I have a net book which is loosing charge every day.
It says while plugged in its now only %37 and its been like that for weeks.
The point i'm making is that your answer is down to cost.
If you can live with the laptop plugged in then so be it.
If you need it mobile then all you can do is buy a new battery.
You don't say how old your laptop is as Batteries can half there power in as little as 12 - 18 months.
I would agree from what you have reported back with it "Is more then likely" the battery.
But if you can live with a lead plugged in live with it that way
Hope that helps, its difficult battery's chargers and laptops to pin it down.
Your laptop can live happily with out a battery