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Thread: Need Actual Roadster Charge times for Optimum Road Trip Calculations

  1. #1
    Senior Member smorgasbord's Avatar
    Join Date
    Jun 2011
    SF Bay Area

    Need Actual Roadster Charge times for Optimum Road Trip Calculations

    I'm trying to work up formulas for calculating how long it'll take me to drive long trips that require at least one charge mid-route.

    Figuring out the Wh/Mile at a given RPM is easy. Figuring out how much of the kW flowing into the car actually end up increasing the kW stored is an unknown to me. Does anyone have information to help me out?

    For instance, the public HPC in Atascadero runs at 240 volts, 70 amps. That's 16.8kW, but if my Roadster's battery's SOC is 22.4kWh, I know that after 2 hours the car won't be full (56kWh). The car charges faster at low SOCs. I know there all sorts of factors, such as the battery temperature (cooling the battery while charging takes away some of the power that would otherwise go into the battery). I'm not looking for exact answers, just good enough to help me plan.

    Any practical information to help me formulate how long it'll take to charge the Roadster given a reasonable number of reasonably known variables would be much appreciated. I'll post my final equations for review and comment here.


  2. #2
    Join Date
    Nov 2011
    Denver, CO
    Ideal velocity formula based on charging rate:

    Model S -> v = (14.286 * q ) ^(1/3) - 0.97
    Roadster -> v = (14.286 * q ) ^(1/3) - 2.46

    q = charge rate in W (not kW)
    v = velocity to travel for minimizing time (mph)
    where q> 1000W, trip requires charging and it takes no time to stop and hookup charging unit.
    I also assumed that it would only be charged to 80% so you wouldn't wait for topping off that last 20% which is much slower.
    so for 16,800 W charging idea velocity is ~59.7mph

    If you need to go a little further than 380miles, then finding out the charge rate that last 20% could make a big difference on optimal velocity.

    I also created a spreadsheet including that it takes about 10 minutes to get off highway and hook up charger.
    Brute force calculations
    @ 380 mile row, column 60mph shows highlighted data:
    "9h(2.7[1])=42 AA"
    9hrs total trip time
    2.7 hours needed for charging
    [1] one stop min
    42mph =average overall speed
    AA = color coding to highlight cell
    TotalHrs (chargeHrs [number of pitstops]) = average mph (A,B,C,D - shading indicator)

    I created a Google Doc template too

  3. #3
    Senior Member smorgasbord's Avatar
    Join Date
    Jun 2011
    SF Bay Area

    Is the charge rate at 20% SOC the same as at 70% SOC? Given a choice of charge points, it might be better to run the battery down before charging, as it might charge faster at lower SOC.

  4. #4

  5. #5
    Join Date
    Nov 2011
    Long Beach, CA & Taos, NM
    My experience is that you get about 75% into the battery. A 70amp charger will get you 50-52 Ideal Miles Per Hour. Based upon my detailed observations the rate of charge (Standard Mode) does NOT drop off until you are nearly full. I have not done detailed observations of the drop off rate in the Range Mode.

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