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120V/20A charges at 9 km/h, which is about ...

Discussion in 'Model S: Battery & Charging' started by scottm, Mar 17, 2015.

  1. scottm

    scottm Active Member

    Jun 13, 2014
    ...twice as fast as my normal walking speed.

    So here's a math problem:

    Assuming the car was completely flat charge, zero rated in tank, will not start.
    I plug it in to a 120/20 outlet using mobile charger, and set out for a walk on level ground and do no break my walking stride.
    How far can I get before the person left behind in the Tesla catches up to me?
  2. Mattias

    Mattias Member

    Mar 15, 2015
    Sure no problem:

    Considering an EPA-rating of 265 miles per charge.

    265 miles=426.5 km

    Charge speed: 9km/h.

    Charge time:

    426.5/9=47.38 hours. (Hope I got this right)

    Walking distance during charging:

    47.38*4 km/h=189.22 km

    Now, start traveling at 70 miles/hour: (112.6km/h)

    Difference in speed is then:


    Time to catch up: Considering the guy stops walking when the car sets off. Keeping him walking means I have to use some formulas that I can't remember by hand...

    The car then has to do 189.2km at the speed 103.6km/h, which is 28.77 m/s.


    Solve for t:


    Which equals:

    t=(189.2*10^3)/28.77=6576.3 Seconds.


    Or 1 hour 49 minutes and 12 seconds.

    That means you could start charging, walk for 47.4 hours (that´s 189.2km) before the car is fully charged.

    Also means that it will take your car 47.4+1.82=49.22 hours before you catch up your walking friend. :)

    - - - Updated - - -

    I made an error in my previous post.

    The Delta-v should be 112.6-4=108.6=30.16ms/s

    That means the correct waiting time is actually "only"

    t=(189.2*10^3)/30.16=1.72 hours.

    1.72 hours is 1 hour, 44 minutes and 30 seconds.

    Total waiting time should then be:

    1.72h(travel time)+47.38h(charge time)=49.1 hours. Which is 49 hours and 6 minutes.
  3. saladman

    saladman Member

    Feb 11, 2015
    Seattle, WA
    Wouldn't the answer be much, much shorter?

    Assuming the car is at zero, 1 hour of charging means the car has 9km of range, but our walking friend has only made it 4km.

    That leaves me just enough time to unplug, go get him, and then come back to the charger. (A round trip of 8km).

    Then we can all do it again. :)
  4. gregincal

    gregincal Active Member

    Oct 26, 2012
    Santa Cruz, CA
    In fact, the answer approaches zero except the amount of time it takes to get enough charge for the car to allow itself to be moved and the time it takes to unplug. Theoretically after one minute the car has enough charge to drive 150 meters, but the walker has only gone 75 meters.
  5. mikeash

    mikeash Active Member

    Oct 26, 2014
    Fairfax, VA, USA
    I'd say you get about 200ft. You cover the first 100ft during the time when the other person is figuring out what the hell is going on. You cover the second 100ft while he sprints after you and either demands an explanation or tackles you, depending on the specifics of the scenario.

    Alternate outcome: if it's your Tesla, the person never catches up to you, because he realizes, hey, free Tesla, and just waits until it has enough charge and drives it back to his house.
  6. cantdecide

    cantdecide Member

    Dec 21, 2012
    Boulder, CO
    Primarily this comes down to the question of how long does it take to get enough charge to start the car, which is a practical question, not mathematical based on any knowledge we have. Also, in practice, temperature matters as well.

    Supposing we ignore those two questions which likely dominate real world experience...
    The answer isn't quite zero... You don't just need the range to meet up with the person, you need to get there while still traveling at walking pace, which is nonzero kinetic energy.
    Hence you really need enough energy to get maybe an extra 100 meters past the walker in order to reach the walker at above walking pace. So maybe more like 2 minutes than one.

    However in general to travel distance of x you don't need range of x, more like x/2 as you can drive slowly to increase range.

    So, lots of factors, but I think hysteresis and energy needed before the car allows you to start would dominate reality.

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