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Homework Help: Check my answer please! simple current/power problem!

  1. May 24, 2013 #1
    The Nissan leaf is powered by 107 hp electric motor, kept alive by a pack of batteries, It takes 79.85 kilowatts to run the motor, assuming 100% efficiency.

    a) The Nissan contains a 24kWh battery pack and its estimated that using a 220V charging dock at home. The leaf can be charged in 8 hours. What the current as the leaf is charging?
    b) if it cost 11 cents perkilowatthour, how much does it cost to fully charge the leaf?

    my answers:
    a) i = P/V =>(24000*8)/220 = 872.7 A
    b)79.851 x 8 hours = 638.808
    638.808 x .11 = $70.26

    is this right?
  2. jcsd
  3. May 24, 2013 #2

    rude man

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    Not right.

    If the battery can supply 24 kW for 1 hr then if it had to be recharged in 1 hr it would take how many amperes to recharge? And if we can take 8 hrs to recharge, would that take more or less current?
  4. May 24, 2013 #3
    i dont understand. (24 kW times 8 hours to recharge)/220V?
  5. May 24, 2013 #4


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    Think about the units for kWh

    = Watts * time(in hours)/1000
    = Current(Amp) * Voltage(volts) * time(in hours)/1000

    PS I believe a Leaf can be charged from a normal domestic socket so 800-900A is unlikely
  6. May 24, 2013 #5
    24000 * 8 hours /1000 = 192 W

    current=192W / 220Volts = 0.87 amps?
  7. May 24, 2013 #6


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    No you missunderstood my post totally.

    I start from the beginning ...

    The battery stores a quantity of energy. I wish they would specify that in Joules but they used kWH instead which is ok.

    kWH means kilo Watt Hours so a sum that gives an answer in kWH will involve multipling power(in Watts) by time(in hours).


    Energy (in KWH) = Power(in Watts) * time(in hours)/1000..................(1)

    but as you know

    Power(in watts) = Current(in Amps) * Voltage(Volts)...........................(2)

    substitute to give..

    Energy (in KWH) = Current(Amp) * Voltage(volts) * time(in hours)/1000

    The question asks for the current so rearrange that to give

    Current = (Energy * 1000) / (Voltage * time)

    Then plug the numbers in.
    Last edited: May 24, 2013
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