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Electricity 101 Course...

Discussion in 'Model S: Battery & Charging' started by jstepy, Oct 3, 2014.

  1. jstepy

    jstepy Member

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    Hello All,

    Forgive my ignorance but I am a geologist that works for big oil and I'm trying to understand my energy usage screen on my new Tesla.

    Now I know some basics like if I use a 100 watt lightbulb for 10 hours that would equal 1kWh of energy on my meter etc. at home right?

    So when I am averaging 350 wH/mile what is that telling me exactly?

    Does my 85 pack battery theoretically contain 85kWh?

    So if I was averaging 1000 wH/mile I could essentially do 85 miles?

    And what's the whole ideal vs. rated thing.

    Thank you for your help
     
  2. rolosrevenge

    rolosrevenge Dr. EVS

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    So 350 Wh/mi is telling you that for every mile you go you are using the equivalent energy of a 100 W bulb for 3.5 hours. Your 85 does in fact mean 85 kWh. If we were averaging 1000 Wh/mi you'd be driving like Brianman and could only go 85 miles. Ideal miles is how far you could go with the current energy if you were going flat at 55 mph. Rated miles is how far it estimates you can go based on your driving behavior. Hope that helps.
     
  3. jstepy

    jstepy Member

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    Thank you Rolos... Just needed some confirmation on what I thought.

    So mid 3's wH seems to be pretty average for most drivers from what I've read.
     
  4. bluetinc

    bluetinc Member

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    Minor correction. "Rated Miles" is not based on your driving behavior, it is a number generated via the EPA testing done, and is about 300 Wh/mi (Ideal is 265 Wh/mi). The only number show that is based on your driving is the predicted number shown on the energy graphs right side.

    Peter

     
  5. jstepy

    jstepy Member

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    May I ask one more question? I have been getting a ton of questions from co-workers who are all big petrol heads and I want to make sure I have all the info down.

    So the yellow kW section on the speedo... if I am driving with the yellow section on the speedo at 20 kW I am essentially using at that time 20,000 watts correct? So if I drive 1 hour with it always on 20 (hypothetical I know) I would essentially use 20 kWh of my pack or just shy of 1/4 of the 85 battery... is that all correct?

    Thanks again
     
  6. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    That would be correct assuming you never slow down and never regenerate any electricity. In practice it's a lot better than that (unless every start is a rocket launch).
     
  7. Kbsilver

    Kbsilver Member

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    And don't forget 85KWH is the technical capacity of the larger battery pack, but you cannot access all of it. The absolute top and bottom ends of the capacity are walled off to ensure longevity of the pack. If I'm not mistaken the USABLE capacity is more like 75kWh (range charge until driven until the car stops running).
     
  8. AoneOne

    AoneOne Member

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    One more thing: one horsepower is approximately 750 Watts, so that 20kW you're using is roughly 27 HP.
     
  9. brianman

    brianman Burrito Founder

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    I wish. The firmware won't let me.

    - - - Updated - - -

    Keep asking, and welcome to the conversation. :)

    - - - Updated - - -

    Another minor correction. From at least a few handfuls of data points, it's been suggested that the "rated consumption rate" can vary across cars and across firmware. It's generally aligned with the EPA 5-cycle test, but we've seen some cars that seem to use ~288 Wh/mi for rated and others as high as ~310 Wh/mi. It's kind of annoying that it seems so... inconsistent.

    - - - Updated - - -

    Since it hasn't been mentioned yet...

    In some markets like (I think) Europe, the "Ideal" word isn't used in the UI. The word "Typical" is used, IIRC.
     
  10. jstepy

    jstepy Member

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    Too funny brianman...

    So another question since you said it was okay. If my battery is completely full and I take a trip and on that the trip meter says I used 20 kWh it's not like it will take exactly 20 kWh from my home to charge it up again... lower amperage etc. is less efficient... so it may take 25 kWh on my home meter to fill up that 20 kWh used?

    I was thinking if I always charge at Super Off-Peak rates that I know are always 9-11 cents a kWh (depending on season) I could easily just take that number from the trip meter and multiply it by the rate and have a good idea on how much it's costing me to charge etc. If I am using an HPWC with a 70 amp breaker (twin charger) is the resistance and loss to charge that big a factor?

    Hope that all made sense.

    Thanks!
     
  11. breser

    breser AutoPilot Nostradamus

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    I've seen a lot of people assume an efficiency of 80% for charging, so your numbers look fine to me (whatever you see in the car divided by .8). Maybe mknox will stop by I've seen him say that he carefully meters all his power usage and he might have a more precise figure. I haven't gotten around to installing metering on my HPWC circuit so I can't give you better figures.
     
  12. mgboyes

    mgboyes Member

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    Various points.

    The usable capacity of the 85kWh battery is about 76kWh, so if you drove at 500Wh/mi you could travel 152 miles.

    The Wh/mi figure you see in the car is only calculated while the vehicle is "on" i.e. power consumption when it's standing idle (vampire load) plus the effect of remote enabling the AC, etc do not count to that figure. Combined with charging inefficiencies you can expect to put 25% more energy into the car than you use while driving it, i.e. if you see 350Wh/mi as your average usage then you are probably actually buying 438Wh/mi of energy from your supplier.

    Charging at a high rate like 70A is more efficient than slow charging from a standard outlet. But most of the charging losses are due to heat generated in the charger units and the battery, plus potentially the power needed to run the AC system to dissipate that heat (the charger units and the battery are both liquid cooled, but it's climate dependent whether or not they need to activate while you're charging).

    Your idea of taking the usage figure from the car and adding 25% should be a pretty decent way of estimating your fuel costs.

    All the different "rated" and "typical" and "ideal" range figures that the car reads in its various different markets are just simple calculations by taking the remaining usable charge in the battery and dividing it by a fixed Wh/mi figure. So in the UK we get 245 "typical" miles, which is about 310Wh/mi, whereas the US has 265 "rated" miles which is about 286Wh/mi.
     
  13. arg

    arg Member

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    Are you sure of that? If we're talking about the values shown on the trip counter screen ('since last charge', 'tripA' , 'tripB' with Wh/mile in each section), I see the opposite of what you are saying.

    Every time I start out, the first couple of miles show a very high Wh/mile, but soon settles down to something close to the average (and no, I don't usually do a 0-60 launch out of the end of my driveway!). So it appears to amortise vampire losses/stationary use of AC over the next few miles.

    The other week just after applying the 6.0 upgrade, this was much worse than usual - I was wondering if something was wrong, as my Wh/mile was still hugely above normal after I'd been driving for 5 minutes; it eventually came down to something reasonable after 10 miles or so. Presumably this was down to the car being fully powered up during the time spent loading the upgrade, and also perhaps me sittting in the car for a few minutes with the aircon on playing with the new features.


    Yes - and the main reason for this confusing difference is that the EU rating scheme produces such an absurdly high number that it's slightly above the US version of 'ideal'.

    Hence you've always got one 'optimistic' setting and one 'reasonably achievable' setting, but 'rated' is always as rated by the local rating agency and fills whichever of those roles it most closely fits.
     
  14. ThosEM

    ThosEM Space Weatherman

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    I *really* wish there were an *option* to display kWh or % on the main energy state of charge (to three digits accuracy). I noticed that the cars used for the Tesla team cross country charging time record run were displaying in %, so this is clearly something preferred by tech-heads like myself. There's a switch in the settings that purports to switch between energy and distance units, but it only applies to the rate of charging display.

    Truly, I have never seen so much confusion generated by a simple "fuel gauge," as seen in the infamous NYT/Broder test. Everyone knows "your mileage may vary" and that electric cars have important sinks of energy other than just driving. But putting up a display of miles or km really seems to shut that awareness off and produce all manner of angst about the nature and reliability of the algorithm used to generate those numbers.

    Very interesting if the "non-driving" energy use gets "charged" against the ensuing driving by the trip calculator. I had wondered about that initial burst of energy and thought perhaps it was caused by battery conditioning. That is certainly the case when starting out with cold soaked (and probably hot soaked) batteries, but the effect continues in spring and fall, so this seems quite credible.
     
  15. CatB

    CatB Member

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    Glad you brought this up. A few weeks ago, I had a 90% charge, drove 178.8 miles using 54 kWh with average energy usage of 302. I had 38 miles of range remaining. Which tells me I don't really have a reasonable range of 250 miles unless I count on driving below zero. Also, I had no idea there is almost 11 kWh of battery reserved.

    As I am also not an EE type, is there a simple way to figure out if/when you may need to check with the Service Center about rebalancing the battery? I read enough to decide that's not something I want to do on my own.
     
  16. Electric700

    Electric700 Member

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    I think it's closer to a 1-5% efficiency loss, so you'd be at a minimum of a 95% efficient charging process. Let's say you are using a 120 kW Supercharger to charge your car, and gained back 170 miles in 30 minutes. Using a value of 350 Wh per mile (actual real-world Model S average energy usage example), that translates to 59.5 kW. But Supercharging used 60 kW (120 kW x 0.5 hours), which is approximately 99% efficient since (59.5 kW)/(60 kW) gives us this percentage. All very respectable numbers I think!
     
  17. heems

    heems Member

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    Amen. ++1. That's what an energy capacity meter should show primarily. Relative to full how much juice do I have. Optionally I can convert to kWh or miles or hills or gallons or whatever... I can also see the rate of consumption and make necessary adjustments. I'd say anyone coming from ICE is inherently trained that way. Rarely I look at the ICE fuel gauge and try to guess my miles left. It's the % available and the rate the needle moves that matter mostly for me.
     
  18. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    #18 Cottonwood, Oct 4, 2014
    Last edited: Oct 4, 2014
    I've found the following numbers to be reasonable for energy vs Rated Miles in my P85 sig with an A battery. Of course, your mileage may vary. Also, remember any energy use while the car is not on, is not calculated or displayed; this includes things like vampire loss and pre-heating/cooling the car.

    • 290 Wh/mi for using Rated Miles from the battery.
    • 300 Wh/mi for putting Rated Miles into the battery with DC from a Supercharger. This corresponds to about a 3.3% charge/discharge loss. 290/96.7% is about 300 Wh/mi.
    • 333 Wh/mi for Rated Miles into the battery from an AC source. This corresponds to 90% conversion efficiency from AC to DC. 300/90% is about 333 Wh/mi.
     
  19. Rheazombi

    Rheazombi Member

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    So where is the thread on hypomiling techniques? lol
     
  20. brianman

    brianman Burrito Founder

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    Interesting.
     

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