Help Please. Building EV from scratch

I am wanting to build a EV from scratch. I am going to try to make this project as light as possible in order to obtain the best range and speed within a reasonable budget.

I am very good a fab work from building and being around race cars most of my life. I am wanting to build the chassis and use a fiberglass racing body, it will be a two seat open cockpit design. It expect the roller to weigh around 1200 lbs (less batteries) maybe less.

I would like a range of 30 miles+ more the better and possibly a top speed of 45 mph.

Here is where I need help.
1 How many batteries will I need (voltage)? And which batteries.
2 What motor? I would like to couple the motor direct to the rear end (no transmission)
3 With two being said. What rear end ratio would I need.
4 What speed control?

Please keep in mind I have a little money to spend, but I do want to watch the budget.

Thanks in advance.


Sounds like a fun project. I’m working on a conversion myself.

You will have no trouble meeting your speed and range requirements. I’m no expert but from everything I’ve read so far here is my opinion.

1 How many batteries will I need (voltage)? And which batteries.
If you honestly would never need to exceed 45 you can run 72-96 volts.

2 What motor? I would like to couple the motor direct to the rear end (no transmission)
This isn’t very common because your acceleration goes into the toilet. You’ll need a lot more amps to get that thing moving quickly. I know the Tesla is setup this way, but they have a very high output AC motor and are able to push serious current with the lithium setup they have.

3 With two being said. What rear end ratio would I need.
Unless you can build your own rear end here I’m not sure you can really find a rear to suite your speed requirements. I looked into this on my conversion and basically removing the tranny would be like always driving between 4th and 5th gear (6 speed tranny). It would be way to high geared. I have a 4.10 rear end. You’d probably need a 6:1 to 8:1 read end. You’d have to do the math based on the circumference of your read end tires to figure it out and then base your cruising speed on the recommended RPMs for your motor (prob around 5k rpms).

4 What speed control?
Curtis seems to be most widely used. So I’m going with them since I hear nothing but good things. Just find one that matches your voltage requirements. The lower the volts on the motor and controller the cheaper it is so that’s good :slight_smile:

Just my two cents, hope it helps…

Thank you for your 2 cents. You have been helpful.

I think this will be a realy cool project. I have been thinking about doing it for some time now, and a good friend of mine who has the same interest said he would bank roll the operation… So I guess now is the time.

I have done a lot of race car fab work and this would be a pretty easy build.

I would really like to do it without the trans, it would be so much cleaner and lighter.

I was asking ragee on his 86 Ranger thread what gears he used, so that I could roughly calculate his overall gearing to maybe use as a starting point.

Any suggestions on part numbers on motors, control, etc.?

I forgot to ask. What is the primary purpose of the build? Commute to work in the city, drive to the grocery store, etc.


Brian No real purpose, just looking to build a fun vehicle to showcase EV’s.

I’m doing a conversion, so I’m not sure how much I can help

but from what i can tell a 72v system is probably the way to go for a cost concerned individual. That is what I am doing. As you go up in voltage the controllers go up in price, but after 72v you equipment seems to get really expensive.

As for speed I am hoping to be able to push my Toyota Celica up to 75 mph. I’m going to use a five speed transmission. So I think you should easily be able to get 45 mph without a transmission.

A good way to check your speed would be to calculate it

If you want I can show you how to do it. Or if you’ll tell me some details about the car I can calculate it for you. (a rough calculation)
The approximate weight (tell me if that includes the batteries and motor)
And the hight and width of the front of the car. As well as the general shape, and size. (to calculate the aerodynamic drag)

With that calculation I can tell you about what your going to need in the way of Amps, Horsepower, and number of batteries (more or less).

Hope I can help

I’ll be back in 10 hours

good night

U4 Thank you. I would like to get by on lower voltage if possible. (lighter)

As I said if it will run a solid 45 mph that would be good. I can travel all around my city and be ok at that speed, most of the time 35-40 would be fine. If it goes faster thats great, but I would rather have a little extra range and acceleration than top speed.

After a little more calulation, I “estimate” the roller to weigh 1000-1100# (less motor and batteries).

And the body is pretty slick. Here is a link to the one I am looking at.

That should really draw some attention.

That looks fun : )

Ok I’ll assume that the front of the car is about 5" X 18"
I can’t tell the exact size, but I shouldn’t be to far off. Aerodynamic drag won’t be that big of a problem at 35-45 mph

1,100 lbs, you should be more efficient than my 2,000 lb car

OK give me an hour or so

Remember that I am being very conservative with my numbers

Well if I got this right, your car will weigh 1100 lbs (w/o batteries and motor).
Our motor weighs about 200 lbs, but it’s huge (your probably not going to use something like that). So lets say your motor weighs 150 lbs (that is still a heavy motor). Now for the batteries, just to get 72v I would recommend six 12v batteries. which weigh any where from 60-85 lbs. I think a 75 lb battery is a good guess, and six of those is 450 lbs. So the total weight should be somewhere around 1,700 lbs.

the equation for aerodynamic drag is Fd = (CdAV^2)/391
Fd is the force due to aerodynamic drag
Cd is the coefficient of drag
A is the area of the front of the car
V is the velocity of the car

the Cd has to be somewhere around .3, but I think it’s better to over estimate. So I’ll use .35.

Fd = [(.35)(90 sq ins)(45mph)^2]/391 = 32.628 lbs
multiply this by 150% (so we have a margin for error) = 48.9 lbs
which is equivalent to 50 lbs

Next the equation for rolling resistance is Fr = CrWcos(theta)
Fr is the Force due to rolling resistance
Cr is the rolling resistance factor
Cr = 0.015 on a hard surface (like concrete, I think you want this one)
Cr = 0.08 on a medium-hard surface
Cr = 0.3 on a soft surface (like sand)
W is weight in lbs (as opposed to Newton’s, Ha)
cos is a basic trig function (won’t go into details)
And theta is a Greek symbol used to represent angles (in this case the angle of your tires, lets assume your going strait, so theta = 0. And the cos(0) = 1)

Fd = (0.015)*(1,700 lbs)*cos(0) = 25.5 lbs
multiplied by 150% = 38.25 lbs
which is equivalent to 40 lbs

So the total drag on the engine should be about 90 lbs, but lets make it easy and say 100 lbs.

The equation for horsepower is HP = F*V/375
F is force in pounds (by now you know F = force)
V is velocity in mph

HP = (100 lbs)*(45 mph)/375 = 12 hp

The equation for Watts is W = HP*745.7
W is Watts
HP is horsepower

W = (12 hp)*745.7 = 8,948 Watts

To get Amps we use I = W/V
I = current in amps
W = Watts
V = volts

I = (8,948)/(72v)
I = 124 amps

One way around the rear end gearing issue would be to use two motors in series to run the driveshaft, with an electrical switch to change the wiring from series to parallel. From what I’ve read here, the switch from series to parallel will be similar to shifting gears with a transmission, so a 2 motor setup would be like having a 2 gear electric transmission while still being a mechanical direct drive. I can’t remember for sure, but think wiring them in series puts half voltage and full current to each motor for low end torque, wiring them in parallel puts full voltage and half current to each for speed. The important thing here is motor mass, two motors that are half the size of the motor you want to use will have more or less the same power, so long as the mass of the innards is the same. The advantage with two motors is having twice the brushes moving power, along with the ability to easily do the series/parallel wiring. Here is where I first learned about doing this in an EV.

For other reference, a fellow in Arkansas used a jet starter motor at 36V, was able to move a 3000lb car at a constant 45MPH on flat ground. Going with a smaller vehicle, and a higher voltage system, no problems.

Hope all this helps.

That doesn’t seem right

Oh my bad, hold on

I think you will want six 12v batteries in series to give you 72v
you are going to want to look for a battery with high Amp hours (AH)

Here is one that some one told me about, the Trojan T-1275. The problem is it is expensive, very much so. But something with 120 AH will do.

Oh and the top speed, and acceleration is linked mostly to the voltage your using. But also important is the rpm of the motor, and the gear ratio. What I calculated is on a 1:1 gear ratio (I think). And pretty sure, that you will get more rage and maybe use less amps, if you down grade the gear ratio. But thats not my field of expertise.

And you can get range by putting more batteries in series to make a higher voltage. because all batteries in series put out the same number of amps. So battery pack with 4 batteries will last twice as long as a battery pack with 2 batteries (assuming there the same kind).

You can get more rage my putting in an extra string of batteries in parallel with the first. However this is not as efficient as getting more range by putting more batteries in a string. Because putting them in series will increase the length of time you can use your batteries, but it will also allow you batteries to get the same performance without giving as many amps.

So, you might want to go with six 12v batteries, and maybe step down the gear ratio a bit.

Fun fact about motors:
combustion engines are rated in a lab under ideal conditions (for marketing)
Electric motors are rated at there worst sustainable performance. (for industry)

In fact the electric motor ratings are a little lower than nominal performance

Electric motors are strongest when stalled (0 rpm), and after start up they generally have 2-3 times the rated horsepower. My friend said that this is a conservative figure too. Because he was getting 4 time the rated horsepower (cannot be sure of the validity of that last calm, but so far I have no reason to doubt)

A combustion engine dose not give any torque until after it builds up its rpm.

The one we are using is a refurbished fork lift motor we got for free. So I didn’t look to hard for motors. I don’t know, You might find some good ones on e-bay.

I don’t know much about controllers, but I’m using a Curtis controller because I haven’t seen anyone complain about them. I think it was Kimberly that lies about how many amps there consolers will take. My friend bought a Kimberly controller rated for 600 amps, he said his gage never picked up more that 300 amps, but the controller burned out. And there are apparently other reports of unjust failure.

Hope this helps, Good luck

I want to thank both of you for the input. It is very helpfull. As I said before I have alot of time in the auto field and race car fab and racing. I also had a Citicar a couple of years ago that I tinkered with and have raced radio control cars since the 70’s and I have a better than most knowledge of electrics. But I just have not been around these large motors and controllers to pick for myself and really value the help.

I want to add. I am willing to increase the voltage some if it helps with the range, speed and all around power. I will have plenty of room for batteries, I just don’t want to completely break the bank buying them. And I will be using a rear axle that I can gear anywhere fron say 2.56 to 4.56 in ratio.

I see that Electric Vehicle USA offers some kits. Would any of these be a good starting point or should I look into others?


I posted this in another thread. It looks like it would eliminate the need for a differential. I don’t know what it costs but I’m sure if you called them they’d tell you.

I don’t know about kits. If they’re cheaper, then go for it. If your still looking for a 72v system, then…

On ebay I found a $300 72v motor controller that takes 400 amps. the seller said it was used, but would run like new. And he has 100% positive reviews.

I also found a $350 electric motor, its only rated at 6 hp, but I think it might work. This guy has 98.9% positive reviews.
That takes care of half the car right there for $650 + shipping

Whats left after that…
a potentiometer

So i think it might be better to by the part individually.
I’d look around before committing to anything; because its much cheaper to change your mind before you have the parts.

By the way, I went to visit this guy
He did a 72v system and got on the news. his goes 45 or 55 mph at top speed
But he said one of the problems he has, is overcoming hills. Just something to think about.

PS sorry if i insulted you, i thought you where like my dad. If it can be welded, epoxied, ground, cut, soldered, or cleaned, he can do it. However if something involves theory, or calculation then its usually my job.

Hope this helps

No you did not insult me at all. I am not known on this site and was simply trying to give a little of my background. The advice is great, I can use all I can get.

That would be great if I could use those components, That would cut the cost a ton.
In your calculations you list the amps as 124. Is this at the 45 mph? And the 12 hp, is this what is needed also for the 45 mph?


I also found a $350 electric motor, its only rated at 6 hp, but I think it might work. This guy has 98.9% positive reviews.
That takes care of half the car right there for $650 + shipping

Hope this helps[/QUOTE]

that motor has no face to mount it to. You’ll have to machine something to support the shaft on the output side with a means to hold a bearing of some sort.

Yes that appears to be an open endbell motor. Would that 400 amp control be enough amperage for what I am wanting? It would be a good buy if so.