Acer C720 does not work from the battery

Can someone suggest a fix to the battery problem I'm having with my Acer C720? The display tells me it's 100% charged (with blue light), but the tablet no longer works unplug. The battery is not too old; it's a replacement but now a couple of years old. I also replaced the motherboard last year, and everything worked fine till last week.

Using only battery power, the computer would suddenly and repeatedly shut down and previous to that after I opened the lid it would sometimes immediately turn off. Now it has completely failed and only turns on when it's plugged in. I took the back cover off and tried turning it on pressing the tiny on/off switch on the motherboard, but nothing happened. I also disconnected the battery and held down the on/off button as is often suggested. Can it be the battery? Thank you.