battery detached but red battery light blinking and laptop shuts down.

I have a Z510 Lenovo laptop. It’s battery doesn’t work so I detached it and took the battery out. but the red light of battery in front of laptop sometimes randomly blinks fast with the possibility of my laptop shutting down. sometimes it shuts down after a hardwork, and sometimes good for a week and sometimes immediately after turning on. what do you think the problem is?