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Calculating Battery requirements for EV

  1. Mar 28, 2010 #1
    I have a motor which i am planning on using in an electric vehicle i am designing.


    and want calculate the number and specs of batteries needed to run it.

    I have the power demand from the motor at any given time and therefore i can use
    P=I.V to calculate the current needed at any given time and then use that in Q = I.t to calculate the battery life in Amp.Hrs.

    From that i guess i can calulate how many batteries i need. I will probably use Lithium Ion Phosphate batteries.

    I assume looking at the spec sheet that the voltage i use is 156 v dc, is this correct?
  2. jcsd
  3. Mar 31, 2010 #2


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    Yes for the wiring configuration given.

    A common rule of thumb for highway capable EVs is about 4 miles per kilowatt-hour of battery energy on board, or 25 kWh of battery capacity for 100 miles. That is for commercial vehicles with regenerative braking. You might assume 3 miles per kWh.

    Also consider temperature. Assuming you go with Li-Ion, unless you provide some kind of thermal management system for your battery pack, expect the discharge energy, and thus your range, to drop by 20% in freezing temperatures (so down to 80 miles on a 25 kWh pack), and more than that if you need to use max power for some reason for an extended time. The batteries should yield a full charge again upon return to mild temperatures.

    Extended operation at high temperatures will negatively impact battery life. For the newer LiFePO batteries, you can expect almost 3000 full depth of discharge cycles before the capacity falls to 80% of new - at moderate temperature operation. LiIon is much more efficient than Lead Acid (i.e. lower internal resistance), but you still want to allow for dissipation of 5-10% of your electrical power lost to heat inside the battery pack. If for some odd reason you want to use Li Ion Cobalt (laptop) chemistry, then you really need to consider safety at high temperatures (ie don't use Li Co), as they're famously prone to thermal runaway if you let them get too hot.

    Apparently people want to look back at deep cycle lead acid batteries for these projects. The numbers are changing, but my last look yields a comparison something like this for the same amount of battery capacity (Lead Acid vs Li Ion):
    Weight: Lead Acid ~3x heavier.
    Cost: Lead Acid 2-3x cheaper initial purchase.
    Cycles: Lead Acid 2x less (i.e replace them 2x faster).
    Last edited: Mar 31, 2010
  4. Apr 1, 2010 #3

    Thanks for the useful info,

    My calculations are coming out as half of that though! I did my calculation based on a 3 part drive cycle consisting of 10 mins on urban, rural and motorway.

    This consumed around 5kW of energy per cycle (half hour long). so for 100 miles i needed around 12Kwh of storage, the averae speed was about 20m/s (50mph) for this period.

    Bearing in mind this is a very lightweight vehicle (410 kilos) with a max power requirement of 33kW does this sound reasonable to you?

    I would aslo say that i used that constant voltage for caluclating the storage requirements (didnt account for stop start voltage changes). I havent accounted for the efficiency of the battery or motor yet either (though i have accounted for efficiencies of drivelines and mechanical workings of car/resitive forces).
  5. Apr 1, 2010 #4


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    No, 12kWh will not take you anywhere close to 100 miles at 50 mph, unless your vehicle is a ~one seater with the drag coefficient of an airplane. Above ~20-30 mph, wind resistance dominates the tractive load for regular cars, so the weight of your vehicle matters little compared to its drag coefficient on the highway. At 50 mph, figure on something like 15kW for your home built vehicle to maintain speed on level road. So 100 miles would require two hours at 50 mph, or 30kWh. Slow down to 15-20 mph in a light vehicle and yes you might make 100 miles w/ 12kWh of capacity, with care.

    Some additional help. This is the energy v speed curve for a typical combustion engine sedan, tank-to-wheel efficiency = 25%:

    Note how drag dominates the rolling resistance.

    At 50 mph (81 kmph) the above vehicle uses ~45 kWh/100km. The same vehicle powered as an EV and with perhaps 75% efficiency would use 1/3 that, or 15 kWh/100km (24 kWh/100 miles). You may do slightly better with your light vehicle.

    90% +/- 5% Ii Ion for rated discharge
    91%+/-3% at rated power. In my experience, assuming no exotic batteries or motors.

    Good luck.
    Last edited: Apr 1, 2010
  6. Apr 2, 2010 #5
    I aggree with you that at 50mph that i would need more energy.

    However if you look at my drive cycle below you will reaslise that it will not do this average speed for 100miles continuous.

    I am trying to compile a spreadsheet of the powers required to accelerte the vehicle and the resistive forces throughout.

    The dominant force is not drag, the force required to accelerate the vehicle is allmost allways dominant.

    Attached Files:

  7. Apr 2, 2010 #6


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    If you want the total energy required for a trip you are now making, you should get a gps, record the trip data, and http://chargecar.org/participate" [Broken].

    They take as input parameters the drag coefficient and weight of your vehicle, so this would make it an ideal tool for analyzing your hypothetical vehicle.

    Here's a snippet of some of the data from one of my trips:

    which gives you 430 watt-hours per mile.

    (actual mileage will vary depending on how you drive and maintain your vehicle)
    Last edited by a moderator: May 4, 2017
  8. Apr 2, 2010 #7


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    Ok, but then it is not an average.

    Units? Speed in km/hour vs seconds? No regen braking to capture that energy when you decelerate?
  9. Apr 3, 2010 #8
    Ok, sorry on that graph, the x axis is seconds, the speed is in kph. Im leaving regen braking out for now, ill wait until i can get a reasonable answer for just normal battery drain. Plus i have no idea about the effienency about how much energy from braking can be converted into electrical energy and stored!

    I will hopefully post up a few more graphs by the end of today.
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