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Rated miles needed at X Wh/mi

Discussion in 'Model S: Battery & Charging' started by tomp, Jun 29, 2015.

  1. tomp

    tomp Member

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    Hi all,

    Please let me know if I am doing this math correctly. I am taking a road trip soon, with 6 people and a roof box. My testing is that my energy usage is 375Wh/mi at highway speeds with the box on and A/C running pretty good. To be safe, I'm going to assume 400Wh/mi.

    What is the most accurate way to convert that to rated miles? In other words, if I assume I travel 100mi at 400Wh/mi, how many rated miles should that take up? My understanding is that rated miles assumes 308Wh/mi, but I've also heard 300Wh/mi and 29x Wh/mi. Thus, I'm assuming I will need 100 x 400/308 = 130 rated miles. Key question is: what number to use? 300? 308? something else?

    Thanks!
     
  2. Oba

    Oba Member

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    It would matter which vehicle that we are talking about.

    81kWh usable / 265 miles EPA = 305.7 Wh per mile
    70kWh usable / 240 miles EPA = 291.7 Wh per mile
    60kWh usable / 208 miles EPA = 288.5 Wh per mile
     
  3. Ampster

    Ampster Member

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    #3 Ampster, Jun 29, 2015
    Last edited: Jun 29, 2015
    No need for conversion. Rated miles is just an assumption used for planning. One assumption you made is that you will use 400 Watts per mile. Use whatever assumption you want. If you choose 400 Watts per mile that will equate to 150 miles on a MS 60 (60,000/400)
    An alternate way would be to convert Watts per mile to its miles per kWhr equivalent (1000/400=2.5) and multiply that by the kWhr capacity of your battery. In the later case it may be a more familiar formula like miles per gallon.
     
  4. tomp

    tomp Member

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    I don't think this is accurate because not all 60kWh (or 85 or 70) are available to use. I think Oba has it right.
     
  5. Ampster

    Ampster Member

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    #5 Ampster, Jun 29, 2015
    Last edited: Jun 29, 2015
    60,000 was an example based on Oba's statement of 60 kWh usable. You can use any capacity that you believe.

    My point was that there is no need to convert the OP'S assumption about 400 Watts per miles into rated miles. Any range calculation is a calculated guess based on the assumptions that you use for capacity and Watts per mile (or miles per kWhr if you prefer to use that).
    Actual results may vary. LOL
     
  6. Duma

    Duma Member

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    I am with Ampster. You can do the calculation directly based on battery capacity without considering rated miles. Much of the "controversy" about the Wh/mi corresponding to the rated miles displayed on the dash relates to how much of the nominal battery capacity (60/70/85 kWh) is available for use before (a) the car shows 0 mi range / 0% battery or (b) the car stops to protect the battery from permanent damage. Based on what I have read in these forums, Tesla may have reduced or removed most of the buffer associated with (a) that was originally available so that (a) and (b) are almost the same with the current software.

    A good rule of thumb appears to be that only 95% of the nominal battery capacity is available for use - 81 kWh for the 85 kWh battery and similar for other capacities. For the 85 kWh battery at 400 Wh/mi that gives a range of 81000/400 = 202.5 miles. Of course you will want to build in a 10-20% buffer to account for the unexpected, from strong head winds to detours to problems with the planned charging location (ie don't plan to use 100% of your estimated range to get to your next charging point.) If your trip involves significant elevation changes, you will want to factor that in as well.

    There are two resources that can help you beyond the basic range calculation. The first is in the car itself, assuming that you have the navigation option. In the energy app there should be a second tab that says trip. This only works once you have a trip to a destination set up. It will show your expected % charge remaining when you reach your destination. This estimate includes elevation changes. Unfortunately, the initial estimate appears to be based on the nominal energy usage for your drive configuration, but as you start driving it corrects for your actual energy usage. If the estimated %charge at the destination drops below your comfort level then you have two options. One is to add a charging stop sooner, say if you were planning to skip a supercharger because you expected to have enough range to reach the next one. The other is to slow down slightly, since even a drop of 5-10 mph at highway speeds dramatically increases range. This is a great way of getting early warning that something that you overlooked is causing you to use more energy than planned, allowing you to take corrective action before you have a crisis.

    The second resource is EVTripPlanner (https://evtripplanner.com/index.php). You can configure this for your car's configuration, including load. The estimates take into account elevation changes (but not wind or rain). Also it does not include your roof box. The best way to model the roof box is to select a planned speed that is higher than your actual planned speed. Plugging your test results into the planner should allow you to determine a good adjustment factor. The planner reports estimated Wh/mi for a projected route, so just adjust to get the observed usage for your test route. This tool is very helpful if your route has significant elevation changes.

    Sounds like you are doing all the right planning to have a successful trip, even if superchargers are a bit sparse along your route. Good luck and have a great trip.
     
  7. tomp

    tomp Member

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    Thanks for all the input. I think my question simply boils down to what is the actual battery capacity. This has been discussed a lot elsewhere, but per Oba (as well as your 95% estimate), it seems I can assume that is ~81kWh.
    We will see how the trip goes. If I'm not on-track - slow down is the thing to do :)

     
  8. Rocky_H

    Rocky_H Member

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    Here's the other tip I'll give for traveling. The "trip" tab of the energy app is good, but it only displays in percent, and I have a hard time thinking in percent of battery. On the main tab of that energy display, it shows your average consumption rate for the past 5, 15, or 30 miles. Then on the right, it will show an estimate of the number of real miles that you can go based on that average consumption rate. That's only looking at your last 30 miles average, so it's not going to have visibility into really big picture elevation things, but it does give you are pretty good estimate of real miles left based on your speed, weather, and heat or A/C use that are affecting your current driving.

    I've used that compared to the miles left to go on Nav to give me a good idea of how much margin I have related to what speed I should go. So aside from the pre-planning estimates, I've found this to be a pretty good fine-tuned estimate to use as I'm doing the actual drive.
     

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