How old is the laptop? Depending on how often you use the laptop the wear and tear is different on the batteries, but over time they loose capacity and you would have to replace them sometime. Sometimes it is possible to find companies specialized in exchanging the battery cells and not the whole battery which can be cheaper. If you bought your laptop between 2010-2012 I would say the batteries are rather old and needs to be replaced if you need the same work time out of them as when they where new.