Replace battery message coming up on all robots

We have a fleet of 24 VexIQ robots along with an equal number of NiCad battery packs (grey bricks). All of them are displaying the “Replace Brain Battery” message at some point during operation. All the batteries are fully charged with a solid green indicator (no flashing lights) on the chager. Pulling the battery and pushing it back in temporarily resets the sytem, but once the robots are on the move again, the red light begins flashing and the message shows up again. They do this even when the batteries are replaced with other fully-charged units.

All robots were working fine when we stored them at the end of May. However, since then, all brains have been updated to the latest firmware (2.1.4) and all sensors and motors have been updated as well. My hypothesis is that there is an amperage spike triggering the message, as it happens only when current is put to the motors. I’m not sure if this is related to the latest firmware release or not. Please help as this is putting both of my classes in a major bind, and I’m out of ideas.

If they are still in warranty, you may be able to get an rma from vex.
Fill out this form and email it to vex. They should be able to help you.

The kits are over 3 years old.

Did they get stored with the batteries in the Brains?

No. We always store the batteries separately during the summer.

One thing I am noticing is that if the firmware is 2.1.4, the battery voltage fluctuates at 54-57% constantly for a fully charged battery (according to the green light on the charger) and never settles down. However, if the firmware is 2.1.2, the same battery fluctuates briefly at 89-90% and then settles in at 89. This is consistent among all of the brains runnng 2.1.4.

When a battery is fully charged according to the charger, what voltage do you see across the battery terminals with a multimeter? It sounds like a dead cell but seems odd in all packs.


Stand by one (20 char)

Edit: My multimeter is at home. A bad cell sounds plausible, but for all batteries?

FWIW, I just checked a brain on 2.1.2 firmware with a charged battery connected one of my built clawbots. I got a pretty interesting fluctuation as well but it didn’t shut the robot down when I put a load on it. The sensors and motors seem to be drawing about 20% capacity while the system is turned on and idle (not running a program). Is that normal? If I have nothing plugged in, they gain that 20% back.

Can you match the Multimeter voltages with what the system page on the brain?


One other symptom…when I run driver control and put a load on any motor, some devices will turn off and come right back on.

@Foster, I’ll do that tomorrow.


OK…I brought in my multimeter this morning and tested all 24 batteries. Of those, only two read lower than 8.7 volts. Everything else was 8.7-9.1. Like all batteries, this is the raw reading across the terminals. When installed, the batteries read around 7.4 v, but the percentage of charge fluctuates rapidly anywhere from 2-4 % in System Info.

Wow, that’s not what I would have expected. @jpearman may be able to shed some light on it.

I’d call VEX tech support ASAP and see what they think. Can you please cycle back when you get an answer?


Exactly what I did. thanks!

Those numbers seem high. What do others see on freshly charged batteries? We usually see max 8.4


Sounds like an iffy cell in the packs, these voltages seem high.

Define freshly charged battery.

  1. If you take the battery off the charger and test it you’ll find a voltage in the mid 8.x range.
  2. If you wait 5 mins, it will have dropped 0.2 volts
  3. If you wait 20 mins from coming off the charger, you’ll see it at the normal 8.2 volts.

A bad cell would give you full voltage at the start and the run less than a min. The op has a huge number of packs that do this, a very strange issue.


I had a strange issue with a brand new brain recently. I had a battery (5 min off the charger) with one robot registering the voltage at over 10V. A multimeter showed 7.2. The same battery in a different brain showed 7.3


@Foster That’s exactly what is happening, but I’m not sure why either. What does seem to happen with some is that the robot will just restart after a current is applied. It seems in some way to be related to the firmware change, as 24 batteries with bad cells is odd indeed.

So what is the solution? Get a new brain? Get a new battery?