Resources for Electrical load information

I would like to find out more information about how to plan for electrical loads. I have a basic knowledge of Volts and Amps, but want to know how to plan for how different components / motors / lights / etc… will impact out battery life or put us at risk of failure.

I understand that resistance plays a big part in motor draw, but would like to teach the team what to look for.

Any info would be great!

The FRC Game sense has a really good primer on motor & gear box selection. You really cannot have one without the other. I will go out on a limb and say motors are the biggest load ion a FRC robot. So understanding them is the first step in controlling loads.

Datasheets are a great place to start in order to judge current needs of a particular component. Most motor datasheets I’ve seen list a free current draw and a stall current draw. Even though the actual current draw will vary from application to application (load on motor, speed, input voltage from speed controller, etc.) those measurements will give you a good place to start, and can be used to get a rough estimate.

As far as other components go, like the control system, the current draw listed in a datasheet is generally pretty reliable - assuming you can find the correct datasheet. IMO it is generally safe to assume that an individual component from the control system wont draw more than a couple amps at max.

Chris,
I like to calculate worse case items. Most people forget that stall current on motors occurs when the motor has electrical power connected and the motor is not moving. (Like when the motor is starting) It is generally accepted that most teams can wire a CIM such that it can draw between 100 and 116 amps in stall per motor. That number will not tell you how long your battery will last but it will tell you a few things about the electrical system. I have talked about the “wire foot” for some time now. My definition is 1 foot of #10 wire drawing 100 amps will drop 0.1 volt. Basing on that, #12 is about twice (actually 1.6) the series resistance so 1 foot of #12 is 2 wire feet. #6 is about half of the series resistance so 1 foot of #6 is 0.5 wire feet (wf). #14 is almost 3 times the #10 per foot.
The critical factor here is the available voltage then determines the actual current supplied to the load. The 116 amps above is calculated on the loss of a standard run of 4 feet of #10 and the series resistance of the speed controller, battery and #6 wiring. Without the series resistance the motor is speced at 131 amps stall at 12 volts.
To make calculations on how long your battery will last, you will have to come up with a model of how the robot is driven. You might for instance, guess that the robot will normally drive for 10 seconds at 30 amps per CIM then stop for 10 seconds while it is trying to score. Then turn for two seconds and drive for ten seconds and then stop to pickup or defend. Doing worse case again, assume that the 18 amp hour battery under our conditions actually will derate to 15 AH or less. You might be able to back into a calculation on battery life. I have seen teams actually do spreadsheet analysis for this and come pretty close to predicting the actual robot behavior. The critical factor is keeping parts of the control system at levels that will prevent them from rebooting. In the previous control system, 5.5 volts was the critical battery level. At that point, the digital side car would cease functioning and all motor control would stop. At 4.5 volts the power supplies would stop functioning and the cRio and/or the radio would reboot. This drop is due to the series resistance of the #6 wiring, and the internal resistance of the battery (11mohms at full charge).
BTW we had a great time in Marceline. Everyone we met was friendly and helpful.

Thanks Al, it was great you stopping by. Just purchased “Behind the Design”, saw it referenced and purchased it online then was surprised when I saw that you wrote it. Think you need to do another, more updated version :). Great place for team members to look for ideas.

Chris,
I only added a some ideas. Vince and Stephanie did all the work.