My LCD display i2c displays fine for first minute then text fades although led light is on and still works
It's an uno with 2 LEDs if I run off 9v battery it has 6.98v left it works for 1 minute but if I connect to usb all works fine I thought usb supplied 5v so battery should be ok it's a standard 9v battery not rechargeable so I'm guessing 400 may is it just battery current and usb is supplying more
The problem is you are not using a serious battery.
And you are using the "Vin" or "barrel jack" which is a bad idea to start with.
Perhaps use a USB "Power Bank". "
A 9V battery with only 6.98V is nearly dead. And little smoke detector 9V batteries are almost useless even when new.