Why does my laptop shut down when I unplug it?

I have an Acer Aspire E5-573G (i3-4005U, 8GB RAM, 920M). About 6 months ago a problem started occuring. Whenever I unplug the power cable, the laptop just instantly shuts down. My first guess was to replace the battery, but that did not do anything. What could be causing the issue here? Originally it was stuck at 31% battery and slowly degraded over time, and no matter how long i plugged it in for, it wouldn’t charge.