12 volts for 12v 20 mm Metal Gearmotor

Robotics and electronics discussion Mechanical design and actuators
1

Hi,

My first project will be …

small 2 wheeled, differential drive, robot car
a third wheel (ball caster) for balance

It will just roam around the house for 15 to 30 minutes and then park for a recharge.

Controlled by a Raspberry Pi and a RoboClaw

3" diameter wheels

My approach to pick an RPM was to walk 15 feet at a speed that I thought was OK for the car.

That took 7 seconds and about 19 revolutions of the wheel.

That equates to about 163 RPM.

My thought is to pick something like this: 140 rpm 12v 20 mm diameter metal gear motor:
https://www.pololu.com/product/3490

Question #1: Because it is a 12v motor, is it appropriate to connect 2 of these 6.0 V NiMH Battery Pack in series:
https://www.pololu.com/category/106/6.0-v-nimh-battery-packs

Question #2: The motor mentioned above has a 12v stall current of 1.6 amps. With two motors, does that mean I ideally need 3.2amps to run for an hour which would mean, besides connecting two of the above batteries in series, I would have to double that and create a second pack to connect in parallel?

Thanks,
Bill

2

Hi, Bill.

To answer your first question, you are correct in that you can connect the two 6V batteries in series to achieve a 12V supply, but probably you should just get a 12 V battery, or you can get the 6V version of this motor.

If you actually needed to power a system that constantly draws 3.2 Amp for one hour, then you would need a power supply rated for 3.2 Amp-hours; however, I think that would be overkill for your setup. Generally, you should only load your motors with about 25% of their stall torque rating, so your average current draw should only be about 25% of the stall current (though you could use our datasheet to make a more accurate approximation). You could parallel two batteries to increase your system’s capacitance, but perhaps one of the large batteries you linked to will be sufficient on its own.

-Ryan

3

Hi Ryan,

re Question #1: connecting 2 6v batteries to get 12 volts.
I will go with your suggestion and get 6v motors for this first experiment.

re Question #2: picking the correct battery.
I will pick the motors and charge ahead and get one of the 6.0 V NiMH Battery Packs.

Then test the robot car.

Hook up a multimeter and have the car do it’s thing for some period of time

and measure the amperage.

Use that to tell me if I need a higher milli Amp hour battery.

Thanks for the info,
Bill