Question about batteries for an electric cart project

[mp19393]

Hi everyone, I am a senior Chemical Engineering student working on a project for a course of mine designing a remote control car. We have made 10 batteries from hand that each produce roughly 1.5 volts each. The car is fairly heavy and we only have a small electric motor to work with, and thus we would like to maximize the torque the motor produces.

My question is, to maximize torque, would we want to put the batteries in series to maximize the voltage applied to the motor or in parallel to maximize the current through the motor? An explanation of why or what either case does physically to the motor would be great. Thanks in advance!

 

[Tom.G]

With a fixed design of an electric motor the current it draws is directly proportional to the torque the motor is delivering. The motor speed is directly proportional to the applied voltage. Note that this is for a given motor design. "Within reason," a motor can be designed for desired speed and torque with given input voltage and available current. Of course you can't expect more output power than you supply, 1HP = 746Watts. To keep down the size of the wire and switches, thus their weight and cost, I suggest a higher voltage motor. Also figure out how you will charge the batteries (if rechargeable), most (all?) are very sensitive to being charged in large banks, either series or parallel.

 

[jim hardy]

What is the nature of your small DC motor ? As Tom.G points out there's more than one way to design them.
For high torque loads like vehicles one often chooses a "Series Wound" DC motor.

Tom also pointed out there's a voltage vs speed characteristic,
You might get fancy and start the cart with batteries in parallel for high current hence torque, then once rolling switch them to series for higher voltage hence speed. Diesel-Electric railway locomotives of the 1960's did something similar with their traction motors.

 

[CWatters]

I suppose we should ask why you want to maximise torque ?

A lot depends on the voltage range the motor is intended for. Model car motors are typically in the 6-7V range so one option might be to put two strings of five cells each in parallel?