What does it mean when the lights go up and down when its charging?

i bin having my JBL boombox for like a year or so and this just stared like a weeks ago when i charge it the lights will go up and down and it says it charged but when i use it it only lass for a 2 or 3 hours if I'm playing rap or rock but if i play old country it say playing for longer. I don't how to fix this I mint need a new battery but ill ask you all before I do that.