[mcfly]

Hi. I am at the brainstorming stage of an idea. I know my way around IC cars but I only know a few basics about EVs.

My question is, how much of an electric drivetrain would it take to power a 4500-lb vehicle (that is the fully loaded figure), but it only needs to work at slow speeds? Like 0-15 mph? Imagine a car or small pickup truck maneuvering around in parking lots. Stop-n-go traffic. (The motor is powering the driveshaft or differential. No sending the electric power through multiple gears with a regular transmission before it reaches the wheels.)

How much of an electric motor would this require (size & weight)?
How much battery capacity (size & weight) would it need for, say, 10-15 minutes of that?

By keeping the speed/performance demands low, does this allow for a much smaller/lighter motor (compared to a typical EV conversion) or does it mostly just let you get away with carrying less battery?

I'm only looking for some very rough wild estimates here.

 

[madderscience]

you would only need maybe 10hp, assuming no steep grades or mud bogging involved. basically, find a modern electric forklift drivetrain and transplant that. 48V AC system probably. Figure battery size based on hours of autonomy drivetrain friction, and rolling resistance of the loaded rig. Aerodynamics won't be a factor. If this thing really only needs to run for 10-15 minutes, you won't need much of a battery. You will probably need a larger battery than otherwise necessary just to get the power density. 60ah at 48v would probably do it if 3C discharge rate capable.

 

[brian_]

The motor is powering the driveshaft or differential. No sending the electric power through multiple gears with a regular transmission before it reaches the wheels.

If you use a typical EV motor, you're talking about taking a motor which needs to run at thousands of rpm to develop maximum power, and forcing it to run at no more than a few hundred rpm. You could plan on a large motor at low speed and voltage, basically using something like a forklift motor the way it was intended (as madderscience suggests), rather than the way most DIY EV builder use them, but the alternative is to use a smaller motor and a single-speed reduction gearbox.

 

[mcfly]

Thanks for the responses.

A fixed amount of gear reduction on the motor is okay. I just don't want to be shifting gears & working a clutch pedal while driving it. I want this to be something you can drive with one foot.

 

[madderscience]

it would be easy enough to take a standard manual transmission and lock it in first or second gear, and remove the shifter and clutch mechanism. If you are ultra concerned about efficiency, you could remove most of the internal gears and just leave the ratio you want. I did this with my 85 MR2 conversion - I removed 5th and reverse ratios. The modifications consisted of a mix of just grinding teeth off of gears and manufacturing spacers to take the place of removed components. Only thing to be careful of is there are still enough moving parts dipping in the oil to sling it around and keep things lubricated.

 

[brian_]

A fixed amount of gear reduction on the motor is okay. I just don't want to be shifting gears & working a clutch pedal while driving it.

One of the simplest ways to do this is to mount a motor over a final drive unit (differential), and connect them with a chain or belt drive to provide additional speed reduction. Someone in this forum did this, I think in a Scion xB, but perhaps in a different vehicle.