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Homework Help: What is the estimated battery life?

  1. Aug 29, 2011 #1
    1. The problem statement, all variables and given/known data
    Electric motor in put power rating is 17.5kw
    Voltage is 48V
    Battery is 7.5kw/h
    Average W needed to maintain motor per sec is 1446.7W

    2. Relevant equations
    Motor: W=I*V > I = 364.58Amps

    3. The attempt at a solution
    My attempt: If 1kw = 3.6 x10^6J > 7.5 * 3.6 x 10^6 = 2.7 x10^7
    Then the battery is 2.7 x10^7 Jouls

    I can't go any further than this. Can anyone help please?

  2. jcsd
  3. Aug 29, 2011 #2


    User Avatar

    Staff: Mentor

    Batteries are rated in Amp*hours, or in this case in kW*h (not kW/h). Power multiplied by time is energy, and the battery initially holds some amount of energy.

    So to work problems like this, you are given the average power supplied to the load, and asked how long the battery will last....
  4. Aug 30, 2011 #3
    Thanks Berkeman, However, the only other information my teacher gave me was
    The average power required to maintain the battery = M*V^2*fr = W
    M= 176kg
    Fr=energy usage fraction= 12%

    Any ideas?
  5. Aug 30, 2011 #4
    Ok this is my answer.
    Aver E = 170kg*8.3^2*0.12=1405.35w or 1.405kw
    The Battery has 7.5kwh so 1.405/7.5= 5.3 hrs of use

    Thanks Peter
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