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"Official" recharge times

Discussion in 'Roadster: Technical' started by TEG, Nov 19, 2008.

  1. rugbymonk

    rugbymonk Roadster #287; S Sig #14

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    Real World recharge times

    So in a real 350-mile road trip scenario (my NJ to Cape Cod trip next Friday) where I have to choose where to stop to recharge, let's assume:
    1. I want to minimize total trip time
    2. an HPC at the 100 mile mark, and
    3. various NEMA 14-50's along the way (e.g. at the 150, 200, and 250 mile marks), Which place(s) would you stop at to minimize "down-time"?
    How about if we change assumed locations? There should be a simple Excel macro to allow us to plug in these various inputs, but I am lousy with Excel. Any of you guys got a good macro to explore these possibilities?

    I assume any HPC 70-amp stop wins over a 50-amp to minimize down-time, but I'd like to see the math or better yet, a plot on chart. Also, would you charge with setting for "maximim distance" if using a NEMA 14-50, or is it not worth the additional charge time when you are less than 220 miles away?


    Finally, car speed: assuming I cannot safely go below 60mph on the 95% highway trip, is it right that even with a single HPC top-off along the way, there is no higher speed that would make sense to minimize the overall trip time (road speed*distance + charge time)?
     
  2. edo

    edo Member

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    I'm not sure if you are saying you can't go less than 60 mph
    assuming no air conditioning (adjust Anc. Adder)
    also assuming 90A rated 70A draw HPC
    assuming 40A 240V available draw from the NEMA 14-50s (Do your homework on this! many places don't have service matching the plug!!)

    According to the Straubel spreadsheet that I previously posted here, you could:

    Travel at 30mph and never stop, driving for 11hrs 40min(probably "risky")
    You could drive at 65 mph (or faster) for the first 100 mile leg, and stop to charge about 2 hrs, then travel the remaining 250 miles at 40 mph (1h32m+2h+6h15m)=9h47m
    (theoretically you should be able to go 50mph for 5hrs instead of 40mph for 6h15m, but there is no margin at all there (result is 1h32m+2h+5h=8h32m)
    Same first leg and charge, traveling then at 60mph, stopping after 200mi, then charging just enough and finishing trip. (1h32m+2h+3h5m+1h34m+46m)=8h57m

    3h34min charge time. I don't think you will find a way to do less if you want to go 60mph.

    I must say, the HPC is not in the optimal location. Just 50 miles would have made all the difference. Note that charging at 56mph is very much preferred. Any charging at less than your driving speed is going to more than double your time!

    if you're taking 95 to 195 to the cape, you can't legally go over 55 mph for a lot of that trip (the speed limit through the ess curve in Providence is 45, for example)
     
  3. bolosky

    bolosky Member

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    There's one additional thing to consider here. The charging rate depends both on the available power and the state of charge of the battery. When charging on the HPC, as the battery gets more and more full its charge rate will drop off. So, you can't assume that you'll get full benefit from the HPC at 100 miles out.

    The optimal charging schedule for your trip is actually pretty clear: stop at the HPC and charge until the current draw drops below the 40A that you'll get from the 14-50s farther down the road. Then, drive until you feel like stopping and charge up enough that you've got enough charge to make it and have sufficient reserve that you're not nervous.

    The real truth about how to have the shortest time trip is that you could draft behind a big truck. It would improve your mileage really significantly and so reduce your charge time. However, I can't recommend that you do it, because it's dangerous and may be illegal. But, it makes a huge difference at 65-70 mph.
     
  4. tomsax

    tomsax Member

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    The tables shown earlier in this thread don't tell you the full story. By just using the time for the full charge, you're combining the charging at the current limit with the time spent close to the end at lower currents. Note also that the table is for a full standard mode charge, a full range mode charge takes longer. In my experience, going from a full standard mode charge to a full range mode charge takes about 1:40. You have to add even more if you're below 10% in range mode when you start the charge.

    On my blog, I have posted more detail about how charging works, how fast you can charge at each amperage limit up to the point where the current starts to taper off, where current tapering starts, and how charging rates effect efficiency:

    Tesla Roadster Charging Rates and Efficiency

    To optimize total drive time, you want to start the day with a full range mode charge (unless this is a drive you do too frequently to use range mode every time), then stop to charge until the current tapers off below the amperage at your next charging stop. If you have different size chargers along the route, you'll want to maximize time spent at the faster chargers.

    I also have a spreadsheet where you can input how far you want to travel between full charges, how much buffer you want to end with, and what charger amperage you'll be using. It then shows total travel time at various driving speeds, based on JB Straubel's speed vs. range spreadsheet.

    View attachment Road Trip Calculator.xls

    To use the spreadsheet, fill in the four values at the top of column C, then look at the Total Time column to see how driving speed affects total driving time.
     
  5. tomsax

    tomsax Member

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    You have to be a little careful there, as there's a certain amount of charging overhead, so lower current limits charge more slowly than you'd expect because a smaller fraction of the power goes into the battery. It's not a noticeable effect until you get down below 240V/32A.

    I think the actual charge rate at 120V/12A is more like 3 miles of range per hour of charging. I've also seen a charging rate of 0 for an hour of charging with a hot battery that would have otherwise lost charge while cooling the pack.

    I haven't actually done the graph for 120V/12A. I should do it and add that to my blog.
     
  6. edo

    edo Member

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    I accounted for this, but I should have included something about this in my assumptions. The charging rate is roughly constant current till 80% (I forget if that is 80% of full range mode charge or of standard mode charge) then tapers off... I guestimated that 2 hrs was about right for the HPC to charge 100 miles...
     
  7. Tdave

    Tdave Member

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    For what it's worth: I was able to get 235 mile range while going 65 mph drafting 1 second behind semi trucks. The trip was 170 miles, but I was on a pace to get 235 miles range. I was pleased with that result.
     
  8. rugbymonk

    rugbymonk Roadster #287; S Sig #14

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    I've been commuting in the Tesla every day for the whole summer. Today I tried drafting as much as possible on my 40 mile commute into NYC. At 65 mph (truck and bus speed) for most of the way, in "max range mode" which has noticeably different acceleration and regen braking, I was able to use only 85 ideal miles on my 80 mile round trip. Pretty good for a novice on a windy day on the NJ Turnpike. Usually, if I am driving 70-75 mph without drafting, I use 100 ideal miles for my 80 mile journey. But for my roadtrip next week, I will definitely use your tips.

    Thanks for all the help and insights on range. Tesla told me they are working on an 70 amp HPC about halfway NY to Boston, so maybe roadtrips in the east will actually be pleasant (not just feasible via RV parks...)
     
  9. dsm363

    dsm363 Roadster + Sig Model S

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    #29 dsm363, Oct 15, 2010
    Last edited: Oct 15, 2010
    I'm not sure if this fits into this thread but I'm going to install an outlet in my garage to use the Universal Mobile Connector charger (instead of buying the High Power Wall Connector charger since I'm leasing my house). Since I can have the electrician install anything that works with the mobile charger, should I do the NEMA 14-50 at 50 Amps? I remember someone talked about charging at 32 amps or something lower might prolong the life of the battery if you didn't need a quick charge and could charge it all night. Thanks.
     
  10. scott451

    scott451 KWH-PWR#1349Sprt,S Sig#96

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    Having a portable UMC is much better than a HPC. I rarely need to charge above 40A. Have your electrician install a NEMA 14-50 with a 50A breaker. Tell him you plan to use the outlet for your "brothers RV" when he visits. :) If you tell him it's for your electric car, it will cost $100 more.

    -Scott
     
  11. DaveD

    DaveD EVs Kick Gas!

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    I recommend that you have the electrician wire it to handle the max rated current of 50 amps. The cost difference between that and smaller diameter wire and a lower-rated breaker is going to be minor. Then set the Tesla to charge at whatever current level, at or under 40A, that you prefer - it'll remember the setting, even distinguishing it from another charging location, since the GPS coordinates are saved as part of the profile.
     
  12. dsm363

    dsm363 Roadster + Sig Model S

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    Thanks. So the circuit can handle up to 50A but the mobile charger can only go up to 40A (if I understood that correctly). I pasted the specs below from the online Tesla store:
    Technical Specs
    Max Current: 40A
    Voltage: 208-240V
    Max Power: 9.6 kW

    That's pretty cool the Tesla remembers how to charge based on GPS.
     
  13. jaanton

    jaanton Roadster NA #1026

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    The UMC and the car negotiate to know that the UMC has 40A max so the car will not use more than that and when the car knows a UMC is connected it will not even give you the options above 40A. The GPS memory has given me problems from time to time. I always check the time and current of the charge system. Sometimes it doesn't remember properly or maybe it does not have a good GPS read when I turn off the car. Once I scheduled the car for a range charge to start in the middle of the night and it lost the range charge mode. Annoying. I had to start a range charge just 30 minutes before leaving and thus didn't get a full charge. Luckily I was fine on my trip. I just got my recall work done which included a firmware upgrade. That upgrade cleared my charge history and the location information.
     
  14. vfx

    vfx Well-Known Member

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    Uncool, Tesla.
     
  15. TEG

    TEG TMC Moderator

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    #35 TEG, Oct 15, 2010
    Last edited: Oct 15, 2010
    Keep in mind "continuous load" and NEC guidelines.
    (For instance, see this: http://ecmweb.com/mag/electric_sizing_circuit_breaker/ )

    Basically if you have a 50AMP breaker, you really shouldn't pull more than 40AMPs continuously for multiple hours through it.
     
  16. Doug_G

    Doug_G Lead Moderator

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    It can be much worse than that.

    I got the HPC, and decided to get my house's service upgraded to make sure I had enough power. I asked a local electrician for a quote. He hadn't seen a car charger before, and started doing research. He figured out what car I was buying. Then he gave me a quote for $7000!!!!

    Fortunately I'm not a complete idiot! I got a couple more quotes, and in the end I paid a fraction of that. Take Scott's advice... if you can, don't tell the electrician what it's for!!!

    (If I was to do it again, I'd have just put a NEMA 14-50 in the garage and bought the UMC instead. I don't need 70A charging in my garage; in fact, I've dialed it back to 40A.)
     
  17. Doug_G

    Doug_G Lead Moderator

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    That's not just good advice, it's a requirement of the electrical code!
     
  18. dsm363

    dsm363 Roadster + Sig Model S

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    Thanks. That's what I'm planning on doing. I would have done the same thing I'm sure but since I'm leasing my house and after reading posts here, I agree that's the way to go.

    The breaker box is on one side of the house and I have a detached garage about 25 feet away. Initial quote was for $800 but he was told it was for an electric car. There are three 120V outlets in my garage and the electrician said something about needing to dig a small trench to run more cable to the breaker box. I only need one 120V outlet in the garage. I was wondering if the wiring for the two other 120V outlets can be combined somehow to make a 240V outlet thus saving any digging? (I'm sure it's clear I have no idea how electrical wiring works).
     
  19. rolosrevenge

    rolosrevenge Dr. EVS

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    For what its worth, if you have a 240V - 50A, set it to charge at 12 A if can (time permitting of course). Losses are i^2*R. Therefore if you charge at a lower current for longer, you do so more efficiently.
     
  20. Doug_G

    Doug_G Lead Moderator

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    You very likely have just one single phase 120V 15A circuit in the garage. It is highly unlikely that you would have two phases available. Even if you did, the wires would have to be rated to carry the necessary current.

    If you can plug into the house instead of the garage, then the installation would be pretty inexpensive.
     

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