# Choosing a Battery

Hey Guys,

I’m new to the robot building world and I need help calculating what size battery I need for my robot.

Here are some of the specs of the robot:

1. Its a 2-wheeled robot, where each wheel is connected to a CIM motor

2. The robot needs to be driven continously for at least 45 min at a speed of 3m/s on one battery charge

3. The robot needs to carry a payload of 50lbs

4. Right now I’m planning on using 12.5" diameter wheels, but this can change

5. The robot will use an Arduino Uno microprocessor

I’ll think thats the basics, if more information is needed I can let you guys know.

Thanks for the help!

Some of the additional info we might need is…

1. terrain, is it flat, paved, uphill, etc.?
2. Is this outside with a wind component?
3. Is there any other electrical load other than two motors and controller?
4. What is the ambient temperature?
6. Have you calculated out the transmission ratio needed to get 12" wheels running at 3 m/sec?
7. What would the target motor speed and torque be for that calculation?

*Will it be going uphill or downhill?

Really, all seriousness aside, it depends on the total weight (not just the payload weight), the motion profile (how much accelerating/decelerating), the drivetrain friction (chains, belts, gearing, bearings), the rolling friction (what type of tires and what type of surface you are driving on), the efficiency of the power electronics, the wire gauges and lengths…

And what’s up with the 2 wheels? Is this a Segway?

1. the terrain will be mostly flat, an occasional incline, but nothing greater than like 5 degrees. It should run on grass, dirt road and gravel road

2. It will be outside so wind could be a factor.

3. I will be using an arduino board to control the robot, but if need be a different battery could be used to power the board

4. The average temp outside is around 65 degrees

5. the payload is in addition to the robot structure, I don’t know how much the actual robot would weigh, but I’d guess it would be around 30 lbs (depending on what type of battery I end up needing)

6. The gear ratio that I calculated was 34:1 and I calculated the robot could then travel at 3.74m/sec

7. I did not calculate the speed of the motors, but I did find the torque to be 80.5 oz-in using the 34:1 gear ratio

And this project is for an internship, and my supervisor/mentor is on vacation for the next 2 weeks, so the plan is for it to be like a segway. It’d be a self-balancing robot.

OK,
This part of the design is not my specialty but calculating the wheel circumference I get about 1 meter. So to travel 3.74 m/s, wheel rpm is about 224 rpm (3.74rev/sec60sec) and motor rpm is 7630 (224rpm34). That is about 2000 rpm higher than the free speed of the CIM. So something needs to change.
There is someone who posted here on CD a few years back that had built a cheap segway of their own. You should search for that series and see if you gain any additional info.
Al

drivetrain/battery calculator, i choose you!

just plug in your planned stats in and it will spit out a result. it also has a mah/ah calculator.

I believe the question on the table is, what “planned stat” should he plug in for this value
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The computer that i’m on has strict internet filter so I can’t access that calculator right now (I’m actually surprised I could get on this site) but if you guys approve of it, I can check it out later.

i just kinda ignore that value and leave it at 70% but here’s an explanation for it.

garbage in, garbage out

were you planning to cool the CIM motors somehow?
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If it does run for 45min I probably should determine a way to cool the motors, I’m sure they will heat up. Especially if the robot is used on a hot summer day. So would you think a fan by each motor would be sufficient? I don’t have a fan in mind that I would use though

I’ve never seen any specs or data on how long you can drive a CIM at different power levels before it overheats and is damaged.

The CIM is a sealed motor; no ventilation. My understanding is that it was designed for intermittent use. It has sufficient mass to absorb a lot of heat for short periods of operation (like a 2-minute FRC match).

An external fan might help somewhat, but the real question is how hard are you going to be driving the motors.
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I suggest you use an empirical method to solve this problem. Get a \$25 flooded lead acid battery designed for riding mowers (I think the size is U1) or the sealed type of that size used for mobility scooters, and see how it does. If it’s too small, then get a normal marine battery, or run two of the U1 size in parallel.

Another way to do it is to research what battery a Segway uses, and how much load it can carry, and how long the run time is on a charge.

I called my mentor and he said that the design requirments state that the robot should run for 45min but in all honesty the robot is only going to be used in short burst (~5min) but it should be able to handle this type of running for a long period of time on one battery charge.

Eric,
Please keep in mind that in a two wheel configuration, the motors will need to be capable of accelerating to a higher speed than your target in order to accomplish balance. As I remember the Segway is driven via toothed belts and max speed on the police version is I believe 24 MPH.

@Eric: In other words, when you’re cruising along at your desired 3 m/s, you need some headroom to increase the voltage.

Suppose you leave 2 volts headroom, so you’re operating at 10 volts at 3 m/s (I have no idea if 2v headroom is enough or too much, this is just for sake of discussion).

CIM free speed at 10v is 4425 rpm. To go 3 m/s at that motor rpm with 12.5" dia wheels, you’d need a gear ratio of 24.5:1, not 34:1.

Of course, at free speed a motor is producing no torque output. So let’s say 90% of 10v free speed, or 3983 rpm CIM speed (again, I do not know if that is sufficient1). Now you need a gear ratio of 22.1:1.

1At 10v and 3983 rpm, each CIM produces about 28.6 ozin torque. Run this through the 22.1:1 geartrain and assume 10% torque loss in the drivetrain and you’ve got about 3 ft-lb torque at each wheel. For a 12.5: dia wheel this translates to about 5.7 lb motive force per wheel, or 11.4 lb total for both wheels. Is that enough to move your 100 lb robot+payload at 3 m/s up a 5% incline on a gravel surface?
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@Ether

Suppose all of these calculations are correct and work for the robot. How do I determine if the battery will last 45min?

PS: I’m an electrical engineer intern so I don’t really know a lot about the mechanical side of things so this might be an easy question for you to answer, but when you say 11.4 lb total motive force does that mean that the force would be able to move 11.4 lbs? so therefore not enough to move my 100lb robot+payload?

The motive force required to move a 100 lb robot is usually not 100 lbs, it’s some percentage of that. But it depends on rolling resistance, how much acceleration you want, whether you’re going up hill or down hill, if you’re turning, etc. And if it’s a balancing robot, you’re going to have a balancing load…you will probably be driving the motors in an oscillating manner to keep the robot upright, so you will be using power just to stand still.

This is very hard to figure out with numbers…which is why I suggested you do it empirically. Try it and see what happens.