Power supply problem with Arduino Mega

Please pardon me for the title of the subject. It does not help much, but I could not find a better one.

I am using an ELEGOO screen "shielded" on top of an Arduino Mega. I tried several test programs provided with the Elegoo screen without any problem. Same positive result with simple programs of my own. Everything works fine as long as I use the USB supply from the PC, but the screen stops working as soon as I switch on the normal power supply of the Arduino (I use a 6V battery). The screen then looks dead.

The problem does not look like coming from the power socket because the Arduino Mega alone seems normally alive. The led on the Arduino is on and the 3.3v voltage on pin 22 checks Ok. To make reasonably certain that the Arduino is ok, I tried a few simple programs like the "Blink" and everything works fine.

To make sure I was not dreaming, I tried the same setup with an Arduino Uno (Elegoo screen + Uno) and it works OK with both power supplies.

Thanks to anybody that could help.

I use a 6V battery

How are you connecting this to the Mega ?

Thanks for your answer.

To use a 6V battery to power an Arduino, I use a 2-wire lead. One has got 2 terminals compatible with the battery terminals (paper clips are ok too),the other end is fitted with a male power plug (5.5 mm external diameter) .

About the problem related yesterday, I think I made some progress. With the Arduino Mega, I measured the tension on the 5V pin. When powered from USB, the tension is 5V, but it drops at 3.2V when powered from the 6V battery. That tension is probably too low to power the shield screen properly.The same test with an Arduino Uno gives 5V in both cases. This explains-I think-why the Arduino Mega was working normally with programs without shield.I will have to buy a new Arduino Mega.

Or you could try using at least 7V into the barrel connector as is always recommended. 6V is too low for the internal regulator. In fact I'm surprised it works on the Uno.