laptop shut down, boots plugged in but only shows backlight when not

When the laptop shut off, i was doing some light gaming while listening to a podcast. It was on a bed, so cooling probably wasn’t the best. It didn’t feel super hot to the touch, though. When its plugged in, it will boot and work fine. When its unplugged, it will shut the laptop off although the backlight will stay on for 5 or so seconds. I’m thinking the battery simply needs to be replaced, but since it was working fine before it shut off, I just want to make sure as a decent battery for this laptop is about $100. Any and all answers are appreciated. Thanks! :)