New coaches, this post is for you. We just went to our third tournament of this In The Zone game season, and encountered two novice teams that were not aware of the need of a 9V backup battery. We’ve encountered *other* rookie teams in past competitions that also did not know about it, so I thought I’d discuss it here specifically.

Buried on page 31 of the 33-page In The Zone game manual, it states the following:

<R13> 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, unless the robot is utilizing the VEX Power Expander, and a single 9V backup battery. Robots utilizing the VEX Power Expander can use a second (2) VEX &.2V Robot Battery of any type.


c. To ensure reliable wireless communication, it is required that all teams connect a charged 9V Backup battery to their VEXnet system using the VEXnet Backup Battery Holder (276-2243).

New coaches can be forgiven if you didn’t see this one, or if you saw it and didn’t fully grasp what was said on page 31, in sub-point c. of item <R13>. You might have seen it in the “101 Things I Wish I’d Known Before My First VEX Tournament,” and glossed over it again, since it’s the second sentence of item #18. And you might not have even known that you can download the Robot Inspection Checklist that the volunteers will be using to inspect your robot at the tournament. And even if you did have this document, you might *still* miss this item—the 16th item in the “VEX Parts Inspection” section.

You may have read “backup battery” in one or more of these instances and perhaps it never quite registered that this is even a thing you need to think about. This is a thing you need to think about.

The Good News

The good news here is that you probably already own one. If you ever purchased a VEX kit or bundle that includes a both cortex and a joystick, you have one. Kits that include only metal pieces, and the “Programming Control Starter Kit” (which does not have a joystick) lack this item. The “Dual Control Starter Kit”, any kit with the word “Competition” in the title, as well as the “VEXnet System Bundle” include the backup battery holder. LOOK AROUND YOUR LAB FOR ONE NOW—not the night before the competition. If you find you don’t have one, or you don’t have one for each robot on your competition team, order it now ($9.99; seems like a lot for a 9V snap connected to a small 2-wire plug, and a little plastic holder, but there you have it).

So What Is This Backup Battery For?

Cortex backup battery portThe 9V backup battery is used to maintain connection to the field control system in a competition in case the robot loses battery power momentarily (jostled battery connection, or tripping one of the cortex’s circuit breakers). The 9V keeps the cortex code running and prevents it from restarting completely.

Theoretically, it saves time to reconnect and theoretically keeps your sensors from resetting back to 0. Motor encoders getting reset to 0 is a bad thing; often causes awfulness, including (in our case) the robot committing suicide by tipping itself over because it thought it was supposed to keep running those motors…

In my area (Northern California), your robot will not pass inspection without one, at any tournament, big or small. It appears that in other areas, the backup battery is only required at state- or regional-level competitions. If you’re unsure, you can email the tournament’s primary contact person (their email is on the RoobtEvents page for your competition) and find out if it’s something that will be required.

Mixed Reviews

For the most part, user reactions to this product are along the lines of “We use one because we have to,” or “I have no idea if it really helps, but better safe than sorry.” That pretty much sums up my team’s attitude about this item.

If it is indeed effective—doing its job to keep you connected—you’ll probably never know it. You *will* know about the other end of the spectrum: when the 9V actually *causes* problems.

Low 9V or Dead 9V = Bad.

From various threads on the VEX Forum (this one and this one, for example), it appears that a low 9V battery is worse than no backup battery at all. Sometimes a low backup battery will *prevent* the VEXnet from connecting at all, or cause a disconnect in the middle of a match, even when the regular robot battery is fully charged and connected. Other times a low 9V will give you a red Robot light on your controller, even with a freshly charged main battery.

Never Off.

My team experienced this one at California State Championships last year; the 9V plugged into the cortex prevents the cortex from *ever* turning off, even if you turn off the cortex power switch, or even if you unplug the robot battery. The on/off switch of the cortex does not impact the power flowing from the 9V. This can have disastrous consequences, as sensors and code do not actually reset for the next match. When the next match starts, your robot may just not move, or will have wacky, robot-suicide-causing effects.

The awesome jpearman of the VEX Forum points out that this never-off thing is by design: the 9V takes over when the regular battery power is not available, but it cannot distinguish between a low main battery, no main battery, or cortex off.

Some VEX Forum users report that if you turn your robot off while you’re still at the field, in a certain order or actions (some combination of joystick power, field control plug, and robot power), that the 9V does in fact turn off. I find that hard to believe, given how it works.

The problem of the 9V keeping the cortex on—and later causing problems—appears to be a frequent occurrence. In fact, some teams report that they plug in the 9V in order to pass inspection, and then simply unplug it for the rest of the day (or plug it in most of the way, but without making an electrical connection, so that it just *looks* like they have a 9V plugged in).

My team, when we had the won’t-turn-off problem (characterized by the auton LCD menu being frozen, even after changing the main battery), we made sure to unplug the 9V in between matches to really, really, make sure the robot was off. And I realize now that I am writing this that we have forgotten to do so this year; I guess it’s time to add this to our pit-area checklist!

Don’t Panic

If you find yourself at a competition and you left this item at home (after reading this post, you have no excuse for not owning one), look around the room for some friendly faces and ask to borrow one. Good candidates include large robotics clubs with several robots—they’re bound to have at least one extra floating around.

The other thing that some teams do in a pinch is share them between robots in your club. Take it off of Robot A and attach it to Robot B. This is definitely a last resort, but a viable option if the alternative is not passing inspection. (It won’t help if Robots A and B are in the same match, though, and the volunteers are checking at the queueing table.)

Take a look around your lab now, and order one today if you don’t own one already!