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How to calculate required battery charge for 300 miles

  1. Aug 18, 2010 #1
    Need help - my brain is mush and the longer I look at it the more I don't understand it...can someone please tell me if this calculation is correct:

    1. The problem statement, all variables and given/known data

    A car is run for 400 hours per year, with a total mileage of 10,000 a year.
    The car uses diesel and consumes 2,000 litres a year.
    A litre of diesel costs £1.10 and holds 38.7MJ of primary energy.

    If the engine is 20% efficient overall, how much energy would need to be stored in 95% efficient batteries to give a range of 300 miles on full battery charge?

    2. Relevant equations

    n/a

    3. The attempt at a solution

    Background Calcs:
    Convert MJ to J: 38700000J
    Total Joules (J x 2000 litres): 77400000000J
    Convert 400 hours to seconds: 1440000s

    Energy 77400000000 J
    Time 1440000 seconds
    = Power 53750 Watts
    53.75 kW
    1 watt = 0.001341022 horsepower
    Convert W to hp: 72.07993734 hp

    Answer (attempt!):

    Primary energy (1 year): 400h*53.75kw=21.5MWh
    Energy to 95% efficient batteries: 21.5 x 100/20 x 100/95 = 113.2MWh
    Therefore 300 miles: 300/10000*113.2=3.4MWh

    I appreciate your help!!!!
    Thanks guys

    :confused: + :zzz: + :cry: = me
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Aug 18, 2010 #2

    Zryn

    User Avatar
    Gold Member

    Does the battery replace the diesel or the engine?

    Either way, look at it from the point of view of a flow of units. For example, taking distance traveled & diesel used and energy in diesel, and then calculating energy per distance (MJ/mile) should give you a good grounding for calculating the energy required to travel some more distance.
     
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