How To: Power Expander

How does the Power Expander work? How does the second battery plug in? Do we need it?


The power expander gives you a third breaker to work with. I’m sure you’ve heard of the two in the Cortex, one covering ports 1-5, the other ports 6-10. This allows you to push your motors 1.5 times harder without having them hit breakers.

How the expander works is that it has four input and four output plugs. Instead of plugging from motor -> motor controller -> Cortex, you do motor -> motor controller -> power expander -> Cortex. The power expander inputs correspond to the outputs on the opposite side and which wires you plug into which Cortex slot also matters. But if you get the coordination correct, the power expander is just like a detour from motor -> Cortex.

The second battery plugs directly into the power expander, just as you would plug a battery into a Cortex.


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The wiki is a great (and underused) resource.

Thank you guys for your (fast) help!

I just purchased one today. Does it come with all the right components to work?

What comes is just the power expander I believe. You need a second battery and extension cables, but that should be it.

Nope, not at all. You need to purchase lots and lots more to use it. :rolleyes:

Kidding. It is self-contained, aside from the battery and extension cables you’d need to use it.

What extension cables? We have a battery.

The same ones you use to connect your motors and sensors to the cortex… the long 3 wire cables…

It uses vex 3 wire extension cables, one for each motor you want to run with it.

Oh, ok good :slight_smile: We have those.

Thanks again for all your help!

Another tip for the Power Expander.

We power our driver -or- our appendage exclusively through the power expander. We do not “randomly” plug stuff in and use it. We also ration the other PWM wires evenly throughout the Cortex so that we do not trip breakers while competing with many motors running simultaneously.

For instance:
Drive motors are on ports 2-5
Lift motors are on ports 9-8 and running off of the Power Expander.

Some people do not know that the Cortex has two “sets” of banks for motor PWM’s, thus having two separate breakers within the cortex. If you even out your load amongst both, you will bring down the likelihood of “tripping” while competing.

I’ll say this one last thing, some people don’t believe that this is an issue. This method has solved many issues that 1193A has had in the past with “tripping” while in a match where many motors are being triggered at one time.

If you have any questions while at the competitions in the spring, feel free to ask the 1193A guys about it.

We pay attention to motor distribution across breakers but our answer is opposite to yours. We do not put our entire lift on one breaker, nor our entire drive. Each system is split up across multiple breakers.

We do this because you have to understand you will not always be using every system in your robot, so if you were just using your drive, to be less demanding of your breakers you want to even that load over as many breakers as possible.

Our motor configuration is 4 drive, 4 lift, 2 intake. Breaker 1 of the cortex powers half the drive and 1/4 of the lift. The same for breaker 2 of the cortex. Our battery expander powers half the lift and the intake. We find that evening the systems out across multiple breakers really helps.

This is basically the same config we used for most (if not all of) last year. We never tripped a Cortex breaker. :wink:

However, we split the drive up over more breakers than the lift. (That is, some of the drive was on Cortex breaker 1, some on Cortex breaker 2, some on Power Expander.) This is because the drive is the most (heavily) used part of the robot, at least for us.


What you have with that configuration is 5 motors on the 1-5 breaker, 4 motors on the expander breaker, and 3 motors (ports 6, 7, and 10) on the 6-10 breaker. This is a very uneven setup; the 1-5 breaker is receiving a lot of current, while the 6-10 breaker has significantly less current running through it. I suggest that you distribute your motors more evenly.

What we have:
Drive in ports 2, 3, 8, and 9, with 3 and 8 in the power expander.
Lift in ports 4, 5, 6, and 7.
Intake in ports 1 and 10, both in the power expander.
That way, our drive is spread across 3 breakers (minimum chance of tripping one), and our lift is spread across 2. So the 1-5 breaker has 3 motors, the expander breaker has 4, and the 6-10 breaker has 3.

Can’t use ports 1 & 10 with the power expander.

I think you may be confused as to how the power expander works. You said that he has 5-4-3 motors on the different breakers, but that adds up to 12. A motor plugged into port x through the battery expander is NOT being powered by port x whatsoever, so your list of 12 motors doesn’t make any sense…

Oops, I had that mixed up. We have 4 and 5 on the expander from the lift, not 1 and 10. :rolleyes:

Had that mixed up, too (my mind must have gone somewhere while I was writing that post). He said he had 2-5 powering his drive, and 9-8 on his lift. Assuming he is using more motors (ports 1, 6, 7, and 10,) then 6 and 7 are probably on the expander, too. That means that he has 5 motors on 1-5, 4 on the expander, and only ONE on the 6-10 breaker. This seems extremely uneven to me. If 6 and 7 aren’t on the expander, then there are still 5 on 1-5, 2 on the expander and 3 on 6-10. This is less uneven, but it the power could still be distributed in a more balanced way.

Will a power expander fix my drive train problem. It stops randomly then I have to wait a bit until it comes back on and works. It is a really fast gearing.

Sounds to me like you know the problem already. You gear ratio is too high, making your motors stall. (not to be confused with tripping breakers.) Stalling is when you’re pushing your motors too hard. Tripping your breakers is when you’re using too much motor power or too many motors at once.

As far as I can tell from the video in your build thread, you seem to be running a 1:1.66 gear ratio for speed. Given your robot is designed to hold 15-20 sacks, depending on the weight of your robot, I would be weary of this. You should try changing the gear ratio to something closer to 1:1 and see if it makes a difference.