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Electric Vehicle Battery specification (very basic)

  1. Apr 6, 2010 #1
    I have an concept electric car, i calculated it will need 15,000 Wh stored in batterys to operate over its required range (100 miles) on a certain drive cycle.

    I am attempting to work out how many batteries i need.

    The motor it is powering has a voltage rating of 156 vDC specs here: http://www.azuredynamics.com/product...oductSheet.pdf

    so does this mean that in Ah this is equivilent to 128 Ah? (P.t=IV.t = 2000 = I.156)

    if i were using say, 3.3v batteries would this mean i need 48 (48x3.3=158).

    And if these battereis say, have 100 Ah each what does this mean? Does this mean i have 100Ah x 48 overall, or just 100 Ah overall?

    Lastly can anyone suggest some good batteries for the purpose, low mass is a priority (as well as low volume).

    This is a low drag, custom car, it will end up being like a lighter version of this thing. http://www.aptera.com/
     
    Last edited: Apr 6, 2010
  2. jcsd
  3. Apr 6, 2010 #2
    If that battery is rated at 3.3V for 100Ah. Then you get 3.3 volts at 100 Amps for an hour. or 3.3 volts at 50 amps for 2 hours. etc etc etc.

    So if you have 48 batteries like that you can supply 48*3.3 volts at 100 Amps for 1 hour.

    And now you see why currently electric cars are cack (battery limited). I'm afraid im not that up on battery tech to reccomend a certain type of battery.
     
  4. Apr 6, 2010 #3

    mgb_phys

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    Yes

    Just 100Ah

    Depends, do you have an insane amount of money or just a ridiculous amount of money?

    A laptop battery is pretty good power/kg for commercially available parts.
    A 12V 8000mAh laptop battery gives almost 100Wh for about $100,
    So you would need 200 of these, which is similar to what a tesla roadster uses.
     
  5. Apr 6, 2010 #4
    Thanks for the info, very useful!

    For my car, 15,000 Wh (equivelent to 100Ah) is enough for motorway driving for 100 miles. This takes about 4.5 hours to do the distance. Does this mean it is supplying 158 volts at 100/4.5 = 22 Amps for one hour?

    What about the peak load on the motor at any single time? During straight line performance testing the motor will demand up to 41 kW (for 1 second though) so does this mean that the current demand for that 1 second is 41000/3600/158 = 0.07 Ah? (So the battery is capable of dealing with a high load in a short space of time).
    200? why? did you mean 100Ah for $100, apparently i need 15,000 Wh overall??
     
  6. Apr 6, 2010 #5
    Er. Lets not get carried away here, that is nowhere near whats needed for sustained driving at motorway speeds.

    I think if we change units you'll get a better idea.

    15 kW is 20 horsepower. You can achieve this for a total of 1 hour.
     
  7. Apr 6, 2010 #6
    Why not? Not really helpful that comment.

    Frontal Area=1.74m^2
    Cd=0.15
    density=1.225
    speeds from 70mph = 31m/s
    motor eff = 0.85%

    Rolling resistance = 88N constant for the purposes here
    Drag = 0.5*1.225*31^2*1.74*0.15 = 153N

    Power = F.v/eff = ((88+153)x31)/0.85 = 8789 W


    so actually it is well above what is required for motorway speeds. for the time it takes to complete 100 miles, and adding in moderate acceleration/deceleration this comes to around 15kWh for the 100 mile distance.

    Please if i am wrong, explain why??
     
  8. Apr 6, 2010 #7

    mgb_phys

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    That's only about 11hp - are you sure you have the frontal area and cd correct? Or is this a custom 1/2 seater low profile design?
    A typical modern salon car needs about 30-50hp to maintain freeway speeds.

    Anyway assuming you only needed 9kW then you could run for about 90 mins on 15Kw/h

    You get the watt-hours (ie energy) for a battery by simply multiplying the capacity (amp-hours) by the voltage ( since power = IV, watts= amps volts).

    So a 12V 80Ah car battery gives you (in theory) 9600 Whours and a 12V 8000 mAhour laptop battery 12*8 = 96WHours.

    You can stack the batteries however you like to get the correct number of volts, but you can't change the energy stored in them.
     
  9. Apr 6, 2010 #8
    Your Cd is very low, unless your building a formula 1 car. The 2010 Prius has a Cd of 0.25 which is very good for a sedan of its size. You're going to require about a minimum of 15kW just to maintain freeway speeds. However, the car does have to accelerate as well, which is going to require a lot more than 15kW.
     
  10. Apr 6, 2010 #9
    Yes it is a low profile two seater, see here the aptera 2e, http://www.aptera.com/
    its basically going to be much like that. I calculated the acceleration based on a european standard drive cycle, it ends up being fairly small but thats where the 15kW for 100 miles comes from (which takes around 90 mins like you said).

    I did the same for urban and rural driving, the accel was bigger there but the drag forces smaller due to a lower speed, and the power comes out 12-14 kW.
     
  11. Apr 6, 2010 #10

    mgb_phys

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    If you don't have too many hills and you have nice long freeway ramps you don't need much power.
    Most 1930s cars in Britain were 12Hp (<10KW) and were a lot less aerodynamic.

    And of course there is the famous (20kW) 2CV.
     
  12. Apr 6, 2010 #11
    Good point! I think most people on here are insulted by my low power quotings, in fact the peak power output of the EV will be around 41kW (55bhp) its just for the average journey energy consumption that im talking about here.

    0-100mph - 30 seconds
    0-60 mph 10 seconds

    However on the current battery capacity i can only do 5 full bore starts before running out of fuel/energy which is shockingly low.
     
  13. Apr 6, 2010 #12

    mgb_phys

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    I think it's <cough>anatomical</cough> some countries like France/Italy manage with 2CV/Fiat 500/scooters, other countries need a 350 V8-Hemi pickup.
    I suppose this is why nobody thinks of Texans as the worlds greatest lovers :tongue:

    That's probably a bit optimistic (or pesimistic if you are planning the performance) a regular euro hatchback will do 0-60mph in more than 10secs - 10secs is hot hatchback/GTi territory.

    You can recover some energy when you slow down - but yes batteries are crap compared to gallons of dead dinosaur!
     
  14. Apr 6, 2010 #13

    Yes but those Golf's weigh about nearly a tonne more than this car, and have a much higher top speed...I think the calculations have it as 10.5 at the moment, though i have yet to add motor efficiency into the equation which will knock it back a bit. Once again the aptera quotes a similar 0-60 time, and i would like to beat it considering this is the same car but lighter!
    Its not really aimed at high performance anyway.

    Do you know anything about how battery efficiency effects the figures, does it limit power output or just range?
     
  15. Apr 6, 2010 #14

    mgb_phys

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    That was my point (sorry I wasn't very clear) there are lots of ordinary everyday ICE powered cars that can't do 0-60 in 10secs. A 0-60 in 15-20secs is perfectly usable as a city vehicle. I had a 1.2l Citroen diesel that probably did about 25secs, you just have to plan a little further ahead to overtake!

    The battery technology will limit how much power you can pull from a battery.
    eg, a 8Ah li-ion can, in theory, put out 1Amp for 8hours or 8A for 1hour or even 500A for 1min - while in practice it would explode.
    Generally though li-ion and lead acid can put out enough amps to run a car.

    The inefficency is when you put power back into the battery (to recharge or regenerate) - the battery heats up - wasting some energy - but more importantly the heat build up can damage the battery, Managing the thermal condition is one of the big problems for electric cars - especially if you want it to work in LA in the summer.

    also if your motor can accelerate from 0-60mph in 10seconds you probably want to be able to strop in a fraction of that time. Ideally you want to reuse all that energy rather than wasting it in brakes, but you motor and battery can only handle the power needed to go from 60-0 in 10secs, the rest you have to dump as heat. Some of the newest electric cars will have a separate capacitor bank to absorb this high rate energy dump and use it to m ore slowly recharge the battery once you have stopped.
     
  16. Apr 6, 2010 #15
    An F1 car had drag Cd between 0.7 - 1.1 depending on downforce setups :P. They are anything but low drag.
     
  17. Apr 6, 2010 #16
    Why didnt you say so in the first place? At least the figures make some sense now.
     
  18. Apr 6, 2010 #17
    Theres so much information to tell about this project, most of the time i forget something.
    again very useful info, cheers!

    I will probably incorporate KERS systems, but i dont have any alogorithms for predicting how much energy is recoverable, are there any general guidelines for how much i can recover from braking, or spinning a generator while decelorating?
     
  19. Apr 6, 2010 #18

    Mech_Engineer

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    Well you can calculate your kinetc energy at a certain speed using:

    [tex]KE = \frac{1}{2}m*v^{2}[/tex]

    If you assume your motor running as a generator is 50% efficient by the time the energy gets to the battery, that means you can recover 50% of the kinetic energy available.
     
  20. Apr 6, 2010 #19

    mheslep

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    Well how long is a 'start'? If a start uses max power at 41kW, five starts for 30 seconds each uses only 1.7 kWh. You mentioned you planned for 15kWh of battery capacity (normal discharge rate), or almost nine times that.
     
  21. Apr 6, 2010 #20

    mheslep

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    Take a look at some battery discharge rate curves, e.g. this lead acid curve family here:
    http://www.batteryuniversity.com/partone-16a.htm [Broken]
    Modern Li-Ion here:
    http://www.thunder-sky.com/pdf/20092201189.pdf [Broken]

    The curves also show that total amount of energy that can be drawn from the battery is not a constant = amps x time. As the amps increase the total energy delivered over time falls. The total energy provided by the battery is proportional to the area under each of those discharge rate curves, the lighter the load the more total energy delivered. This is why batteries are rated discharge current x time, C=Amp-hours, as opposed to straight energy in power x time (Watt-hours) which could indicate any discharge current. If the battery is rated at 10-AH (1C), it provides some total energy X = volts(t) * t * 10A. Discharge it twice as fast at 20A (2C), and it provides some energy less than X.

    This effect, modeled as in internal battery resistance, limits both the total power output due to the internal voltage drop and the total energy delivered,as the internal resistance burns stored energy as heat during discharge.

    Note from the curves that Li-Ion is much more efficient than Lead Acid, with little voltage drop due to higher loads as it depletes.
     
    Last edited by a moderator: May 4, 2017
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