MacBook Pro (late-2008) Battery Drains Quickly

Hi, I have a late-2008 MacBook Pro and I've noticed that when the battery is at 100%, it takes only between 2 to 2.5 hours to drain it. The battery was replaced in 2013, so it doesn't seem likely that only 3 years later it's failing again. The computer was recently wiped so there isn't much background activity (I've looked at Activity Monitor). I've also done all the steps to make the battery life last longer but it doesn't make much difference.