Portable charger for V5

Introducing a life hack for the teams who was lucky to receive V5 bundle, but are still waiting for for the second V5 battery to arrive.

This portable charging solution can be used while waiting in the queue or at the practice fields to top off your V5 battery. This will give a second life to your old Cortex batteries and a couple of power extension cables (or you could crimp your own tamiya connectors).


When fully charged two 7.2v batteries will give you closer to 8.0v so their output when connected in series should be enough to match output of V5 charging adapter, rated at 16v / 1.1A.

This took about 5 minutes to build and the most important step was to ensure that output polarity matches what V5 battery expects: positive on the inner terminal, negative on the outer.


I used an old 9v phone charger (for the plug), small twist-on wire connectors, and an optional digital voltmeter to monitor battery status. The parts cost less than $2 if bought online (would be slightly more expensive from a local hardware or hobby store). I would assume that you will need to use some soldering, electric tape, and hot glue (or 3D print a case) to make it more robust in the chaotic competition environment.


Wow, very clever idea. What would happen if you somehow shorted the circuit? Could it potentially ruin the battery?

V5 battery has built-in protection in the charging circuit between its input and the battery cells. If you disconnect 7.2v batteries the meter will show 0v on the V5 input.

Tamiya connectors on 7.2v batteries are asymmetric and cannot be plugged wrong way. Even though 7.2v batteries have no protection circuits, if everything is built correctly there should be no risks of shorting.

Thats great! Thanks for the explanation.

I think this is illegal under R14b:
"V5 Robot Batteries may only be charged by the V5 Robot Battery Charger "

Also the inspection checklist specifically calls for making sure you only use VEX chargers:

If you used the V5 battery charger and managed to plug it into a portable battery then this would be legal I think, but you will probably want to make an official Q&A to make sure on all of this stuff I said.


I applaud your level of knowledge of the game rules. I knew that VEX don’t want anybody to use anything but official VEX chargers, but I didn’t know it was referenced both in game manual and inspection checklist.

This charger idea would be irrelevant if VEX had provided teams with timely shipped second V5 batteries. Similar to our discussion in the broken motor inserts thread, legality of these workarounds depends on the precedence of G2 over other game rules.

<G2> When reading and applying the various rules in this document, please remember that
common sense always applies in the VEX Robotics Competition.

My interpretation of common sense tells me that if VEX Robotics experiences technical and logistical issues with V5 rollout, the best strategy is to find workarounds that will help everyone to go through the V5 transition with less stress and frustration.

We could complain all day long about various issues, but that is unlikely to cause faster shipments of V5 components, magically fix dying ports on V5 brains, or protect weak micro USB ports.

Now, GDC has a choice of either officially allow exceptions for a common sense workarounds to make everyone life easier or to stick to the letter of the rules. If they chose the second option that will do nothing to reduce the level of stress and frustration among both VRC community and VEX employees and, in essence, will punish early V5 adopters.

On the other hand, if they choose to relax the rules, this will show that they take community concerns seriously and indeed care about quality of the end user experience as is stated in the company values.

To be clear I wasn’t trying to put this idea down I like it and agree with your thoughts, I just don’t want anyone to get in trouble because they were just trying their best to work around the problem of too few batteries.
I think that a safer solution would be for all tournaments to provide power strips in the queuing and inspection areas. Safer because you know that if everyone tries to make this charger someone will do it wrong and then their battery or something will catch on fire.

<R14b> exists because inspectors are not expected to be electrical engineers that can make sure your charger is VEX identical. If you really want to charge V5 batteries away from a wall outlet, you can get an inverter and a rechargeable 12V battery to run it. It will be a lot safer than splicing two batteries together on your own.

As for battery and charger availability, spare batteries and chargers were not on the list of items that VEX has frozen orders on. As far as I’m aware, VEX is still taking and trying to fill those orders.

At Worlds teams have been caught with non-VEX chargers and suffered the consequences.

Please heed <R14b> and don’t bring such contraptions to competitions.

<G2> would indicate that you have the common sense to read the plain written rules and adhere to them. <G2> is not intended to circumvent rules.

Moreover, it could be considered a violation of RECF Code of Conduct as you are gaming the system to an unfair advantage over other teams in an unsportsmanlike manner. There is no functional offering from VEX that permits portable chargers of main batteries either for Cortex or V5.

Since the charging circuit is internal, is there really any advantage to be gained by pumping more or less voltage into the battery? One would think that the internal charging circuitry would sort this out so that it wouldn’t be an issue.

As for the portability argument, yes it does allow for portable functionality that is not present in the official charger, however I can (and plan to at my first competition) easily replicate this by using a portable battery and inverter and the official charger. However, such a solution is fairly expensive, and technik3k’s solution would lower the cost of such functionality, thus allowing more teams to gain such an advantage.

I agreee that it is currently technically illegal, but I implore the GDC to seriously consider making it legal.

If you have an opaque box with a charging cable sticking out, so long as it does not catch fire, i am not going to ask what is in the box. You can charge your batteries with a hamster wheel or a nuclear reactor if you want to.

Technically illegal, yes. But the charger does not go in the field during a match, so it really is not a GDC/RECF concern. VEX batteries have their own firmware now. It is the responsibility of VEX and the battery to ensure that what is done to it is legal.

We are not doing random inspections of teams peripheral equipment. We are not doing airport security searches. The rules, as they are written, are tools for the EP’s to ensure a fair and safe competition. They are not the word of God.

Illegal charging devices were a huge concern to the RECF and GDC at Worlds some years back when teams would overcharge their batteries. As mentioned, when found, those teams are dealt with.

I am not going to cherry pick what is safe/not safe as an EP or inspector. Rules say only VEX Chargers, end of story.

I’m not sure we have FRC LRI level of inspectors for every event. Teams still overcharged batteries at events.

Teams still used illegal chargers outside of the premises of worlds.

I won’t do inspections, but if I catch it at an event I’m running I’ll tell the team to remove it.

You mean it is impossible to overcharge batteries with the garbage chargers provided by vex?

We both know you can, you just cannot do it as safely.

The whole outrage over the use of actual quality charging systems is ridiculous, its basically “pearl clutching” for the busy body robot nerds.

I was just responding to the remark that one should not care about using sanctioned chargers, GDC has made it clear that EPs are to adhere to the rules. They have their reasons, safety is one of them.

Our goal is to host a successful event and make sure that matches run on time and everyone goes home at a reasonable time.

We are not there to rummage through teams pits in some attempt to keep people honest. If the GDC cares so much they can police it themselves. Unless some kid has their hobby charger setting openly on a desk and I happen to see it they’ll probobly get away with it. That sort of thing is the responsibility of the teams mentors and the community.

With that said this is completely irrelevant to the OP. This student mearly created a UPS with old VEX hardware. We let teams do this in the past, I’ve seen mobile carts with battery banks in them so teams could keep their batteries juiced. Sure it looks like the end result was a capstone project at the Jerry-Rigg University, but it really isn’t any different. It’s not entering the field and it isn’t changing how the battery is being charged beyond not being plugged into a wall.

…and stay away from my pearls!

It has been made clear to me that as an EP you are expected to run events following rules for competition and judging guide. That is how we run events so when teams go to Worlds they have followed the same criteria and constraints. We run on time and teams have fun too. You can do both :slight_smile:

Not doing a thorough sweep of everyones pits is not breaking the rules. Having it brought to my attention and letting it pass is breaking the rules. PLease see my comment about this being the responsibility of team mentors and the community. You see something, you say something. If the GDC wants this policed they should add it on the inspection check list. “please bring your charging system with you to be inspected”

I am not going to be the TSA and demand to see into everyone’s luggage. That’s ridiculous.

If you want a legal solution for portable charging (that doesn’t violate R14b), you can buy a battery with an AC outlet or make your own out of a battery connected to an inverter. This way, you’ll be charging with an official VEX charger. It is certainly not as efficient as DC to DC like the OP (this setup is DC to AC to DC) but it is legal.