3 CIM Drive Train Battery Draw

We’ve only ever used 4 CIMs on our drive train before this year so I was thinking, with 2 extra CIMs, have teams noticed a shortened battery life?

Specifically, when your’re running all out in a match, has it been an issue?

We haven’t noticed a big difference with ours but we are running 2 cims and 1 mini cim…our winch has the other 2 cims so we use all jags with CAN; we can then use current sensing so we can slow the line power while drawing back. We also have optimized auto shifting to help be most efficient :wink: . We love the acceleration though::safety::

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

It wouldn’t “run-out” but more drain a little faster and depending on how a team makes their mechanism, it could alter its effectiveness.

Last year, we used a 3 CIM drive train (6 total), with 2 BAG motors and 1 Andymark motor, all at once and we did not find any performance drop, throughout any match. We trust it enough to use it again this year, if that makes a difference.

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.

By 8 CIMs you mean 6 CIMs + 2 MiniCIMs?

It might be quite rare, but it is possible. 95’s 4-wheel swerve robot in 2002 could easily drain a battery in a single match if everything wasn’t just perfect. A reasonably high, steady, current draw can definitely deplete an FRC battery in 2.5 minutes or less.

We used 4 CIMs for driving the robot and another 4 to pull it up the pyramid.

I know of several teams that have tripped their main breaker with six CIM drive trains in addition to other motors. If you don’t monitor current at some point in your design and build phase, this will turn out to be a risky choice. Drawing 120 amps continuous for 2 minutes will not only draw the battery down, it will at a minimum shorten it’s life and likely will cause some internal damage. Even to assume that the Amp Hour rating will be derated by 40% by the higher draw leaves you potentially, with just a few minutes before the terminal voltage falls below the critical 5.5 volts when the DSC stops working or the 4.5 volts when the cRio power supply goes away.

Still not being clear… 4 CIMs and 4 MiniCIMs??

Hmmm, wasn’t the breaker that year at 80A? I suspect the RC or radio was losing power as the battery was drawn down. No dedicated supply for the radio or backup battery for the RC to prevent it from resetting.

It was a 60A main breaker. Like the current 120A main breaker it was tolerant of large over-currents for prolonged periods of time. I don’t recall that robot ever blowing a main breaker.

Don’t get me wrong, resetting also happened (at around 6V IIRC). More than once. The next year we got back-up batteries… and not long after that we got 120A main breakers too.

As a student on the pit crew for that robot I distinctly recall checking battery voltages after matches (where the robot was driven a lot) and reading 9-10V floating on the battery.

The big SNAFU with that robot is that we used worm gears to get lots of gear reduction in the small space on our swerve modules, and if anything was a tiny bit out of alignment the drag from those gear-sets could cause ~40-60A no-load draw, and it could be more if the steering motors were active.

Are you sure it was 8 CIM’s?

Is that your final answer :wink:

The CIM’s each only draw how much power is required to accelerate the bot. 3 CIMs only become useful if your robot is playing tug-of-war (or push of war) :D. The CIMs are rated at 67A/hr? That doesn’t mean that they will always use 67 amps. That is what will happen on full load, and I think you have other things to worry about by then! :wink:

We ran in our 3 CIM gearboxes over the weekend. Measured with a DC clamp on ammeter the total currrent draw for all 3 motors on an unloaded transmission was 9.2 amps. The funny thing is when checked individually 1 motor was pulling 4.4 amps, the next was at 3.3 amps and the 3rd was at 1.5amps.

When the next transmission was run in we found similar readings as well. The same motor positions in both transmissions had the same basic readings inasmuch that they went from a high current draw to a lower current draw based on their mounting position. The readings only varied by a few tenths of an amp.

After checking gear lash there were no significant differences when run again. These are the AndyMark 3 CIM upgrades for the AM14U chassis…

Straight up wrong. We’re rocking 3 CIMs in our DT and have 0 intent on pushing anyone. They provide more torque which allows faster acceleration given proper gearing. And let me just say, this beast drives far different than anything I’ve played with.

Caveat lector:

The CIM’s each only draw how much power is required to accelerate the bot. 3 CIMs only become useful if your robot is playing tug-of-war (or push of war) . The CIMs are rated at 67A/hr? That doesn’t mean that they will always use 67 amps. That is what will happen on full load

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

The past three years I’ve been involved with a couple teams using 6 motor drives:

2011

Cybergnomes#2013: 4x CIM + 2x RS775 on Single speed 8x8 drivetrain, 1xRS550 arm stage 1, 1xRS375 arm stage 2, pneumatic gripper.

2012

Cybergnomes#2013: 4x CIM + 2x RS775 on Single speed 8x8 drivetrain, 2x RS550 shooter, 1x AM9012 Conveyor, 1x Window motor pickup retract, 1x RS545 feeder, 1x RS375 Azimuth control.

Ramferno#3756: 4x CIM + 2x FP0673 on single speed 10x10 drivetrain, 2x RS550 shooter, 1x window motor feeder, 1x RS550 conveyor, pneumatic pickup retract.

2013

Cybergnomes#2013: 6 CIM drive single speed on 6x6 drivetrain, 2 MiniCIM on climber, 1 MiniCIM Shooter, BAG Motor Azimuth control.

Never once did we experience a main breaker trip. All machines since they were single speed drivetrains were geared to operate at the limit of traction. They had great acceleration, good controllable speed and phenomenal torque. As far as pushing power, they were on-par with the 2 speed 4 motor drivetrains. However, when in a prolonged shoving match they did eat much more power than the 2 speed drives. The behavior we noticed is that if the drivetrain managed to stall (was designed to break wheels free first but on a weakened battery it would occasionally happen), since no three DC motors are exactly the same (why BrianC was getting different current draws during no load) it seemed that the 40A auto-resetting breakers would trip in a cascade fashion. One would go (highest draw motor) which would increase the load on the other two whose breakers would respectively follow suit.

Personally, I think 3 motor 2 speed is a waste of money unless you can generate enough traction through mechanical interaction with the carpet to warrant it. Single speed 3 motor has the benefit of being able to compete with multiple ratio transmissions if geared properly because we are all limited by weight. Remember: force of friction (traction) = normal force (weight) x coefficient of friction (gum rubber on carpet in best case). Since we can’t use studs or an apparatus to “claw” at the carpet, every robot is more or less limited in traction by its weight. Where the single speed shines is its ability to go whistling across the field, HIT a robot thus exceeding its static forces of traction with the carpet by overwhelming it with momentum and then “freight train” it across the field without ever letting up or shifting.

That being said… 3 motor 2 speed would allow you to wrestle with other robots without using as much power and with very little worry about tripping any breakers.

Instead you can worry about peeling the tread off your wheels and incurring penalties for damaging the field surface.

For this year, we are again building some very power-hungry robots and I am somewhat concerned about power draw myself but if past experience is any indicator, there shouldn’t be too much to worry about. We may be implementing some current-saving techniques like shutting down the compressor or even two drive motors while reloading the primary weapon but all in all I would not be too worried.

One thing to mention though, keep an eye on your batteries. 6 motor drives WILL bring a frail battery to its knees during competition so be wary of that old grey battery that your team has had kicking around from 3+ seasons ago.

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.