VEX battery charger blinking

Happy New Year and hope everyone’s VEX Season is going well.

Our team has a confusion on the batteries. When we plug in our batteries onto the charger, it charges (solid red light). However in about 5 min, the red light starts to blink. Does that mean that there’s a problem with the battery itself?

We are using the 7.2V, 2000 mAh Ni-Cd battery. It happens with a couple of our batteries and sometimes both batteries are blinking.

Also, it is common if we are using all four of our high strength motors for the battery to burn out in just a minute. We have four high strength motors and 5 regular motors. But I think that’s because our battery is not fully charged b/c of the blinking light. How fast does your battery burn out if using all 4 high strength motors?


I assume you are using the old charger, holding two batteries. Try checking the voltage on those blinking batteries using the old transmitter or a multimeter.

I remember something (or someone) stating that if the batteries are charging with a solid red light, and blink after a while, then the batteries are charged. The blinking indicates trickle charge. When you first got the batteries, did you fully charge them? Try draining the battery, and then charging it.

As for that motor problem, our E team had that problem at one of earlier tournaments. Are you sure you aren’t placing too much load on the motors? If possible, please describe how you are using the 393 motors?

Thank you and I will check the batteries.

We’re using the high strength motors for lifting our claw up.

Below is a picture of it

You didn’t remove that plastic part off of the 393 motor right?

A little birdy told me that those protect the motor from sudden surges of voltage. I can’t fully confirm this though.

I noticed that when we took them off, our robot would run shorter than expected.

I can’t see your drive gearing clearly, but it looked close to 1:1. Everything seems to be in order.

Do you have a power expander anywhere? Mid Pacific suffered major problems without it during West Oahu (We were their alliance). Their robot would run for half the match, then stop because of the demand of battery. At East Oahu, they used the power expander and the problem was solved.

I’m willing to bet it’s all a battery problem rather than a motor problem. Try plugging in only one battery at a time, and make sure the battery is pressing down on the little button tabs on the charger.

Oh completely forgot about this too. Make sure the battery is fully in the cradle when you charge it. Those little buttons help the charger indicate if the battery is a transmitter battery, or a robot battery.

The blinking can occur if placed improperly. When you don’t place the 7.2 robot batteries properly in the charger, you are telling the charger that it is a 9.6 battery, (Can’t really explain from here-) thus blinking.

The blinking means that the battery still needs to be depleted more before recharging
Unlike Li-Po batteries, Ni-Cd and NiMH batteries require near-complete drain prior to recharge (unless the user wants to cut down the battery life significantly; making the new maximum at however much depletion there was)
Most chargers, therefore, have built-in sensors to detect how depleted a battery is
If the battery is, say, half depleted, the battery would be better off being depleted all the way then recharged to 100% instead of cutting the life in half
The blinking red light indicates that the battery is not being charged to prevent the horrible “memory effect”
This is why you observe the battery seeming to drain rapidly
Unless the batteries have been forced to charge when they were only mid-way depleted, the batteries should charge up to its normal maximum
The “forcing” with Vex batteries and chargers can be done by connecting a quarter-full 7.2V battery to the charger without hitting the pressure sensor (the quarter-full 7.2V may look like a nearly depleted 9.6V)

Thank you so much for your help. I will try them out next week.

Also, would you recommend the power expander? The only confusion I have is how to program it or how to use it.

I’d revisit this question once you get your batteries in order, as there’s a pretty good chance it’s unrelated to the performance issues you’ve been seeing. The “blinking light” indicates some problem with charging.

Our teams have switched directly to the Cortex Microcontroller, so I do not know the specifics on power expanders, but from my understanding, there is no difference on the coding side
The power expanders work like speed controllers (like the Victor motor speed controllers; except it’s like 4 controllers in one)
The input from the microcontroller is only a signal (white lead on the PWM), but the power comes from the battery that is connected to it (so think of a transistor with the end leads being connected to battery #2 and a motor, and the middle lead hooked up to the microcontroller)
Since the motors you directly hook up to the microcontroller are “listening” to the same signals (the power coming from the microcontroller in this case, however), the coding will look exactly the same
The wiring would be slightly different, though
So in other words, the power expander is just a glorified PWM extension cable/block

The PIC microcontroller can only power 8 or so motors at 100% simultaneously, so the power expander came out (for teams that wanted to use 10 motors at 100% power simultaneously)
The Cortex can handle 10 motors (actually more), so normally, people won’t use the power expander with it
The power expander will indeed aid situations where you have 2 moderately charged batteries, but I can’t guarantee how much of an aid that would be
New batteries might be a better investment (maybe it’s a good opportunity to convert to the NiMH batteries; although money might be an issue as with all situations)

I believe you are confusing the Power Expander with the two 2-wire ports on the Cortex. If you click here you will see the Power Expander, it is used simply to add an extra battery, swapping up to 4 motors from the Cortex’s battery onto the Power Expander’s.


I do understand that the power expander simply adds a battery
I was pointing out that because the power expander draws current from the second battery to power the 4 motors, the signal will be the only thing drawing current from the first battery (for those 4 motors), thereby allowing the PIC to power 10 motors at full power without hitting the internal breakers

The internal breakers don’t restrict the number of motors that can receive any power, nor does the Power Expander completely prevent hitting them.

It is completely possible to power 10 motors from a single Cortex microcontroller, without needing a Power Expander. Also I have seen problems with using the Power Expander this year, as it seems to trip much easier than the Cortex’s internal breakers. Because of this and also to save weight, our team is thinking of leaving the Power Expander out of our new slight redesign of our robot.


Really now? What kind of load does your robot do to trip the breakers, because our B team puts loads such as movable goals (literally), and the bulk of their robot when hanging. They have never tripped their breakers (not that I know of…).

Makes me think more and more…

I’m willing to bet it doesn’t trip more easily, but rather teams are too happy to put more than one or two super-limit-pushing loads on the Expander under the thought that “4 motors on one battery means we can put double the load on them”, which isn’t the case.

I still haven’t a clue how you guys are stalling these motors left and right. The 393 is relatively easier to trip, yes, but it’s not impossible to avoid.

When the internal breakers hit, the entire microcontroller shuts off
Breakers will hit with excessive current passing the breaker at any given point
More motors=more current draw=eventually trip breakers (if none are shutoff or reduced prior to activation of another)
The use of the power expander will have 4 motors’ current draw go through the expander NOT the microcontroller power manager
This means less current draw from the microcontroller and thus no more hitting breakers=more motors allowable to be powered simultaneously

I said the PIC (with its more sensitive breakers) cannot handle the load of 10 motors
The power expander was released prior to the release of the Cortex
It was meant to relieve the “electrical stress” put on the PIC microcontroller
Cortex has a better system already incorporated in it, thus not requiring a power expander to power 10 motors (unless a team wishes to reduce the number of battery recharge runs)

Your implication is that the Power Expander is mandatory for powering all 10 motors, which is untrue. You just need to be a bit more careful and a bit less extreme.

Each of the control systems can support 10 motors at one time. The trick is to balance the load so that you don’t exceed the internal over current protection.

The motor extender allows you to use a second battery to run part of your motor load. This allows you to run your motors at a higher load.

A side benefit is that under low voltage conditions (a brownout where the voltage drops below 5 volt) your Cortex (depending on the 9V battery) and the PIC will reset. If you split the load to the two battery packs your chances for an under voltage (and reset) condition is less likely to occur.


  • Don’t stall the motors.
  • Gears are your friend, you change the current needed by taking a few extra seconds to move the load.
  • With a driver / operator it may make sense to move some of the motors to the motor extender.
  • VEX is an engineers game. There are limitations to the equipment, so you need to do the math to make the equipment work for you. If you just want to do brute force you won’t be as successful.

you cant use the old chargers with the new batteries, it will mess then up badly