TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

Battery % vs kWH

Discussion in 'Model S: Battery & Charging' started by RAW84, Jul 23, 2015.

  1. RAW84

    RAW84 Member

    Joined:
    Oct 6, 2014
    Messages:
    340
    Location:
    Boston
    I'm sure this has been discussed before, but I'm trying to get a formula that relates SOC to kWh. Basically, I want to know if I charge from x% to y% how much kWh did I add back into the battery?

    I have a P85D btw, but it would be nice to know this for all battery sizes.
     
  2. jerry33

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

    Joined:
    Mar 8, 2012
    Messages:
    12,753
    Location:
    Texas
    Because the upper and lower limits that Tesla allows aren't known, no formula is possible. (However there is plenty of speculation)
     
  3. RAW84

    RAW84 Member

    Joined:
    Oct 6, 2014
    Messages:
    340
    Location:
    Boston
    I'd be interested to read some of the speculation.

    Given that we can get the kWh used (while driving), seems we should be able to back into that a fairly good approximation by keeping idle time to a minimum.
     
  4. Matteo

    Matteo Member

    Joined:
    Jul 6, 2015
    Messages:
    167
    Location:
    EU
    #4 Matteo, Jul 23, 2015
    Last edited: Jul 23, 2015
    Hi RAW84,

    In the middle of this page there is an image that shows usable capacity is 75.9 kWh for the 85 kWh pack until the rated range shows zero.
    Tesla Model S Buyers', Delivery and Owners' Guide

    Let's say you charge to 90%, does this mean you have 0.9*75.9= 68.31 kWh energy stored in your battery until rated range shows zero? No it doesn't mean that because you need to consider degradation too. There are two ways to calculate degradation.

    Method 1: Calculate capacity from rated range in North America or from typical range everywhere else.
    Method 2: Calculate capacity from trip consumption data.

    The following survey by forum member Merijn calculates your battery capacity using both methods if you answer all questions including the optional trip data questions.
    Tesla Battery Survey (old name MaxRange) - Google Sheets

    On that document there is also a small calculator if you open the sheet "Range Types Explained" and scroll down a little.
     
  5. RAW84

    RAW84 Member

    Joined:
    Oct 6, 2014
    Messages:
    340
    Location:
    Boston
    Thanks, Matteo! Very helpful!

    I wish Tesla just recorded energy input...
     
  6. jerry33

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

    Joined:
    Mar 8, 2012
    Messages:
    12,753
    Location:
    Texas
    The problem with using rated range is that Tesla changes the algorithm frequently. Ideal range appears to be more stable.
     
  7. mknox

    mknox Well-Known Member

    Joined:
    Aug 7, 2012
    Messages:
    8,561
    Location:
    Toronto, ON
    I put together a little spreadsheet for myself to track this. I went back to my iPhone photos and found some shots of the screens or captures of the app and from the photo got the date to get some history as well. I had some other data points from trips that I'd logged. Some of my entries have only Rated while others have only Ideal, but now, every so often, I update it with both. What I can say for myself is that other than the general downward trend, my Rated Miles don't seem to change at all. I have even noted, contrary to the reports of others, that my Rated Miles number does not change even by 1 over any firmware update I've had on my car. I only have a bit of history with Ideal Miles, but don't see much variation there yet.
     
  8. llavalle

    llavalle Member

    Joined:
    Sep 9, 2013
    Messages:
    554
    Location:
    Somewhere around Montreal in Quebec, Canada
    I do the following math :


    1-First note the %SOC
    2-set your car to ideal or rated (like mknox, I have yet to see a difference in my car)
    3-Open the energy graph and note the 3 set of values (don't know them in miles but on my car it's 10, 25 and 50km). Note the average wh/km (or mi) and km or mi projected.
    Example :
    191 Wh/km for 245km projected 186 Wh/km for 251km projected
    202 Wh/km for 231km projected

    191 Wh/km for 245km = 46795Wh left
    186 Wh/km for 251km = 46686Wh left
    202 Wh/km for 231km = 46662Wh left

    4-Round of the number : around 46.7kWh


    In that case, my car was at 59%SOC. So, according to this calculation, 59% (which might be rounded or simply truncated, I can't say) is around 46.7kWh left in the battery.

    Now, since the %SOC does not have digits, you have to do this a few times to get a real guesstimate...

    My spreadsheet tells me that each 10% SOC is between 7.7 and 7.9kWh.. and total usable kWh in my 85D is between 77 and 79kWh. This seems to match the rating of my car. The 85D's rated Wh/km is around 180Wh/km*. Rated range is 435km. 435km * 180Wh/km = 78300Wh or 78.3kWh.

    I think this is a solid estimation. Then, you can start speculating : How much energy there really is in a 85kWh pack? How much is kept in reserve for anti-bricking... we have no clues. We'd have to accuratly measure the total energy available on a brand new model S cell (with a very slow discharge rate) then multiply that by the number of cells

    *How do I know this? I did 2 things : pixel count the screen to estimate the height of the "rated" line in the energy graph according to the scale on the Y axis AND drove the car until the RATED and Projected were the same value --> average was then 180Wh/km.
     
  9. jerry33

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

    Joined:
    Mar 8, 2012
    Messages:
    12,753
    Location:
    Texas
    I suspect you get those results because of the distance you drive every day. The variable ones (like mine) are from those who don't use a big percentage of the battery on a regular basis.
     
  10. RAW84

    RAW84 Member

    Joined:
    Oct 6, 2014
    Messages:
    340
    Location:
    Boston
    So I've been logging the trip kWh as soon as I see it change battery percentage...any reason this wouldn't be accurate?

    I saw 0.7 kWh per 1%, now obviously, due to rounding, I can't know where that lies between 0.65 and 0.74999 kWh. However, if I increase the delta in percentage reduction observed, I should be able gain another digit, and that should be accurate enough for my purposes.

    This should negate any issues with battery degradation (it would just have to be retested every so often).

    Or am I missing something?
     
  11. llavalle

    llavalle Member

    Joined:
    Sep 9, 2013
    Messages:
    554
    Location:
    Somewhere around Montreal in Quebec, Canada
    If you do this while driving : all good... But the trip kWh only records while you drive and does not account for vampire drain and possibly HVAC while parked (unsure about that one)
     
  12. RAW84

    RAW84 Member

    Joined:
    Oct 6, 2014
    Messages:
    340
    Location:
    Boston
    Yes, I'd do this only in one trip. And I'd record right after seeing the % change, so I know I'm always at the upper bound of a % bin.
     
  13. Matteo

    Matteo Member

    Joined:
    Jul 6, 2015
    Messages:
    167
    Location:
    EU
    RAW84,

    Turning range mode on or off effects rated range displayed. That might be the reason. On average whatever rated range is displayed when range mode is off needs to be multiplied by 1.013 to estimate what rated range would be if range mode was on. Or you can just turn on range mode and see the difference yourself.
     
  14. RAW84

    RAW84 Member

    Joined:
    Oct 6, 2014
    Messages:
    340
    Location:
    Boston
    I'm looking at battery %, that shouldn't be affected by range mode, right?
     
  15. Matteo

    Matteo Member

    Joined:
    Jul 6, 2015
    Messages:
    167
    Location:
    EU
    #15 Matteo, Jul 24, 2015
    Last edited: Jul 24, 2015
    RAW84, you are right. I misunderstood the topic. The percentage shouldn't be affected. Your method could be a good one to record degradation over long time. If I understand correctly your method is like this: Set range display to percentage instead rated range. Start driving. The moment percentage drops lets say from 89 to 88%, record kWh consumption. It might say something like 2.4 kWh. At the end of the trip the moment it drops from let's say 77 to 76% record kWh consumption. Assuming it is 11.5 kWh, it would mean 11.5-2.4=9.1 kWh consumption equals 88-76= 12%=0.12 of battery capacity until percentage shows zero (without going into reds). Therefore 100%= 9.1/0.12= 75.83 kWh
     
  16. RAW84

    RAW84 Member

    Joined:
    Oct 6, 2014
    Messages:
    340
    Location:
    Boston
    ^^^^^ Exactly^^^^
     
  17. Johan

    Johan Took a TSLA bear test. Came back negative.

    Joined:
    Feb 9, 2012
    Messages:
    6,890
    Location:
    Drammen, Norway
    #17 Johan, Jul 25, 2015
    Last edited: Jul 25, 2015
    Seems legit. Only problem is you're likely going to find that the 1% between say 89 to 88 is different than the 1% between 19 to 18. Why do I say this? Well, the main reason Tesla aren't telling us the actual kWh in the battery isn't only that they don't want to come clean that it's not 85000 Wh (which is what they're marketing) but just as much the fact that they don't know. That's right: THE CAR DOESN'T KNOW. What is displayed as either rated/ideal or % is just an estimate that the car is doing based on voltage.

    You have to understand the basic electrical facts: as SOC drops voltage drops, but not necessarily completely linearly or completely predictably. This has to do with the fact that voltage measured will be affected by many variables: are you measuring under light or heavy load? At what temperature? Is the temperature rising or falling? Is the load continuous or more on-off? Etc.

    If the car displayed voltage we would have access to the most raw data there is. You will get this on the display plugging in to a SuperCharger, but I'm unsure if what is displayed is pack voltage or supplied voltage. In reality you can't know because as the SC hooks up the voltage of the system is pushed up and a positive delta in voltage gets created (with the SC side being higher voltage than the battery side) - this is what creates the flux of electrons/energy in to the battery.

    Tl;dr: Forget extremely detailed SOC calculations: they are inherently impossible to perform.
     
  18. PoweredByRain

    PoweredByRain Member

    Joined:
    Jan 5, 2014
    Messages:
    647
    Location:
    Victoria, BC
    #18 PoweredByRain, Jul 25, 2015
    Last edited: Jul 25, 2015
    I have an 85 kWh Model S. I once drove 400 km, with 40 km showing remaining, and... I wish I could remember the exact kWh consumed, but I remember calculating that it implied the total capacity from fully-charged to 0 range remaining is 75 kWh. This figure has been used by a number of people. I was a bit disappointed to learn that we really don't have access to 85 kWh, but that's life.

    So, to answer your original question: 1% = 0.75 kWh.
     
  19. Matteo

    Matteo Member

    Joined:
    Jul 6, 2015
    Messages:
    167
    Location:
    EU
  20. RAW84

    RAW84 Member

    Joined:
    Oct 6, 2014
    Messages:
    340
    Location:
    Boston
    So, I started at 100% at my house and ended at 24% at a Supercharger this past Saturday (also my first supercharging trip! Yay!)

    Em5dvb1.jpg
    In the table I'm using the same percentage as the base (99%, 98%, 97%, etc), the column outside is comparing to the line above (typically, 1 percent increments)

    I'm seeing 0.735 kWh/%. I'm not sure what that means in terms of the value in the sheet Matteo posted. I have a P85D, which lists the useable energy at 75.9 kWh. I'm not sure if that means 100%-0% = 75.9 kWh, or if there's some energy available beyond 0% (I should also note that my December 2014 P85D has 100% at 250, not the 253 listed for a brand new one)

    Johan, those are some great points. I should note that the main reason I'm after how much energy is going into the car is so I can track things like degradation, vampire loss and charging efficiency. I don't think it needs to be super accurate.
     

Share This Page