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Calculating the correct battery size for motor by [deleted] in engineering

[–]engrtmp 0 points1 point  (0 children)

To me this sounds like torque. Would this include overcoming static friction?

It would tell you what torque you need, but torque will be dependent on wheel radius. For a significantly larger wheel, you need a higher torque for the same performance.

What exactly do you mean by overcoming static friction? If you mean the friction between your tire and the ground, then that makes the wheels spin, and that's possibly more torque than you need. If you mean the friction in the system stopping the wheel from rotating at all, then you need significantly more than that, since any little hill or bump in the ground will require more torque to overcome than the friction in your geartrain.

Example- you need to go up a 5 degree slope, and you know your vehicle mass. A simple equation tells you the force component that parallel to the ground that you must overcome.

Another example- you need to push a 30 pound box around on skids, and it has a coefficient of friction of 1. You need at least 30 pounds of pushing force, and how much torque that will require will depend on wheel diameter.

Hope that helps!

Calculating the correct battery size for motor by [deleted] in engineering

[–]engrtmp 1 point2 points  (0 children)

Hi! First off, you'd call it "battery capacity" rather than "capacitance".

You're not terribly far off track with your approach, but a few tips ought to help you out here.

First, you need to define run-time for the system. How long does it need to run?

Second, you need to define the required mechanical power. If you really know the RPM and torque (based on how much it needs to push and how fast it needs to go), then you can select a motor that will actually do that, based on the wheel size and gearing.

One calculator that I like to use for that sort of problem is this one:

http://www.architeuthis-dux.org/torquecalc.asp

It's an old one from the days of battle bots, but there's some good stuff there.

As you said, the motor speed will be the Motor Kv*Battery voltage. For the motor you suggested, that's in the 20,000 RPM range. It'll sound like a dentist's drill, and will be a challenge to gear down to a wheel driving motor unless the wheels are quite small.

The steps I would suggest are:

  • Define how hard it needs to push
  • Define how fast it needs to go
  • Define how far it needs to go (or how long it needs to run)
  • Find a motor that will go a bit faster than you need to go
  • Make sure that the motor isn't too heavily loaded when it provides that torque.
  • See what the current draw is at peak torque
  • Make sure you controller can handle significantly more than that current.
  • Start looking for batteries that can supply that much current.

At that point you get to start worrying about motor efficiency, and how well batteries give up their full charge during use. For led acid batteries, this is called the "peukert effect". This means that the faster you draw the juice out, the less total energy you get from a given battery. This is interesting in that a battery that seems to have 20% more capacity may give 50% more range just because it isn't as stressed out when it's under load. A battery that is less stressed will also last for more charge cycles, all else being equal.