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Can't get the numbers to add up

Discussion in 'Model S: Battery & Charging' started by sillydriver, Oct 15, 2015.

  1. sillydriver

    sillydriver Member

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    I apologize if this has been covered elsewhere, but I couldn't find anything. I have a P85D -- or at least it performs like one. The percentage charge meter implies the pack is less than 70 KWh. See the photo below. I went for a 40 mile drive after charging to 90% as I always do. It was reading 89% as I left the garage. As you can see, I had 71% charge after using 11.9 KWh at the end of my drive an hour later. So I used 89 - 71 = 18% of capacity. Dividing the 11.9 KWh total energy used by .18 = 66 KWh total capacity, not 85. This is an example, but the relation between total energy used and percentage capacity used has been very consistent. This time I used sport and range mode to see if the effect was the same, and it is. I normally drive in insane mode without range mode. I used 18% of the battery to drive 39.6 miles, implying a 220 mile range on 100% charge. Using my trip meter-A 319 KWh/mi it implies a 100% range of 220 * 301/319 = 207 miles. Not good, I think. I haven't pursued this up until now because I use the car only for day trips under 100miles round trip, so in a way it's academic. I have never had the charge meter below about 40%, and I have never used a supercharger. But my question is whether anyone else has seen this kind of relationship between charge percentage and total energy used in what is supposed to be a 85 KWh car, or knows of an explanation. Thanks!

    IMG_2472.JPG
     
  2. Father Bill

    Father Bill Member

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    An MS 85(P/D/RWD) all have a part of the battery that is reserved. It prevents the battery from being deep discharged and damaged. I think I read somewhere that the actual usable capacity is somewhere in the range of 76 KwH. That may help your calculations.

    Peace,

    Father Bill
     
  3. Johan

    Johan Took a TSLA bear test. Came back negative.

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  4. jerry33

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

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    Although it's been said before, I guess it bears repeating as people keep posting it. This image is someone's guess about the A battery pack. It likely doesn't correctly map the battery.
     
  5. sillydriver

    sillydriver Member

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    Thanks to both of you. It looks like 100% on the charge meter should correspond to 76 KWh, so my P85D is falling short at an implied 66 KWh, but not as bad as I feared. I wonder whether the charge meter behavior could be non-linear, so at high charge levels 1% is a smaller number of KWh, and at low percentage charge levels it is a higher number. I'm not sure why they would do that but it could explain the remaining discrepancy, since I keep my car at high states of charge.
     
  6. eye.surgeon

    eye.surgeon Member

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    You're thinking about it too much. You have plenty of range for what you need, just drive and enjoy, the battery will become like the fuel tank in your ICE, you don't obsess over the size of it.
     
  7. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Still bugs me though, to this day, that Tesla will give you a detailed and accurate Wh/mile (or km) number but when you try to use that number to do calculations you quickly end up realizing you're nowhere near being able to access the full capacity of your "85kWh battery".

    I'm not sure what would have been better, but if they could start over one way to do it would be to sell it as the Model S 208 (60kWh) and 265 (85 kWh) models (with the designator being EPA max range. Thinking about it though this is not a very attractive or good solution.
     
  8. sillydriver

    sillydriver Member

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    Before I retired I was a portfolio manager and analyst with a large investment firm. Before that was physics at MIT. I've spent my entire life 'thinking about it too much', and I can't stop now! I try to have a proper Buddhist attitude of non-attachment to meaningless things, but it's hard for me to be driving along and notice an inconsistency between numbers on the screen in front of me and then not think about them too much. I think it's an absolutely wonderful car, by the way.
     
  9. S'toon

    S'toon Knows where his towel is

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    If it helps, all EV manufacturers do this. Most just don't tell you specifics of how many KWh, just the % of "state of charge."
     
  10. Troy

    Troy Member

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    #10 Troy, Oct 15, 2015
    Last edited: Oct 15, 2015
    Your numbers don't add up. Is it possible that you parked somewhere for a while after you read 89% and before you read 71%? If you repeat the test, I recommend a few things:

    1. Take a screenshot at the beginning of the trip when you start driving.
    2. Take a screenshot at the end of the trip when you stop driving. The car must never be parked between these two screenshots.
    3. It looks like you can't use trip meter A or B for the test trip. In that case you need to eliminate the time after the car stops charging and you start driving. The car consumes lots of energy while parked. Any consumption that happens while in park mode, is never included in any trip meter but it will reduce the percentage or rated range displayed. The consumption happens because of battery thermal management, connectivity and battery balancing. Unplug while charge continues and immediately start driving.

    I think the 75.9 kWh number is accurate. That means your average range after you started using trip meter A is 75900 Wh/ 319 Wh/mi= 238 miles.
     
  11. apacheguy

    apacheguy Sig 255, VIN 320

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    Well, they can start by accounting for all loads including vampire draw.
     
  12. sillydriver

    sillydriver Member

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    In know about vampire draw, which is why I charged the car, took a 40 mile round trip to a store (living out in the country, things are far away) and checked the numbers as I drove into my garage a little over an hour later. I didn't take a picture at the beginning because I thought this was just going to be another one of many tests for my own consumption (this time in range mode). But when I pulled into my garage I thought 'why not -- I'll put this on the forum' and snapped a picture.

    Since v7.0 is now scheduled to download at 2 am overnight, I expect I'll play with that for a while and then do another test with the new UI and with pictures at both ends. I am not too disturbed by this anomaly, but want to have data before I take the car in for service (which will be the first time I take it in for service) to get the ludicrous upgrade and LTE upgrade. I assume that will happen in the next few months. If the numbers are still out of whack, I'm going to talk about it with them because I don't want to pay $7500 to upgrade the fuse, etc. in a pack that is significantly out of spec. That is actually my main concern.
     
  13. Polly Wog

    Polly Wog Member

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    #13 Polly Wog, Oct 15, 2015
    Last edited: Oct 15, 2015
    I don't think any of us really know exactly how Tesla decides what loads get incorporated into the energy meter and when. We know that the motor energy is always used, but we also know that most of the ancillary systems (A/C, Heat, Radio, MCU, etc.) are not incorporated while you aren't moving. Regardless, there are a few ways (some previously mentioned) to get a more accurate calculation. Go on an extended drive while tracking your distance and energy usage - all while not using any ancillary systems (no A/C, no Heat, no Radio), and not stopping. You'll get the most accurate results in this way.

    When we buy a Tesla, we receive a specific battery capacity with a specific EPA rated range. That EPA rated range is not based on the total battery capacity, but rather the "usable" portion of that capacity. As many have stated, that usable capacity, for the 85, is around 76 kW (some places identified as 75.9 kW, but we have proof that it can be at least 76.8 kW). For the standard 85, that translates as 290 wh/mi to get to 265 miles rated range.
     
  14. Troy

    Troy Member

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    Polly Wog,

    The difference between rated range and actual range is partly because the EPA tests calculate all capacity available to the user including after you go below 0 rated range. EPA does not get involved in what the car displays. Imagine a scenario like this:

    The car gets 265 miles EPA score. The car displays 265 miles on the dashboard but after you drop to 17 miles, the digits turn to red and a message pops up saying charge immediately.

    Tesla could have implemented things that way. But instead they chose to take the portion from 17 rated miles to 265 rated miles and display this as 0-265 and hide completely the 17 miles at the beginning. On this page, above that famous battery graph Nick Howe explains this well.
     
  15. apacheguy

    apacheguy Sig 255, VIN 320

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    @Troy - It is widely accepted on TMC that there is no zero mile protection. Charge now means charge now.
     
  16. Polly Wog

    Polly Wog Member

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    @Troy, I'm sorry if that happened to you. My experience has been different. Referencing a different example (see picture below), if I take 76.8 kWh and divide by 265 miles, I get 289.8 wh/mi. I also can say that when I have been able to maintain an average of 290 wh/mi while driving with no accessories on (no radio or climate control), I have been able to get exactly the rated range (100 miles of driving while using 100 miles of rated range). I haven't taken my Tesla down to 0 rated miles, but I have been as low as 9 with no issues.

    Tesla500mile.jpg
     
  17. Troy

    Troy Member

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    apacheguy,

    There are two different things.

    1. The zero mile protection zone should be considered non existent in terms of relying on it. Therefore the consensus on TMC is that people don't say things like "there is 17 rated miles range after zero" to newcomers because the range could be much less. It is better to assume there is no additional capacity because it appears to be different in each car. It could be just 1 mile or 17 miles or something in between. Why is this number different in different cars? That's unknown.

    2. However zero mile protection zone exists in terms of how EPA tests are performed. The test shows all capacity until the car stops moving. The way the tests are performed, they put the car on a dyno for 90 minutes and measure consumption. Then the consumption is extrapolated to all capacity that is available to the driver.

    - - - Updated - - -

    You forgot to include "Imagine a scenario like this" part. I was talking about a fictitious example to describe how Tesla could have implemented the 265 EPA score.
     
  18. Candleflame

    Candleflame Member

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    The uaeable capacity will drop the faster you drive as there will be some kwh lost due to thermal loss. Available capacity at 60mph is probably 76kwh but this will drop to maybe 73kwh at 100mph or so.
     
  19. sillydriver

    sillydriver Member

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    I think you are right about this. As you say, they don't include auxiliaries when the car is not moving. I have sat many times with AC and sound waiting to pick up my kid at school, and there is no consumption on the total energy meter. It seems odd that it would still be excluded when you are moving, but the test is to do just as you suggest. I can't do the proper experiment immediately, but soon. Also, I got v7.0 overnight, so I have other experiments to do.
     
  20. cantdecide

    cantdecide Member

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    Measuring battery percentage is a difficult scientific/engineering problem. The numbers we see are best guesses. A few weeks ago my 100% was 252, now back to 259. You might get closer to the 75kwh calculated figure if you range charge a couple of times, then take it right down near 0... To reduce some of the random error and battery balance error.
     

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