Analysis of Vex Batteries and Chargers

At my most recent competition i was shocked when my battery which had been on the charger all night simply would not run my robot in the first match (should have tested). The next battery checked out before the next match but only powered me through half of the next match. Perplexed we charged both competition batteries alternately on two chargers assuming there was a problem with either the batteries or the chargers. The result was the same no matter the combination. Both batteries stopped charging normally after a few minuets (blinking LED) and would barely last an entire round.
A friend of mine who is a EE and does work in industry and for the military has been helping me out this year offered to take the stuff home and put it on his bench to see what the problems were caused by. This is the email he sent me. I hope it can help someone else out. I know some have asked for the performance of the batteries in the past and this has one fully drained though it is not recommended to do yourself. Keep in mind he is a battlebots builder and is not a huge fan of some of the Vex equipment. He is constantly critiquing electronics this way and likes what vex does even if not the equipment sometimes.
Anyways here’s the Email.
The batteries are fine. The charger is fine. The problem is that the Batteries MUST be in the charger cradle. There is a switch there at the top of the holder that detects the 7.2V battery pack. There is no good reason for it! Their thinking is to charge the 9.6V pack at 700mA and the 7.2V pack at 1A. However there is not much difference between the currents and they should have just picked one. If switching the current was all they were doing, it would not be a problem but they are also trying to be “smart” and they assume that if the switch is not engaged then they are charging the 9.6V battery pack and if the voltage is something else then it turns off thinking something is wrong. There is no good reason for doing this. Chargers do not need to know what the voltage “should” be in order to properly and safely charge a battery. FYI the blinking light on the charger means something is wrong.

For your future information as a designer. You cannot add an led to a product without labeling what it means. And you cannot add a hidden switch and make assumptions about what the operator will do without labeling it’s function.

Here is one of the initial discharge plot of your battery at 2A. Note the capacity was only about 1AH but it is old and may not have been charged all the way.
Note you can see the steps where the voltage drops. Each step is one of the cells running out of charge. In general is is not good to keep discharging a battery after a cell is dead because it gets reverse voltage across it. However I wanted to see just how balanced the cells were and to do it this one time will not hurt the batteries too much. As you can see that the weakest battery had about 25% less capacity than the strongest one.

This next plot shows a the Vex charger properly charging the battery:

This plot shows what the Vex charger was doing for us when the switch was not engaged:

I will attach the images in case the IMG doesn’t work here.

I had this problem too, but I figured it out. It really messed up my battery when it was charged at the wrong current. It was sorta stupid to put that there.

The instructions that come with the chargers are pretty specific about seating the batteries. Reading the manual is usually a good thing.

I’m not an EE, but I disagree with your friend’s comments about the VEX battery charger. I believe it was designed to be safe and easy to use for anybody. It was designed to allow the user to put either battery into either slot and charge that battery correctly. The switch is an easy way for the charger to know which battery is in the slot and the proper voltage and current to charge it at. If the battery is not fully seated in the slot and the switch isn’t depressed, the charger would need to know if the battery doesn’t match the switch setting. If not, the battery could be damaged by charging it at the wrong voltage and current. The blinking LED simply lets you know that something is wrong.

I believe these features are there to protect the user and the batteries. How else would you design an economical charger that is easy to use and charges 2 batteries of different voltages? You could design one with switches to make the settings manually or make each slot dedicated to charging a particular battery, but that would make it more complicated to use or not as flexible.

This is just my opinion.

id dont start EE school till next fall so i can’t really say whats best for the batteries. I agree they are safe and economical. Ease of use is opinion and I’m not sold either way on where I stand. They obviously do work maybe not intuitively enough though. I just wish i hadn’t lost the stupid manual. LOL

I was just thinking that they were pretty easy to use from the stand point of a beginner who isn’t really familar with electronics. You just put a battery in, make sure it is seated properly, and plug the charger in. - no switches or settings to make.

Sorry about your matches though. It’s always some little thing that seems to go wrong.

I was thinking that they could have two more prongs on the battery. they could have a resistor. A different resistor for a different battery. The charger senses the resistance and charges it the right way.

A custom battery connector might make things a tad easier, but it would drive prices up and reduce compatibility with other (non-Vex) products. I’ve always liked the fact that Vex uses industry-standard connectors and protocols.

A simpler approach would have been to make the battery charger cradle have a 7.2V side and a 9.6V side. Each side would only fit the battery type it is designed for. No hidden button, and no confusion. Of course it is nice to have the flexibility to charge two 7.2V bat’s at the same time, but of course you are really only charging one at a time since they take turns.

I do agree that the LED should be labeled with a small sticker that explains the three states.


  • Dean

We have ten chargers and I don’t even know how many batteries. It’s perfect the way it is – we stuff batteries in the chargers overnight, and then can put in the second wave the next day. The flexibility of charging both types of batteries in the same device far outweighs the hassle of remembering to seat the battery in the cradle. Don’t change it!

I think that if any changes were
Made it would be just to add lables. This the first generation of vex chargers maybe in the next design there would only be small changes but I don’t think anyone should expect that in the near future with all the cool stuff they’re working on now.

Try the Manual [Inventors Guide]( small.pdf), found on the page VEX Power Pack.