3 CIM Drive Train Battery Draw

[yash101]

Originally Posted by kevin.li.rit

Well, unless the battery has reduced capacity I still don't think you'll run out.

You have a 120A breaker on the robot which is already limiting your current draw. A fully charged 17AH battery is not going to run out in 2.5 minutes

Unless you have 3 XU's like 1706 XD. That would draw some power!

[yash101]

Originally Posted by robohead456

Last year my team used 8 CIM motors in addition to a window motor. The battery lasted throughout the match, but to be sure, we charged it immediately after and used a different battery for the next match.

I thought there was a 6 CIM limit?!

Originally Posted by Ether

Caveat lector:



... The rest of the post was straight up wrong as well.

What's wrong in it? Maybe the 67AH, but that has a question mark of uncertainty on it. It's just go get the point.

Originally Posted by JamesCH95

Please do a LOT more homework before posting information like this. What you posted was terribly incorrect information.

6 cims can accelerate a robot faster than 4 cims, so they draw more power total.

Cims are not 'rated for 67A/hr'...motors aren't rated by charge capacity, batteries are.

Cims won't always use 67A, but at stall (maximum torque) they can draw 133A. At peak power output they will draw 68A.

Isn't that what I said?! Also, I was referring to the draw (i don't know exactly how to express it). However, I think it should have been understood by the context.

[Chris_Ely]

Originally Posted by yash101

What's wrong in it? Maybe the 67AH, but that has a question mark of uncertainty on it. It's just go get the point.

Isn't that what I said?! Also, I was referring to the draw (i don't know exactly how to express it). However, I think it should have been understood by the context.

I could not understand what you said.
Please do research before you post. If you don't know something, then research and ask questions rather than risk posting false information as fact.
DC motors current draw is inversely proportional to speed. When speed is zero (i.e. stall), current draw will be greatest. When speed is greatest (i.e. free speed), current draw will be least. In the case of CIM, it will draw 133 amps at stall, and 2.7 amps at free speed. It doesn't draw 67 amps per hour, nor 67 amp*hours. Take a look at motor curves. This Instructable explains it well.

[magnets]

Originally Posted by yash101

Isn't that what I said?! Also, I was referring to the draw (i don't know exactly how to express it). However, I think it should have been understood by the context.

No, you said that a 6 CIM drive only makes a difference in a pushing match, which isn't true.

[Al Skierkiewicz]

Guys,
When you take a battery directly off a charger and add it to your robot, the dashboard will report a terminal voltage of something higher than 12 volts. This will rapidly fall to a nominal voltage once you start to draw some current. This is a well documented condition, even here on CD. This is not an indication that the battery is any better than another battery, simply that it has some "surface charge" from the battery charger. Lead Acid batteries require a charger to have a higher voltage to push charge current into the battery.

[Danny Diaz]

We have 2 CIMs on each side of our 8-wheel DT (one of the AndyMark systems) and we're sucking battery so fast we can only go about 3-4 minutes before we see noticeable voltage drops on the batteries and the system becomes sluggish. We've not been battery hogs like this in ... ever. Yes, everything has been lubed properly.

<shrug>

-Danny

[Al Skierkiewicz]

Danny,
Something that doesn't come up in these discussions that should be. When you charge a warm battery, it sometimes fools the charger. To make sure you get max charge, let the battery cool down after you take it off the robot for 30 minutes or more. Also, lots of traction/friction with the floor will eat batteries in turns. If you are doing a lot of turning, this will also affect battery life.

[Ether]

Originally Posted by Danny Diaz

We have 2 CIMs on each side of our 8-wheel DT (one of the AndyMark systems) and we're sucking battery so fast we can only go about 3-4 minutes before we see noticeable voltage drops on the batteries and the system becomes sluggish. We've not been battery hogs like this in ... ever. Yes, everything has been lubed properly.

<shrug>

What is your gear ratio (from motor all the way to the wheels) and wheel diameter?

[fox46]

we're sucking battery so fast we can only go about 3-4 minutes before we see noticeable voltage drops on the batteries and the system becomes sluggish.

Your problem may lie in your batteries. How old are they and have you load tested them lately?

[PandaHatMan]

The question should be rephrased.

It's not "should we use 3 CIMS instead of 2?" but "Do we have weight for 3 CIMS?"

From my experience, a 6 CIM drivetrain performs better period. One of the things it improves very noticeably is turning.

[Ether]

Originally Posted by fox46

Your problem may lie in your batteries.

Good point.

How old are they and have you load tested them lately?

Testing the Ahr capacity would be more relevant to the situation Danny has described.

I've seen 12v lead-acid batteries pass load tests with flying colors but fail Ahr capacity testing miserably.

[RoundTabler]

Originally Posted by Ether

Good point.
Testing the Ahr capacity would be more relevant to the situation Danny has described.

I've seen 12v lead-acid batteries pass load tests with flying colors but fail Ahr capacity testing miserably.

Yes. We ahr test our batteries every year. Normally we only use the ones that have 10+ ah. We have a couple batteries from last year we are only using for practice, because we trashed them last year. we had 6 cims, and 2 rs550's, which can use a lot of current.

[Al Skierkiewicz]

I would class 10 AH as trashed and recyclable. What are you using to make the rating determination?

[Ryan Dognaux]

Thanks to Al & the other field management volunteers for talking to our team on Friday & Saturday at the end of the Central Illinois Regional and spending a lot of their time helping us troubleshoot our robot issues. Some of our sporadic performance at CIR was due to what we now think were power issues. We're running a 6 CIM all gear drive through the 3CIM VexPro ball shifters, with an on-board compressor as well that runs for a lot of the match. We switched our jaguar motor controllers to ramp mode to try to help with the immediate 'shock' to the system but that only seemed to be a partial fix. In some matches our CRIO & radio lost power for very short periods of time, effectively taking us out of the remainder of the match as our CRIO rebooted & loaded robot code. We also have some wiring issues (battery & main breaker too far away from the PD board) that we need to fix that may help with this, but we're seriously considering removing 2 CIMs from our drive just so we can function properly at the St. Louis regional.

We also tried to be aware of which batteries we were using when we were having these issues on the field. One of the times we reset occurred while using a 2014 battery; all of our batteries are less than 3 years old & were load tested & shown to be in good condition.

Another mechanical option we're going to implement is removing our outside traction wheels & replacing them with omni wheels, making turning much easier on the robot. Hopefully the combination of all of these changes will yield better results for us at the St. Louis regional.

[z4t143]

I'd like to add my thanks to Big Al and the FMS crew at CIL for their help!

One additional question for Al ... other than the weight penalty, is there any other consideration or FRC rule implication to moving to larger gauge wire on the Battery to PDB loop?

Section 4.8 of the game manual and related figures all reference 6 AWG Wire (min).

It may be easier for us to move to larger wire gauge at this point versus re-arranging components. Thoughts?