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Confusion regarding battery capacity

Discussion in 'Model S' started by sumitkgarg, May 28, 2018.

  1. sumitkgarg

    sumitkgarg Member

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    Hello all... I have some confusion regarding how much usable battery capacity my 2017 75kWh RWD car has based on the trip information from my drive today.

    We left with 100% charge, did not stop anywhere on the way and arrived the SC with 5% charge remaining. The trip stats for this trip as reported by the car are:
    - 4:10 minutes drive time
    - 230 miles driven
    - 63 kWh battery used
    - 274 Wh/mile average.

    If I were to trust these numbers, since 95% capacity of my battery equals 63 kWh, 100% calculates to 66.3 kWh as compared to 75 kWh (or close). What is it that I am missing, or is the true usage battery capacity on a 75kWh actually close to 66kWh?

    Thanks!

    IMG_9829.jpg
     
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  2. DJP31

    DJP31 Member

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    Interesting figures and I can’t give you a helpful answer, other than to comment that the usable battery is supposed to be 72.6kWh.
     
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  3. thetees

    thetees Member

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    Usable capacity is 72.6kWh. Your confusion arises because the 274 Wh/mile reported does not account for power used by systems other than the motor i.e. HVAC etc.
     
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  4. cezdoc

    cezdoc Member

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    Hi there, I've done similar calculations with my RWD MS 70 so not identical but it may be helpful to compare notes.

    First up my data gives a fairly consistent nominal capacity, so as far as I'm concerned the car does track the vast majority of energy use so long as it's turned on & the driver remains in their seat. You already said that you didn't stop on the way so the car should have tracked all the energy used - and the trip stats confirm this with matching figures for "since last charge" & "since 12:41" (wow, there's no way I could keep going without a break for over 4 hours!). Even if you had stopped you won't miss much energy use unless the car is running a heater or cooler at full power or is left for several hours (you can check this by comparing the battery SOC immediately before and after a driving break).

    Second up my data: I've been doing the same as you: using the trip stats alongside a note of the battery SOC before/after single continuous trips, then multiplying up to 100%. I've logged about 14 trips where I used over 50%, and the back-calculated capacity has varied from 63.7 - 65.7 kWh for my MS 70, with an average over 2 years of ~64.5 +/- 0.5 kWh. My biggest single trip was from 99% to 4% using 61.6 kWh giving a nominal capacity of 64.8 kWh. I should also add that my mileage is similar to yours - now around 15,000.

    Conclusions? Well you might hope for 5 kWh extra capacity when comparing your 75 to my 70 so your nominal 66.3 kWh although higher than my averages does look a bit on the low side - but could be within normal error bars (especially as these are different generation battery packs). I've read that the faster the battery discharges, the less efficient it gets (more losses due to heat?). Your 274 Wh/mi average at <50 mph for the trip is pretty gentle though so I would have expected your capacity to be on the higher side/less affected by thermal losses. I wouldn't therefore be unduly concerned by these figures.

    Is this important? Probably not. The Nav energy estimates are very accurate and provide plenty of warning to slow down etc. if it looks like you don't have enough energy to reach your next stop, and things like weather, load, driving style, tyre pressure etc. have more impact on range than the nominal battery capacity of the car. Still, you could try tracking trip data for more long trips to see how much scatter there is in the battery capacity estimates.
     
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  5. Robotpedlr

    Robotpedlr Supporting Member

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    Are you sure this is correct? Because wh/mile goes up when using the Heater. If havoc use was not included in wh/mile, you would expect it not to change.
     
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  6. DJP31

    DJP31 Member

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    I don’t think that’s correct. If I turn the heating on from the app for a few minutes before getting into the car, when I do get in and move off the Wh/m is through the roof, e.g. 1,200 Wh/m. This then settles down as the journey progresses.

    In fact thinking about it some more, I’m sure @mgboyes tested this by turning his heating on for 1/2 hour or so, and then once the car had been driven a few hundred yards the display showed something crazy like 12,000 Wh/m.
     
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  7. SuisseDriver

    SuisseDriver Member

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    That is a very interesting thread and indeed something I wanted to ask.
    I do not have the exact figures with me but I did exactly the same type of calculation on my december 2017 S75D and I found that 100% ~ 67-68kWh, sometimes 69kWh but not more.

    I do not understand either as we are speaking of cases like now in spring where outside temperatur is 15-20°c, HVAC usage is therefore super light and limited and there is no way that auxiliairies systems are consuming 3-4kWh. So I cannot understand how to link the 72,7kWh of "available" capacity with what really seems to be available for round trips...
     
  8. sumitkgarg

    sumitkgarg Member

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    I am not sure if this is correct. IMO the Wh/mile usage does factor in cooling/heating energy usage just based on my experience...
     
  9. sumitkgarg

    sumitkgarg Member

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    Btw - how does regeneration factor into these trip stats? Because there is a non-trivial amount of regeneration involved in a 4+ hour trip, the 100% battery should be more than 72.6 kWh (assuming that is the usable battery available), no? Anyone who knows how this works?
     
  10. MorrisonHiker

    MorrisonHiker S 90D 2018.46.2

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    Hopefully someday there will be a software update that breaks things down more so we get a better idea of where energy is being used.

    In my friend's old 2013 Ford Fusion Hybrid, it showed an image of the car which indicated how much was used for driving, climate, etc.

    While an image isn't really needed, seeing a breakdown of the numbers would be useful for many of us.
    [​IMG]
     
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  11. sorka

    sorka Active Member

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    Such as? It accounts for HVAC? I find there's about a 1.5% discrepancy between reported usage vs actual difference in starting vs ending capacity.

    To the OP, consider using TM-Spy or one of the other apps that will tell you exactly what the the BMS is reporting for usable capacity which does not include the anti brick buffer.
     
  12. BlueRocket

    BlueRocket S90D HW2 MCU1 42.3

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  13. sorka

    sorka Active Member

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    Also, keep in mind that the value that is reported as kWh consumed uses 100% of regen collected to offset that value but if the regen efficiency is only 90%, then you've lost 10% of that value reported as a discrepancy between what was collected and what the battery actually has left.

    This can vary a lot *even* on level ground? Why? Take my wife vs me. When I drive on level ground, I never use regen. If I have to slow down, I'll do so through coasting rather than dipping into regen like if slowing from 70 to 65 MPH.

    My wife on the other hand will vary her throttle constantly to keep the speed she wants so she's constantly regen braking just a little every 20 to 60 seconds.
     
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  14. SuisseDriver

    SuisseDriver Member

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    Ok, I have one data point fresh from yesterday from my wife doing a trip:
    • usage 90% (100% to 100%)
    • 306.6km
    • 58kWh
    • 189 Wh/km
    It was mostly highway (~270km) with speed between 120-125km/h (Switzerland) and 130-140km/h (France) and the rest was city driving (and 3 stops).

    It means 65kWh usable which is really small considering the car is from december (6 months old) and I am also puzzled by those numbers.
     
  15. cezdoc

    cezdoc Member

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    I don't think this looks unusual - you're estimating 65 kWh vs 66.3 kWh estimated by @sumitkgarg. As you made some stops there will have been some energy usage missed by the car which would make up some of the gap.
    The thread link suggested by @BlueRocket above says a 75 battery has 72.6 kWh usable but there is also plenty of discussion on why the car's own estimate of power used won't match this, in particular the way it calculates power use by integrating energy data means it _always_ underestimates total power used due to rounding errors. And @sorka points out that charging inefficiencies during regen could be another source of "missing power" - you have a D model which can regen more than the RWD versions. I hadn't thought of this before - it would be interesting to see what happens on a trip with regen set to low & ideally with lots of steady driving on flat ground.

    Scaling/comparing my own figures, I have a 70 which should have 68.8 kWh usable but the highest I've ever calculated is 65.7. The 75 is actually 72.6 kWh (+3.8 kWh more than a 70) usable so using my benchmark of 64.5 kWh I'd expect a MS 75 to average out around 64.4 + 3.8 = 68.3 kWh.
     
  16. Don85D

    Don85D Member

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    I'm not sure that fussing over this discrepancy has much value. As long as our 85D provides the utility that we need it's working as planned. We charge the car using sunshine which makes extreme efficiency not a concern. Mostly I watch battery capacity in km on the dash and compare that to projected capacity in km on the screen. They are never the same but always explainable at a high level. The comparison is the first level of warning that battery capacity may need attention during the remainder of a journey.

    I also use the motorcycle technique for fuel capacity where a charging stop is planned if the battery gets close to a 'reserve' capacity. I use 100km as the reserve in anticipation that a charging station selected may be out of service and range to reach the next station is required.

    Older motorcycles didn't have a fuel gauge but by resetting the odometer at each fill up and having a reserve amount in the tank there never was a problem.

    We run our Model S the same way as our motorcycle and any battery degradation over the life of the car is not worth worrying about at least until the car cannot perform its daily duties. As for long distance travel, free supercharging for life and an ever expanding network of chargers also makes reduced battery capacity over time not that important.

    I'm expecting our 85D to be a long term keeper. There has been no noticeable degradation in battery capacity over the past 3 years and if it doesn't match 85 kW total capacity exactly I'm not concerned.

    Just another approach for your consideration.
     
  17. sumitkgarg

    sumitkgarg Member

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    Sorry, but rounding errors with what? Even if I assume that the remaining battery in my case was 6% (rather than 5%), the usable capacity is still 67 kWh, not 72.6 kWh. Also, assuming that the trip stats factor in any regen, the usable capacity gets even worse. 63 kW battery used with regen would be less than 63 kW without regen, no?
     
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  18. sumitkgarg

    sumitkgarg Member

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    Sure, but that is not the point of this post. I am genuinely trying to understand how the trip stat calculations work and why doesn't it add up to whatever Tesla advertises.
     
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  19. murphyS90D

    murphyS90D Member

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    Part of the battery is reserved to prevent complete discharge which would likely destroy the battery. You have no access to that part of the battery. You will never get the nominal battery rating for your use. If the battery is at 100% regen is not possible and you better be ready with the brakes to stop the car.
     
  20. redy

    redy Member

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    Isn't that the reason why there is only 72.6 Kwh usable? Or am I understanding it wrong?
     
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