Rescuing my 11 yo Dell Latitude E5440 - The Soldier of All Laptops

I have a Dell Latitude E5440. I got it in 2014, so the laptop is 11 years old. Today, the laptop won't start up unless it is plugged in with the charger. If I disconnect the charger, then the laptop just turns off instantaneously (ie. whole system powers off in just a few nano seconds). I rebooted the machine and pressed F12 to enter a diagnostic screen. I ran the diagnostic, which took about 1 hour and found zero errors. The diagnostic said my laptop is completely healthy.

The battery on the laptop is relatively new, because it was replaced sometime last year.

Is there a way to know why this won't work unless plugged in?

I love this laptop. It is still my primary laptop when I'm on the road. It is the most durable laptop I have ever owned.