Vex brain is blinking red with full battery

Hello! The problem I am having is that when I take a fresh battery off the charger and put it in my robot. Then I drive it for a few minutes then the brain starts blinking red when it says the battery is still full.

Here’s a picture:

Try another battery, if not that it could be that the battery is overheated

If your motors are under too much stress, it can cause the battery voltage to drop, causing the brain to think that the battery is low and turn the green light red. To solve this, try gearing down your motors (particularly the ones under the highest load, e.g. the drivetrain).

Also, batteries don’t last forever, so if it is an old battery, you might just want to get a new one. But if this was the case I’m pretty sure the battery icon on the brain would not be full, so I’d try what I mentioned above.


The voltage of all batteries drops under a given load. Larger loads cause the battery voltage to drop further. Usually as soon as this load is removed from the battery, the battery voltage will bounce back to near what it was before (minus the slightly lost capacity). Older batteries cannot handle large loads as well as well as newer batteries.

The Robot Brain LED color is “latching”, and tied to battery voltage. The icon on the Robot Brain LCD screen is the instantaneous voltage.

What this means is that if at any point the voltage of the battery drops into the red (dead) zone, the LED will remain that color until it is powered off, but the Robot Brain LCD Screen will always show the current voltage level. So if the battery was highly loaded, but then stopped, the LED can switch to red, while the LCD screen afterwards shows it’s still “fully charged”.


Just make sure you check the voltage percentage in the settings menu under system info. Your battery can be at 50% and still show a full charge in the battery image. I will always advise against trusting that image. We have had many students fall for that trap and use uncharged batteries thinking they were full and they couldn’t play a match properly.

Best of luck