high strength motors and battery power

Do you think a robot with out the power expander be fine using 4 high strength motors with 6 regular motors? How long do you think the robot would be able to run?
Sorry I know I know that you have to test. But we are meeting tommorow and I need to tell them that we either do not need the power expander although it would be good or yes you do need it because are robot wont run for a full match.
The reason is that we want to save money by ordering the cortex micro controllers and any other parts we need?

If you’re interested in saving money, you could wait on buying the Cortex. It is not required for use with the High Strength Motors, but it is certainly nice.

it depends on how your using the motors
since the 4 new motors convert to 8 of the old ones
you are actually running 14 old motors
so power expander would be recommended
it depends how extreme you want your robot to go :slight_smile:

It’s ok i am pretty shock, but are advisor is buying cortex microcontrollers for 2 of his teams so it is ok. Do you think that it would be possible to use no power expander and last atleast 2 matches?
If you use the old microcontroller you would need 4 motor controllers to use the new high strength motors

That’s not how DC motors work…

You’ll see on each motor’s spec sheet that they draw roughly the same current at free speed and stall, operating at the same voltages. Thus a High Strength Motor under loads proportional (not equal) to a normal motor will have roughly the same current draw (i.e. either motor will draw x current at y% of stall, though different loads will bring them to y% of stall).

I would say “probably” since I’ve run 10 motors through >1 match without a Power Expander.

So is it possible to last 2 matches with no power expander and 4 high strength motors and 6 regular motors.

That depends largely on how much each of the motors is used. When my team was practicing with a power expander, we could easily practice for 5 minutes before we had to change the batteries. We also had a lot of old batteries that didn’t hold a charge very well. So I think that you will be fine without the power expander if you use new batteries (i.e. barely used NiCd batteries or the new Lithium batteries). I would just make sure to switch batteries before every match.

You could always try your robot without the power expander and if it doesn’t work buy the power expander.

you will be fine
at world we had the power expander and we swapped out our batteries after 3 matches
we were taking the most out of the motors too :confused:

This message is about using the NiCad batteries:

With the V0.5 microcontroller and ordinary motors, and a single NiCad battery that is in good enough shape to hold 1500-2000 mAh, you can run for several matches before exhausting the battery.

During each match, and over the course of the several matches, as the voltage the battery is able to sustain under loads drops a bit, your motors will spin a bit more slowly; but the battery will have plenty of stored electrons and will be able to supply them at a reasonable voltage.

Either attempt to maximize motor speed by going to the extreme of using a new fully charge battery for every match, or design your robot around what a good battery can supply in the large middle-region of its charged-to-discharged output characteristics.

Search and you will find more than one earlier discussion of this topic. In one of those I went through the calculations to show people that the NiCads definitely hold plenty of coulombs (electrons) for several matches.

Having enough storage capacity for several matches is not the limiting factor for the NiCads. How your design uses the motors is. Don’t nearly stall them all at the same time because that will try to pull an unrealistic amount of current out of the battery (and through the microcontroller’s current-limiting circuits/breakers) at that moment.

Searching is your friend.


It also depends on what kind of robot you have… Last year, my team had a holomonic drive system, and we had to change the batteries every single match because it was so sensitive…

It appears we agree

Chris this sounds a little fishy to me.

The three wires are not spec’d on the Vex site but some sellers have quoted them drawing 5ma to 1 amp vs the two wire .15 amps to 1.8 amps… The 5ma is probably way too low for the three wire… probably like you say its closer to the i_free of the two wire.

Edit: I just put a fluke on a three wire : got (stall,free) = (.6-.8 amp, .13 amp)
The two wire with a 29 motor controller gave (2.5 amp, .16)

So, the i_frees are pretty close but the 2.5 amp two wire stall was surprising.
Plugging the measured values into a spreadsheet gave peak efficiencies of .7 and .55 for the three and two wire resp.
If we assume (1 amp, .13 amp) for the three wire and the spec (1.8 amp, .15 amp) two wire then the resp. peak efficiencies are .55 and .7 … they are just reversed so who knows.

Stalling the motors and measuring currents is probably not kosher relative to spec values but it does give some indication.

If the motors had the same currents and voltages then the two wire would have to be about twice the efficiency at peak power… which I doubt.

The Vex 3 wires are spec’d on the website, that’s where I got the information. I would consider it reasonable that the new motor being twice as efficient, considering one was picked out 5 years prior and prices could have dropped.

I find it interesting the 2-wire’s measured value is different from the web site.

I expected to find the spec currents here…http://www.vexrobotics.com/products/accessories/motion/276-2163.html but they are in the inventor Motor document. 1.6 amps stall, .14 amps free. But the motor has the 1 amp current limiter that will trip over time.

I think based upon the 5 amp stall current measure by QUASAR https://vexforum.com/showpost.php?p=126739&postcount=45 that the 1.8 spec two wire is perhaps a little low. I think my 2.5 amp fluke measurement was probably an average over a few seconds after the PTC kicked in.

but they are in the inventor Motor document. 1.6 amps stall, .14 amps free. But the motor has the 1 amp current limiter that will trip over time.

Both motors have this, I think. Regardless, it still proves my original point.