Mindstorms EV3 - Continuous low battery alarm My son is facing an issue with his Mindstorms EV3. Every time he turns it on a low battery alarm is displayed and it shut downs.
This happens regardless of the batteries, we have tried 12 different rechargeable batteries and 6 brand new alkaline batteries.
Also, I noticed it takes a lot to start up, I don't remember it being so slow before.
The sequence is something like:

Starting up: it takes 26 seconds
Green light: it is on for 2.5 seconds
Orange light with low battery alarm: it is on for 8 seconds
Shut down: then it shuts down and it takes 20 seconds to complete

I have tried disconnecting every cable, and even opened it up and disconnected the display flex cable, but still the same behaviour. I have double checked the button in the battery compartment to make sure it is not stuck or anything similar, to avoid the brick thinking the Lego battery pack is in place.
Given that it stays on for such a short time frame, I am not able to update the firmware.
I have seen similar posts but none with a solution other than returning the brick to Lego, which is not an option to us given it is no longer under warranty.
Any clues or help?
Thanks!
Update: we made it work, after trying many things, see below.

Opened the brick and inspected it for obvious signs of problems but saw nothing. Even tried with the screen disconnected.

We tried, as suggested in the comment above, with a firmware update. Even though the brick will stay on for only a few seconds, it was enough for the computer to recognize it and see were able to begin the update. It completed without problem, showing that the batteries were ok. After restarting it behaved in the same way. We tried firmware 1.09 and 1.10.

We tried ev3dev, from an SD card. It will also shutdown once it finished booting. But after many tries, it booted and displayed the main ev3dev menu, on which we could see the voltage being 5.10, getting lower gradually until it reached 4.60 and it shutdown.

It seemed obvious it was a hardware issue so we decided to desolder the battery contacts and inspect the board properly. There must be something in the circuit sensing voltage, maybe a resistor going bad and being unable to divide voltage properly. We immediately saw the sign of acid going through the board, with that green/blue colour. We cleaned it with a wire brush, resoldered and it worked! Both the brick's firmware and ev3dev worked perfectly, now seeing 9.10 volts.


I hope this helps someone else with the same problem.
Cheers