I’m working on a small robotics project, and since I’m also I member of the FIRST community, I thought I would see what you guys have to say.
Here’s the situation. I have a small robot 16"x16"x24" (small compared to FRC) and weighs about 45 lbs. I’m using mecanum wheels for the drive and 4 bane bot rs550 motors. The robot must also be able to throw a ball, so I’m using 2 more rs550 motors for a baseball pitcher style throwing wheels, and one more rs550 for the ball feeding mecanism.
My question is: Will a 12v lead-acid battery with 7.2 amp-h be sufficient for this application? I will be able to change the battery every few minutes or so just like in FIRST, so I just need to know if it will be enough power to get the job done.
Thanks for your help! Below may be a couple useful links regarding the battery and motors.
7.2 A-H means you can draw ~70A constantly for up to 6 minutes. Depending on the gearbox you’re using with those RS-550s and how fast you run them, they’ll probably pull from 5-20A each. If this if a competitive robot, your drive motors will likely pull more current and should be what you’re worrying about. If it’s for demonstration or something, then I’d mostly worry about the RS-550s. In which case you could probably run that shooter continuously for at least 10 minutes or so before you had problems.
So yes, I think a 7.2A-h battery would probably suffice. Is there a particuar reason you’re going with lead-acid and not, say, a 12V, 10A-h NiMH battery pack? That NiMH pack is about 60% the weight, with about 40% more A-h, after all. Of course, it costs about twice as much as well…
No, the only reason I am using the lead-acid battery is because it is a battery that we already have several of. It’s a small robotic football competition with my school. Because I the plays are very short, I will have time between plays to change the battery if need be. I just want to make sure that while I am using it, I’ll be able to provide enough power to the motors to accurately throw the ball and not have too much change in voltage.
Okay, so you are suggesting controlling the motors based off of feed back from encoders? That’s a good idea to help keep it consistent. I just want to make sure that I have the voltage available to give to the thrower wheels. Do you think that will be an issue?
Design your thrower so that the thrower motor needs only 9 volts to throw properly. That will give your speed controller some headroom to hold the thrower speed constant.
Using the spreadsheet together with your estimate of the duty cycle, you can get an idea of the amps you’ll be drawing from the battery.
Then you can compare that to the discharge curves in the datasheet for the battery:
… and you can determine if the battery can handle it.
You mentioned that the batteries are left over from a previous project. Have they been properly stored? If not, they may have substantially diminished capacity. You should charge them completely, then run a capacity test.
I can send you details how to do a home-brew capacity test if you don’t have access to the proper test equipment.
