery to raise the voltage from V1 to V2. If it in fact took 95 times ∆Q then you know the capacity of your battery is 95 and once you know the capacity you can predict range, calculate wH/mi numbers and determine percentage of capacity any voltage corresponds to. This is I believe, how Tesla proceeds but I do not know that.


    That one will keep me awake tonight!

    The car can measure (directly) battery volts and battery amps pretty accurately, can convert them to numbers in an A/D converter (if the sensors don't already do that) multiply those numbers and accumulate. The result is an accurate measurement of the energy (Whr) taken from the battery over some period of time and the car can measure distance really accurately (I'll get to that in another post). Thus its WH/mi data is really good and can be used for prediction of ETA at a next waypoint as long as nothing the car doesn't know about (it does know about hills - it doesn't know about an imminent thunderstorm) pops up. So that covers en-route prediction. When it comes to determining how efficiently you drove it then uses "rated" Wh/mi, not the Wh/mi it estimated for the last segment you drove.


    If they did that it reflects that they don't understand how the car measures its power consumption.

    I did note that you phrased the charging measurement description in a way that hinted that they used the power delivered to the car during charging. This is more than the power delivered to the battery as the charger is only about 90% efficient. One could do things like that if one were concerned with calculating his power bill but most of us are more interested in knowing about the power drawn from the battery per mile driven than how much grid energy it took.
     
  3. ajdelange

    ajdelange Well-Known Member

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    #103 ajdelange, Sep 5, 2019
    Last edited: Sep 6, 2019
    Always a good thing!

    Lot's of questions here. First, the Tesla has a GPS receiver (and it may be Glonass capable as well) plus a cell phone receiver and WiFi. Between all those it is capable of very accurate position and speed determination. When my SO goes to the grocery store I can see which parking spot she is in. I don't know how a radar detector would measure speed using radar as they are passive devices. I would not be at all surprised if they incorporated a GPS/Glonass chip but I don't know what to think about the radar detector disagreeing with the speedo. The police radar is different. I have, whenever I notice them, found them to agree pretty closely with what the car displays.

    Were I you I think I'd get my SO to drive while I compared the speedometer to Waze or the direct access to the GPS chip feature of my smart phone. Were I to find a 5 mph discrepancy I'd be at the SC.

    Now believe it or not there are auto speedometers that do not use GPS for odometer or speedometer functions. Lots of them, I think. I don't know whether this is the case with Tesla or not. If the car has to be reprogrammed when tire size is changes that's a good indication that it is. I think your concern may be with the distance used in Wh/mi computations. I am pretty sure those are GPS derived as I get a little map of every trip from TeslaFi.
     
  4. Hmp10

    Hmp10 Well-Known Member

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    My Escorts do have GPS chips. They reference databases of traffic camera locations and sound an alert when nearing one. They also give me a voice alert when they lose or reacquire the satellite signal. I thought they were using radar to calculate speed, but apparently they're doing it with GPS.

    By the way, I'm here to learn, too, and very much enjoy your posts. I'm still not sure most drivers would duplicate your experience with range in your Tesla, but I do have a streak of stubbornness.
     
    Mjhirsch78 likes this.
  5. ajdelange

    ajdelange Well-Known Member

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    None will because at this particular moment I am king of the hill (though I don't know why) at least among subscribers to Stats:
    IMG_1195 2.png

    But the histogram (wish he'd present the cumulative data) shows that about 1/4 of users are getting better than rated (100% efficiency) performance.
     

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