VEX rules for robot construction at first glance lead you to believe that you can only use one battery on your robot. From the 2015-16 Nothing But Net game manual:

The only allowable sources of electrical power for a VEX Robotics Competition Robot is any single (1) VEX 7.2V Robot Battery Pack of any type . . .

But if you keep reading, the next part of the sentence says:

unless the robot is utilizing the VEX Power Expander…Robots utilizing the VEX Power Expander can use a second (2) VEX 7.2V Robot Battery of any type.

Power ExpanderWait. What? You can use a second battery? Yes, that’s right, you can use 2 batteries on your robot if you buy the $49.99 VEX Power Expander. The power expander is a small widget that’s connected to your second battery, the cortex, and the motors you want to run off the second battery.

Here is VEX’s diagram of how it works:

Power_Expander

Whaaa???

Here’s a simpler diagram:

Simpler diagram of power expander

So what’s the point of the power expander, exactly, and why does it have to be set up this way? The main point is that it prevents you from daisy-chaining your batteries together, which would double your voltage and indeed be an unfair advantage (and at the same time would risk damaging your components). The power expander gives you a way to run up to 4 motors off of a dedicated second battery.

Use an Extension Cable!

When you buy the power expander, be sure to also acquire a $5 battery extension cable, just like you’d use on the cortex to keep the battery plug from being over-used. In our rookie year, we found out the hard way that plugging & unplugging the battery from the power expander was taxing on the wires of the device—at California state championships in the middle of the day one of the wires on our expander plug broke off and we did not have an extra! Our very nice pit-area neighbors, the Dancing Pandas of CART, lent us a new one ASAP and we zip-tied it onto our robot and were back in action. We now own an extra, and always use a battery extension cable. 

Setup

This is all great, but how do I set this up in real life? What are those slots with letters on them for?

Along with the power expander and battery extension cable, you’ll need to purchase some short extension wires, if you don’t already have a supply. I recommend the shortest size that VEX sells, 3-Wire Extension Cable 6″ (4-pack). To attach the power expander to your robot and get it working, a diagram should help:

Cortex, power expander, motor wiring

The slots on the power expander are labeled A–D, which is different than the cortex’s numbered ports. In your motor configuration screen (RobotC and easyC), you’ll still assign your power-expander motors to normal, numbered slots, and you’ll still write your program referencing those numbers, the same as if the motors were indeed plugged directly into the cortex.

On the power expander, you’ll see letters A through D running down the middle of the device, with a bank of ports on each side. One bank has pins sticking out of it (for a female plug) and the other bank has holes (for a male plug). This is great, because it means you can only plug things in the right way here; there’s no getting things backward. 

A little daisy chain will run from the cortex to the power expander to the motor. First, a short extension wire runs from the numbered cortex slot to a lettered slot on the power expander. You can use any lettered slot you wish, just make sure that your wires are not all jumbled on top/around each other. You’ll end up with a lot of wires in a small area here, and you’ll need to do your best to keep things neat and usable.

Move to the slot on the other side of that letter, and plug in the appropriate motor. The important step here is to make sure both incoming and outgoing wires are next to each other, in the same lettered slot, so that the “pass through” of information actually finds its way from the cortex to the motor. For example, in the image above there’s an extension wire running from cortex Port 3 to the bank of slots to the left of letter A. Motor 3 is then plugged into the matching right-hand slot next to A.

Slide It on Over

Power expander sliding coverI’m not sure when it dawned on me that the exterior grey casing of the power expander slides back & forth, by a fraction of an inch. (It’s easier to see this action when it’s empty & not attached to your robot.) On the cortex, there are those little black plastic “covers” on the side of the banks of ports that keep the plugs in place & not yanked out of the cortex accidentally. On the power expander, the entire grey outside of the casing slides a teeny bit to cover up the edges of the plugs to keep them from being pulled out.

The various plugs need to be pretty thoroughly pushed in in order for the casing to slide. If it seems like it doesn’t work, try pushing the motor wires & extension wires down some more.

Robot Design & the Power Expander

In your robot design, consider spreading out your motors between the power expander and Cortex. When plugging motors directly into the Cortex, one wants to spread out the motors between ports 1-5 and ports 6-10, since each half of the motor ports is regulated by a separate 4-amp circuit breaker. Spreading out the motors—particularly motors that will run at the same time—reduces the likelihood that too much current will be drawn from either group (tripping the circuit breaker).

The power expander also has a 4-amp circuit breaker that controls all of the motors plugged into the device. jpearman on the VEX Forum ran extensive tests on the PTC component that is inside the power expander. As with a motor’s circuit breaker (PTC), tripping the power expander’s circuit breaker results in reduced capacity for those motors until the PTC has had a chance to cool down and reset; in fact, it takes over 30 minutes for the power expander to return to its full abilities, so avoiding tripping it is extremely important on a competition robot.

How does one avoid pulling too much current through the power expander? By spreading out the load, just like one would do on the Cortex itself. If all of the motors connected to the power expander run simultaneously, like for a lift, then a high load will be placed on the power expander every time the lift is used. If that lift is under heavy load or stress, then even more current is trying to be pulled through the power expander, increasing the likelihood that the circuit breaker will trip. Instead, distribute the lift’s motors between the power expander, ports 1-5, and/or ports 6-10, so that no single group of ports is overloaded.

That Little Light

The power expander has a single light on it, that lets you know 2 pieces of information: the power level of the battery (via color), and the status of the power expander’s 4-amp circuit breaker, or PTC (via flashing).

Color Battery Status Flashing – Fast
Green Good PTC* was tripped
Yellow Low PTC was tripped
Red Nearly-dead PTC was tripped
    Flashing – Slow
Red any PTC tripped right now

* In VEX, the PTC is the circuit breaker that’s built into the motor, cortex, or power expander.

Displaying Voltage on an LCD Screen

This topic is described in this section of my post about batteries; on the VEX Forum, a very nice user, kypyro, has RobotC code that can be used directly (4th comment in this thread).

Updates

  • 2/14/18 – added links for displaying voltage on an LCD screen.
  • 10/11/17 – added “Setup” section with wiring diagram.
  • 9/18/17 – dded a chart of the power expander status light.
  • 6/20/17 – post was edited to add mention of the battery extension cable, plus the section on Robot Design, which discusses spreading out motors between the Cortex and power expander.
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