Insignia Flex Battery replacement questions and powering on/off

A Year or so ago the battery power % for the tablet would fluctuate while be plugged in and when it was unplugged. Pretty much showing me that it was time to replace the battery.

Before it died out tho, it stopped responding to the charger and would no longer charge at all nor did the plugged in icon show up. For what ever reason I never bothered to see if it was the port as I assumed that the battery just stopped working and now it's completely dead. The port looks to be ok and after opening it up i see no damage to it at all.

My question is this:

If the battery is dead wouldn't I still be able to turn it on and use it if it's plugged in?

I have no way of testing power as I lack the tools and do not want to waste money on a part I may not need. ( I have SO many broken screen tablets lol)

Any thoughts on this would be awesome.