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Chewing up time savings at the Supercharger

Discussion in 'Model S: Driving Dynamics' started by WSE51, Dec 4, 2014.

  1. WSE51

    WSE51 Member

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    Has anyone published real world examples (rather than theoretical calculations) of the tradeoff between shorter driving times going fast vs. longer recharging times at Superchargers?

    My overall charging speed on two round trips between Barstow CA and Nephi UT (i.e. Barstow, Las Vegas, St. George, Beaver and Nephi Superchargers) averages about 175 Rated Miles per hour. Of course, charging speed depends upon starting Rated Miles and how full I want the battery to end up at. I typically roll into a Supercharger with 30 to 80 Rated Miles and charge enough to have 40 Rated Miles above what EV Trip Planner says is required for the next leg. I have an "A" battery which limits my supercharging speed to some extent. Also I can't control whether I will have to share my SC circuit with another car, though I rarely have to except for Barstow. It is impossible to control for wind and temperature changes in making comparisons in just two round trips of 700 miles.

    Given all these limitations on the quality of my testing, my experience is that Rated Miles used increased so much going 85 mph vs. 65 mph that over 90% of the time savings on the highway were given up to increased charging times at the Supercharger. Net net I gained very little time by going so fast. As I result of my testing I plan to drive no more than 70 mph, not only is it generally safer (as long as you are not holding up traffic) but the perceived time is shorter because you are stopped at the SC for less time.

    If anyone else has done this comparison, please let me know.
     
  2. Zextraterrestrial

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    I think at high speeds you need to figure out how many miles per hour you are actually burning and if the supercharge average rate that will give you enough charge to get to the next destination is more than what you use in rated miles per hour, drive faster.

    Probably somewhere between 85 and 100 mph(could probably use the range graph and figure it out) and trying to get to 0 for the beginning of the SC charge would yield the fastest trips. not sure exactly what speed and wind and temp would make a bit of a difference.


    for example, say if you have a 120 mile trip between SCs and you go 60mph you will get ~ rated and not need much charge, go 75 save ~1/2 hour use a bit more power but that time charging will more than make up for it
     
  3. WSE51

    WSE51 Member

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    Zestraterrestrial, thanks.

    Here are my back-of-the-envelope calculations for a 120 mile trip:

    65 mph: 110.8 minutes driving time, 128 Rated Miles used, 43.9 minutes charging time to replenish the 128 RM, total time 153.7 minutes
    Net driving + charging speed is 46.8 miles per hour

    85 mph: 84.7 minutes driving time, 166 Rated Miles used, 56.9 minutes charging time to replenish the 166 RM, total time 141.6 minutes
    Net driving + charging speed is 50.8 miles per hour.

    going faster saved 26 minutes driving and cost 13 minutes in charging. So I was wrong in my initial post to say 90% of the savings were chewed up in charging time, in this example it is 50%
     
  4. AoneOne

    AoneOne Member

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    Instead of keeping your battery full, you should only charge enough to get you to the next supercharger at your chosen speed. If you could always charge from an empty battery, then you would get the full 340 MPH charging rate and the extra 38 miles of charge will only take about 7 minutes extra:

    65 MPH: 128/170*30 = 22.6 minutes charging, 110.8 minutes driving, total of 133.4 minutes, for a net speed of 54 MPH.

    85 MPH: 166/170*30 = 29.3 minutes charging, 84.7 minutes driving, total of 114.0 minutes, for a net speed of 63 MPH.

    In practice, planning to supercharge from an empty battery isn't wise, so you can't quite get this result. Instead, you just stop charging as soon as you have a comfortable buffer for your chosen speed and range. Drive relatively fast, arrive with modest remaining range (20-50 miles), and recharge enough to do it again. So long as the superchargers are reasonably close and convenient, there's little reason to slow down.
     
  5. LMB

    LMB Member

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    A related issue is how much time is burned getting off the highway to and from the Supercharger. We see times as low as a couple minutes (Connecticut Superchargers in highway service plazas) to fifteen minutes or more for Superchargers that are not directly on the main route (e.g. Hamilton, NJ). When the chargers are close enough together, it makes sense to skip the latter even if charging times are somewhat longer.
     
  6. WSE51

    WSE51 Member

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    Aoneone, I agree with you in theory, but in practice it hasn't worked that way for me. First, only having a 20 mile buffer is risky, especially in the winter where winds and temperature can really impact projected miles. I plan on a 40 mile buffer (above EVPlanner-projected RM consumption) and still have had a few nail-biters. The higher the buffer, the more RM in the battery at the start of charging and the slower the charge rate. Second, I find real world charging speeds are much slower ... I do have an "A" battery pack ... it starts out at nearly 300 RM/hr but very quickly declines and averages about 175 RM/hr for me, and I am not generally charging beyond 200-210 RM.
     
  7. ckessel

    ckessel Active Member

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    The short answer is, drive as fast as you can to minimize total trip time if you've got SC access for the trip.

    Without superchargers, the rule of thumb was to minimize total trip time don't drive faster than you can charge. So, if you can only charge 30 miles per hour (standard 40 amp/220) then you want to take it easy in the slow lane. It's not quite true since wind resistance isn't linear, but it's close enough for the normal 40-80mph speeds you use on road trips.

    Superchargers charge at hundreds of miles per hour...so, by the rule of thumb you should drive hundreds of miles per hour :tongue:
     
  8. MikeL

    MikeL some guy

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    quote of the day
     
  9. Blu Zap

    Blu Zap Grinning member

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    "Superchargers charge at hundreds of miles per hour...so, by the rule of thumb you should drive hundreds of miles per hour" [​IMG]

    Dang, just what I needed, another justification to speed! :tongue:
     
  10. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    That is true until you go deep into the taper. The conversion for rated mph to DC kW into the battery is 300W-hr/mi. 100 rated mph charge rate happens when the taper is down to 30 kW into the battery, 75 mph=>22.5 kw, 50 mph=>15kW, etc.

    It's only heuristic, and not a proven optimal solution, but if you roughly drive the speed of your ending charge rate in the taper, you minimize total travel time. Do not use the displayed, session average mph charge rate for this; you must use the calculated, instantaneous charge rate of kW/0.3 or V*A/300.

    Once you need to charge deep into the taper, you can reduce overall drive time by slowing down. Tesla seems to be using the guideline of 133 miles as the max distance between Superchargers, so you can get there faster by speeding up, and don't have to slow down (much).
     
  11. AoneOne

    AoneOne Member

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    For me, in practice, driving faster has worked well. For you, driving slower and charging less is more pleasant than the converse. There's no rule that we all have to enjoy the same driving style. Of course, we all have different preferences and are subject to varying external conditions, including the spacing, convenience, and amenities associated with the specific superchargers, the weather, use of climate control, and the use of range mode, the size, type, and condition of the battery, and the personal preferences of the driver and any traveling companions. With all of these variables, there's no one optimal way to drive.

    So, drive slower if you like. No one will complain that you're not maximizing some arbitrary measure of efficiency.
     
  12. AndreyATC

    AndreyATC Member

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    Simple rule:
    For Supercharger-only trip, drive as fast as it permits, assuming you have enough charge to make it to the next SC
    Charge speed is always faster than driving
    For non-Supercharging trip, drive as efficient as possible, since charging is always slower than drive (CHADEMO might be exception)
     
  13. liuping

    liuping Active Member

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    #13 liuping, Dec 5, 2014
    Last edited: Dec 6, 2014
    In a recent trip back to San Diego from the Bay area, I had to decide while at Tejon Ranch, whether to plan on stopping at SJC or driving all the way home.

    In theory getting the minimum charge and stopping in SJC was best, but there are real world variable that come into play. It takes time to get to/from the SJC site (adds about 10 minutes total), it's frequently full (add unknown minutes), or at a minimum 1/2 full meaning you get a slow charge (so the theoretical 15 minutes turns into 30), etc.

    I decided to spend the extra time at Tejon to get to 250 RM (It was full, but no one was waiting. I would have moved if anyone else showed up).

    Sometimes spending a known 30 minutes is better than possibly saving 10 minutes or loosing an additional 30 at the next stop.
     
  14. Chris TX

    Chris TX Active Member

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    The only reason I might drive a little slower, or at least right at the speed limit, is if I could skip a Supercharger stop altogether. Otherwise, I drive fast.
     
  15. jtg

    jtg Member

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    Ok you Tesla experts and math majors. Please explain what one should do pulling a trailer. Assume a big hit on range due to extra drag and mass. It would be similar to winter driving with a headwind. At what point should one slow down assuming Superchargers every 130 miles apart? Thanks for your answers.
     
  16. ckessel

    ckessel Active Member

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    I doubt the answer is going to change. You'll use more energy per mile, but you're still going to be better off driving faster than slower if saving time is the goal.

    The only time driving faster while towing a trailer might get dicey is uphill in bad weather. And I say that only because you might need nearly a full range charge to reach the next SC at all.
     
  17. Zextraterrestrial

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    for graphical confirmation of this
    IMO the most you really ever want to charge to at an SC is ~ 90%SOC if it is enough range to get to the next SC going 'fast'

    charge.jpg
     

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  18. Owner

    Owner Active Member

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    Having visited a ton of superchargers, I've tried to do some of this myself but it is almost impossible to really do this on the road with extreme accuracy because of changing weather and simply traffic or not.

    I think as others have pointed out your probably arriving with a bit too much of a buffer, which applies to all batteries.

    But I think part of this is just experience on each leg of the road. And those in Utah vary so much. It so hard to say "drive 75 mph" or anything. There was a stretch in Utah where the speed limit was 75 but I was trying to get to Las Vegas when there was no supercharger in St. George....

    So rarely can you drive at speed X on any road... Accurate rules just don't quite work.

    I don't personally like to go down to zero. Its a bit too nerve racking even if I know the "tank" has more juice.

    So my best advice would be to maybe target more like to arrive with 25 miles left so you'll have a buffer and put on some music or a great book.
     
  19. ckessel

    ckessel Active Member

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    FWIW, I always target 150% of the mileage. SCs are typically 120 miles apart, so I target 180. Now, of course, often I'll do better Wh/mile than that so I'll arrive with 40-50 miles, which just means less time at that SC to get to the 180 mark. If it was a brutal trip (weather/elevation) I'll pull in with sub 20-mile range, but there's no nervousness since I easily know 30-40% through that leg if I need to slow down. As of yet, at 150%, I've never come close to worrying about making it.
     
  20. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    150% is a good, safe number most of the time. Conditions that can burn through that are big climbs (add 6 rated miles per 1,000 feet net gain), high speed (much over 75 mph can really suck on the battery), and frigid weather. EV Trip Planner accounts for all of these if you put in the correct parameters. Items not in EV Trip Planner that can get you are headwinds, wet/slushy roads, not using standard regen on big descents, and sloppy, twitchy driving.
     

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