battery life is getting shorter and shorter

Hi! some months ago I lost my original battery charger, and bought a second hand one, which I thought was compatible. It is a Dell charger, and on the back it reads

"input: 19.5V (19.5V) 4.62A(4.62A)".

Quickly after buying this new charger, however, the battery life started getting shorter, and now sometimes it looks as if the charger was just keeping the pc alive, but as soon as i unplug it the pc turns off instantaneously, just like a desktop would do. This anyway doesn't happen all the time.

What should I do? any suggestions? shall i look for a new charger? is it very costly to buy a new battery?

I care for this pc, and battery apart it still works very well. I wouldn't ike to have to dispose of it!

Thanks!