Choosing a Battery

[badtkee]

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!

[Al Skierkiewicz]

bad,
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?
5. Is the payload in addition to robot structure?
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?

[Ether]

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?

[badtkee]

Originally Posted by Al Skierkiewicz

bad,
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?
5. Is the payload in addition to robot structure?
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?

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.

[Al Skierkiewicz]

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/sec*60sec) and motor rpm is 7630 (224rpm*34). 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

[AlexH]

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.

[Ether]

Originally Posted by AlexH

just plug in your planned stats

I believe the question on the table is, what "planned stat" should he plug in for this value

[badtkee]

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.

[AlexH]

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

[Ether]

Originally Posted by AlexH

i just kinda ignore that value

garbage in, garbage out

[Ether]

Originally Posted by badtkee

• occasional incline..like 5 degrees
• It should run on grass, dirt road and gravel road
• wind could be a factor
• self-balancing robot
• driven continuously for at least 45 min at a speed of 3m/s on one battery charge

were you planning to cool the CIM motors somehow?

[badtkee]

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

[Ether]

Originally Posted by badtkee

So would you think a fan by each motor would be sufficient?

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.

[MrForbes]

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.

[MrForbes]

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.