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#1

# Formulas

Range Estimation:
Code:
```
range[km]=250 x capacity[kWh] / (mass[kg]^0.6)
-or-
range[miles]=250 x capacity[kWh] / (mass[lbs]^0.6)```
Wire Temperature? :
Code:
```Thot = Tcold + (K x ((Rhot - Rcold) / Rcold))

where K = 256.4 for copper
Thot = hot temperature in deg.C
Tcold = cold temperature in deg.C
Rhot = resistance at hot temperature in ohms
Rcold = resistance at cold temperature in ohms```
Horsepower:
Code:
`Hp=(torque*RPM)/5252`
AWG to Metric Conversion

Current Limiting Formula for LEDs

Amps, Volts, Watts and Watt-Hours 101

 Contributors: rbgrn Created by rbgrn, 08-07-2007 at 03:07 PM Last edited by rbgrn, 12-19-2007 at 11:40 PM 10 Comments , 15600 Views Edit Advanced Edit History
#2
09-24-2007, 08:21 PM
 sl.whiteside NTEAA Member Join Date: Sep 2007 Posts: 5
Re: Formulas

Is the torque in ft-lb, in-lb, or n-m?
#3
09-25-2007, 08:16 AM
 rbgrn Administrator Join Date: Jul 2007 Posts: 691 Blog Entries: 13
Re: Formulas

Quote:
 Originally Posted by sl.whiteside Is the torque in ft-lb, in-lb, or n-m?
Ft-Lb I believe.
__________________
Green Web Publishing, LLC
#4
09-25-2007, 04:55 PM
 sl.whiteside NTEAA Member Join Date: Sep 2007 Posts: 5
Re: Formulas

Quote:
 Originally Posted by rbgrn Ft-Lb I believe.
Robert,
I confirmed after checking my Pocket Ref by Thomas J. Glover, 2nd Edition.
It is Ft-LB.
__________________
-Steve

'98 Ranger EV Conversion
#5
09-26-2007, 09:15 AM
 rbgrn Administrator Join Date: Jul 2007 Posts: 691 Blog Entries: 13
Re: Formulas

I love that book, btw.
__________________
Green Web Publishing, LLC
#6
01-01-2008, 07:43 AM
 Thalass Senior Member Join Date: Dec 2007 Location: Perth, Western Australia, Australia Posts: 110
Re: Formulas

In the range equasion (range[km]=250 x capacity[kWh] / (mass[kg]^0.6) ), what is the "0.6"? It looks like me to be CoD, but there's no real reason for that, just a hunch.
#7
05-16-2008, 08:21 AM
 weelliott Member Join Date: May 2008 Posts: 37
Re: Formulas

I am not familiar with the formulas, but I can tell you that it is not from Coefficient of drag. The fact that the exponent is under 1 means that the mass has a relation that factors it in less than what it would factor in as if you were looking purely at acceleration--where weight matters.

This equation is very simplified. If you were to develop an equation for range that assumed that the car were traveling on a flat surface at a constant speed, weight would not matter at all, but drag area would drive the results.

Conversely, if you were to base range on a stop and go city type environment where it is mostly acceleration and aerodynamic losses are relatively minimal compared to acceleration and Rolling Resistance, then the weight of the car would matter more, and that exponent would be 1. so it looks like some assumptions about an average driving cycle have been made and the 0.6 exponent has been used to reflect that compromise. It looks as though it uses some stop and go or some hills.

The fact that Cd is not in the equation suggests to me that this assumed driving cycle doesn't have much traveling over 45 mph.

So I am really curious. What is the assumed cycle? Is there an assume value for drag area? (Drag area is just Cd times frontal area, the two variables specific to a car's design.)
#8
06-17-2008, 08:02 PM
 Vwbeamer Member Join Date: Jun 2008 Posts: 51
Re: Formulas

^ is powers. You need a scientific calculator, it will have a ^ key.

BTW the formula is very optimistic. for example a 2500 lb car with a 10KW power pack would go 91 miles according to the formula.
#9
06-20-2010, 07:33 AM
 Franky.EV Member Join Date: Feb 2010 Posts: 74
Re: Formulas

I've try to put all formulas in a single oooCalc spreadsheet.

Look at :

It takes into account : grade, DOD, motor torque, gear, shift time.

Last edited by Franky.EV; 06-22-2010 at 11:40 AM.
#10
09-09-2010, 06:56 AM
 Crash Senior Member Join Date: Oct 2009 Location: Los Angeles, CA Posts: 122
Re: Formulas

I don't think that range formula is all that accurate...

I was trying to figure out the range a 3000Lbs Corvette with a 40.2kWh pack would go and it's saying something around 82mi... That doesn't sound right at all unless I'm doing my math wrong:

range = (250 * 40.2) / (3000 ^ 0.6) = 82.394mi

That really doesn't sound right.

Tesla Roadster is 2723Lbs with a 53kWh battery pack (I know this has changed lately, but lets go along with the old numbers for now.)

Using the same formula I get 115.130 which is 135miles less than what Tesla claims.

I think that 0.6 is the wrong number. It should probably be closer to 0.5. 0.5 would actually put the range for the Tesla closer to 250mi.

Running the numbers with the 3000Lbs Corvette and a 40.2kWh pack comes out to 183mi. That makes more sense to me. Maybe still a little high of an estimate, but sounds more accurate than 82 miles out of a 40kWh battery pack.

EDIT: I forgot to mention, the original formula is suggesting that my car would use 490wh/mi.

Last edited by Crash; 09-09-2010 at 07:09 AM.

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