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Roadster SoC <-> Mileage <-> Mode decoder

Discussion in 'Roadster: Technical' started by gregd, Jun 14, 2017.

  1. gregd

    gregd Active Member

    Dec 31, 2014
    Hi folks,

    I'm trying to wrap my mind around the Roadster's Ideal Miles display, and how that corresponds to the state of the battery in Range vs Standard vs Performance mode. This is for both charging and driving. So, 6 combinations. By digging through logs and poking at the car, I've figured out a bunch of it, but would like for someone to look over my shoulder and let me know if I've missed something. (Apologies in advance for the stream-of-consciousness format.)

    I'm defining SoC as the battery charge you see in the logs. I believe this is a raw number, i.e. it's not dependent on charging or driving mode. I see that my car charges to about 84% in Standard mode, and 94% in Range mode. I'm also guessing that 0% SoC in the logs isn't actually brick-level, but that there is an undefined (and inaccessible) reserve below this. There is a drive to "-1%" SoC in the logs from before I bought the car, and it (obviously) survived. (Thankfully, they charged it later that evening.) But it also appears that the top few % of battery capacity is not hidden, as I never see more than 95% in the logs.

    The calculation of Ideal Range appears to be directly related to "available" SoC, with "available" dependent on mode. 11% SoC at the bottom in Standard mode (see derivation below) becomes zero range. Range mode takes it all the way to zero (but with an always hidden anti-brick reserve below that). I also note that a full charge in standard range mode displays as 95% on the battery icon. So, the battery icon is approximating the "Available" SoC, not actual. I'm guessing that 5% is the region where Regen is still available to charge the battery.

    For my car, with a CAC at about 140, Standard mode charge gets me to an Ideal Miles of 167, Range mode to about 214. When I first got the car two years ago, it arrived with 6 miles of ideal range (and a very nervous delivery driver...), and the logs show it was at 14% SoC at the time. This was in Standard mode (he didn't know about Range mode). Since a range mode charge to 94% SoC is about 214mi, 1% of SoC nets about 2.2 miles of range. Cross-checking, 1% of 140AH is 1.4AH, times 375 volts is 525wh. At 235wh/mi (from the car's trip display), that should net about... 2.2 miles. Yea!

    With 1% of SoC equating to something around 2 miles of ideal range with my battery, that means the zero ideal miles is at about 11% SoC in Standard mode, based on the delivery event of 6 miles at 14% SoC. I think that's about what I've read elsewhere, that standard mode hides the bottom 10%-ish of the battery.

    Further cross-check, if I charge the car to 166 in Standard mode, and then flip to drive in Range mode, I start out at 189 miles Ideal, so 23 miles different. At 2 miles per SoC%, that's about 11%, which squares against this hidden range at the bottom when in Standard mode.

    Ok, so I think this all makes sense.

    Now for Performance mode. The only effect I can see is that when charging in Standard mode, and flipping to Performance to drive, the battery icon "full" indicator drops down by one pixel lower than Range mode (approximately 84%). The ideal range number doesn't change compared with Standard mode, indicating that the majority of the effects of putting the car in Performance mode are on the actual driving behavior (acceleration, temp limits), not battery charging SoC nor hidden areas for discharge. Yes? Why then the reduction in apparent battery charge with the icon? Is it cosmetic, assuming that I will be driving hard, or is there something else going on?

    Final calculations... At 2 miles per %SoC, the additional 10% SoC at the top that is available for charging in Range mode should equate to another 20 miles of range. Add to this the 11% (22 miles) at the bottom, suggests that the total range difference between Standard and Range mode should be 42 miles. So, 167+42 is 209, which compares to 214 that I see with the car. Close enough?

    Did I miss anything in this?


    p.s. For extra credit, could someone walk me through this analysis for a 3.0 battery?
    • Informative x 1
  2. dhrivnak

    dhrivnak Active Member

    Jan 8, 2011
    NE Tennessee
    That sounds right to me and is similar to what I see. I will find out about the 3.0 in about 2 weeks. The base thing with the 3.0 is you get about 50% more miles.
  3. Tesla 940

    Tesla 940 Member

    Nov 2, 2011
    Long Beach, CA & Taos, NM
    Based on nearly 10k miles of recorded history - my weighted average wh/m is 229. On trips in excess of 100 miles the wh/m has ranged from 227wh/m to 232 wh/m.

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