Formula ? - DIY Electric Car Forums

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#1
05-12-2012, 12:15 PM
 EVEngineeer Senior Member Join Date: Apr 2012 Location: Florida Posts: 236
Formula ?

I need help in understanding the formula range[miles]=250 x capacity[kWh] / (mass[lbs]^0.6)

Where does the 250 come from?
What number do I use for the kWh? Motor?
How can I find how much the vehicle weighs exactly, if that is what the mass number is?
Where does the ^0.6 come from?

#2
05-12-2012, 12:55 PM
 Woodsmith Admin: 'one of many' Join Date: Jun 2008 Location: Manchester UK Posts: 4,838
Re: Formula ?

Quote:
 Originally Posted by EVEngineeer I need help in understanding the formula range[miles]=250 x capacity[kWh] / (mass[lbs]^0.6) Where does the 250 come from? What number do I use for the kWh? Motor? How can I find how much the vehicle weighs exactly, if that is what the mass number is? Where does the ^0.6 come from?
Where did you get that from?

I suspect that it might be an approximation that includes some constants to allow for some unknowns and some conversion from imperial to metric to simplify the calculations.

I tend to convert my input data to SI units then calculate and convert the output back to imperial.
#3
05-12-2012, 04:37 PM
 nulluser Junior Member Join Date: Mar 2012 Location: SE Michigan Posts: 15
Re: Formula ?

That formula is just a best fit empirical model of battery capacities, range data, and vehicle weight.

It is just to give a rough idea.
#4
05-12-2012, 06:53 PM
 ruckus Senior Member Join Date: Apr 2009 Posts: 654
Re: Formula ?

Don't use that formula! It is very flawed by only considering mass.

If you are stopping and starting or pulling hills mass is important, but if you are on flat highway Aerodynamics is WAY more important than mass.

Go here:

http://ecomodder.com/forum/tool-aero...resistance.php

This tool is much better. At the magic speed of 60mph your KW needed is also your battery amp hours needed to drive for 60 minutes and go 60 miles.

Using the stock parameters (a small car) you can see it would take about 10kw/hrs to go 60 miles. That is 100% dead. So 12kw might do it. Add hills, your buddy, and some wind and you better make it 15kw.

Bottom line, get as many batts as you can fit/afford.

Good luck.
#5
05-12-2012, 07:20 PM
 EVEngineeer Senior Member Join Date: Apr 2012 Location: Florida Posts: 236
Re: Formula ?

Ok, great answers. I really appreciate it. I am going to get an aerodynamic shaped larger car, like a 2006 or newer Honda Civic. I can get one really cheap, plus I would make modifications to it, like switching out the side mirrors for something more aerodynamic. I would also put those full cover hubcaps on the wheels and maybe add those covers that are part of the car. I would also like to add recessed solar panels on the car, so it does not affect the shape of it.
#6
05-12-2012, 07:24 PM
 EVEngineeer Senior Member Join Date: Apr 2012 Location: Florida Posts: 236
Re: Formula ?

What do you people think I should get in order to achieve 65mph for ~100 miles range? I drive a little less than that every day at that speed. I drive a little city to at 45mph for about 25 miles range every day.
#7
05-12-2012, 10:40 PM
 EVEngineeer Senior Member Join Date: Apr 2012 Location: Florida Posts: 236
Re: Formula ?

What is the price range are we talking about here and why exactly? Is there ways of cutting costs? Could you explain why I would need the parts that you suggested please? Thank you. Also, I do not currently have the skills, but I know plenty of people who do, so I would have other people do it for me.
#8
05-12-2012, 10:40 PM
 tomofreno Senior Member Join Date: Mar 2009 Posts: 2,318
Re: Formula ?

Quote:
 Originally Posted by EVEngineeer What do you people think I should get in order to achieve 65mph for ~100 miles range? I drive a little less than that every day at that speed. I drive a little city to at 45mph for about 25 miles range every day.
With a Civic you likely will get around 300 Wh/mile +/- 10% at 65 mph. So 100 miles would require about 30kWh. You want to discharge to no more than 20%, and if you do it everyday 30% will extend your pack life from 2000 to 3000 cycles according to the manufacturers (who knows really?). So 30 x 1.2 = 36kWh pack. If you want to drive that 25 city miles at < 45 mph, well add around 5kWh for a 41kWh pack, minimum, 45kWh would be better. If you use the motor Gotti suggested and a Zilla or Soliton Jr controller you could use say 71 cells for a nominal 227V pack (and set the controller to limit motor voltage to around 160 - 170V so you don't damage it). Then to get 45kW you would need 200Ah cells. At around \$1.25/Ah delivered that would cost around \$17.8k. Then just add motor, controller, charger, DC/DC converter, vacuum pump, stronger vehicle springs, bms,...costs, and figure out where you will put all those cells in a Civic. With the about 900 lb pack you may use more like 330-340 Wh/mile at 65 mph.

Last edited by tomofreno; 05-12-2012 at 10:43 PM.
#9
05-12-2012, 10:51 PM
 ruckus Senior Member Join Date: Apr 2009 Posts: 654
Re: Formula ?

Here is a fairly good list of aero drag with a few mistakes.
http://en.wikipedia.org/wiki/Automob...ag_coefficient

Don't forget you have to multiply the drag coefficient by the frontal area.
scroll down the wiki page for a list of square areas.

Honda CRX, Eagle Talon (or other DSM), Mazda Rx7, Nissan 240sx, Dodge Daytona, in that order more or less.

You can find a junk DSM with a blown motor and maybe already gutted with roll cage for pennies on the dollar... while a rusted out honda is like gold bricks.

All these light small cars need a full cage and racing seats/belts or when you wreck it's KYAB.
#10
05-12-2012, 10:58 PM
 ruckus Senior Member Join Date: Apr 2009 Posts: 654
Re: Formula ?

These guys are pulling your leg...

I drove the 37 Jaguar 90 miles the other day and only used about 16kw of the 25kw pack. It is small and light, but definitely not aerodynamic. A civic/Talon would have used much less..

But I wasn't going 70...Speed Kills (your batteries)

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