This has been going on for a while now (early 2011), but the Cortex always displays a red LED for the robot battery, even when the battery is fully charged and giving off readings of over 8V on the multimeter. The issue doesn’t really affect much else, but it’s pretty annoying.
This happens regardless of whether the 9V is plugged in or not, and when it is, the 9V is charged.
Any ideas?
Kennenth,
Wow that’s strange…
What Master Code version do you have on the Cortex?
Also, if you are using a VEXnet Competition Switch, you will need to connect a back-up battery on the Cortex. Without the back-up battery the ROBOT LED will always be red.
-Eli
Thanks for the reply!
Lets see… Master CPU Firmware is on v3.16, from robotC 3.04v, and started back when we used whatever Master Firmware robotC 2.32 had.
The issue occurs regardless of whether the competition switch is used or not.
When you download a program the LED should be green.
Do you get a blinking green LED when downloading code?
No, the LED stays red when downloading code onto the cortex, although they do blink.
This is very strange.
So the Cortex is fully functional, except for the Robot LED status… correct?
-Eli
Yeah. It doesn’t affect the performance of the Cortex in anyway. It’s only a bit bothersome to not know whether the battery is fresh or depleted without breaking out the voltmeter.
Overall not a huge deal by any means; just curious if there was any solution for the issue 
Kennenth,
I’m not sure why your Cortex fails to read your battery voltage.
We would have to physically troubleshoot the Cortex, to see if we find anything wrong with the PCB board.
Sorry about the inconvenience.
No problem, thanks for the responses!