Hypothetical Scenario

All of my equations and coefficients came from the book
"Build Your Own Electric Vehicle" by Bob Brant (If you wanted to know)

I’m trying to calculate the amp draw for a hypothetical scenario for
a car that should weigh about 2,000 lbs
can make it to 75 mph
run for 25 miles
and use a 72v system

I decided to calculate how many watts it would take to maintain 75 mph

To maintain 75 mph the motor needs to run just enough to counteract wind resistance and the drag from the weals

Fr = CrWcos@ (pretend that “@” is theta or phi, whichever you prefer)
Fr is the rolling resistance force W is the weight
@ is the angle of your tires which will be 0 (we are not turning)
and Cr is the Rolling resistance factor (which the book stated was 0.018)
so i got
Fr = (0.018)*(2,000)*cos(0) = 36 lbs

Fd = (CrAV^2) / 391 (that is "(CrAVV) / 391")
Fd = aerodynamic drag V = speed
A = area of the front of the car (which i measured to be 17.5 ft^2)
Cd = coefficient of drag (which was given to be .42)
so i got
Fd = ((.42)
(17.5)*(75^2)) / 391 = 105.7 lbs

Total force = 141.7 lbs

HP = F*V / 375
so i got
HP = (141.7 lbs) * (75 mph) /375 = 28.34

W (“W” now means watts) = HP*745.7
so i got
W = (28.34) * (745.7) = 21133

A = W/V (V is volts)
so i got
A = (21133 ) / (72) = 293.5 amps

I don’t know if all of this equations are accurate or not. If you have a better way to calculate this or any other comments; they are welcome.

And if my math is right; can i run a 72v motor with six 12v batteries. That are rated at 125 AH each. I guess my question is, will six batteries with 125 AH each put in series run a motor that takes 294 amps.

I would be great full for any comments on this post, Thanks

Hay, is anyone going to comment

…Is there anyone on this forum

#1 At night and on a Sunday, people are actually going to sleep instead of doing your homework. So next time be patient. If you’re not going to be patient when you should, you’re going to go mad and mess up something good.

#2 If you calculated according to the book everyone else uses, add 50% to every “negative” value you calculate and begin to overcompensate. That is a general idea to get better results than what you’re building for.

#3 Is your car 2000lbs before or after batteries? I mean, six 12Vs will add at least 420lbs, so how are your calculations?

#4 If you want faster results with calculations about ev’s go to the EV Calculator. It will speed up the process and is fairly accurate. Once you have some ideas down, get to work.

#5 Are you looking for 75mph for 25 miles or 25 miles range overall?


Sorry I’m not scheduled to work much this week so I don’t quite have a good sense of time, and i was just a bit depressed sorry

but thanks, i did not know there was a calculator available

I don’t know how accurate the weight is… but I think i can get close to that

Thanks again

Oh and i need 25 miles overall

I only need to go 75 for a sort period of time

When I drive my Rubicon 75 mph, I get about 12 mpg. When I slow down to 55, I get 18.5 mpg… I have found that IF I adopt the attidude of I’ll get there when I get there, the 55 mpg is actually relaxing. I don’t have to worry about passing anyone !!

The same idea applies to your EV project. Fast = reduced range and an increase in cost / weight. Think about this, if you designed it to only go 65 - 70 mpg tops, then you will increase your range and you will know 75 is not in the cards :slight_smile: The H1 had a top speed of 70 mph with the diesel, but when you are driving 8,000 pounds of steel that will go through almost anything, speed is not your mission.

I am afraid that if i went sixty on the highway during rush-hour, I’d be shot. I noticed you saw my other post (sorry to post twice) but as i said there this car is for my dad.

On the way to work he said he could go not much slower than 60. but on the way home he has to go pretty fast, or wait a hour or two before he goes home.

But I’m thinking if the setup we’re plaining right now works. Then it is going to take him forever to build up to 75 mph

I’m thinking we need to upgrade to a 144v system. Is there any chance the 72v system will work?

I am certain some Detroiters are less than happy I am going our minimum allowed speed limit at 55, but $6,250 verses $4,050 / year just to communte to work means for $2,200 I can live with it…

But, if you choose the right car - what are you going to use ? - and the right conversion, it is possible to easily meet your needs. Check out “new dawn” conversion of his Saturn… he can go 75 and he also has a 50 mile range.


My dad found a Toyota Celica (I think i spelled that right ?)

Down here in Dallas road rage is a big problem (I do not like to drive down here)
And if you are going 20 over the limit. You will still have people passing you like you where standing still. Going The Speed Limit makes people mad (not joking)