My battery is dead and I want to change it

So, I've this lap and it's battery is dead and I repaired is from some local repairing dude and he just made my laptop direct through the charging so now I'm using it on directly on charging can anyone help. The whole situation was my laptop got water damaged and wasn't turning on so he said the display is gone and he changed something and said the battery is also gone for good so he is now saying you've to change the battery .