Your Mac's battery hasn't reached its maximum cycle count, which for your model is 1000 cycles (see Mac notebooks: Determining battery cycle count):
Other factors may decrease battery life, so double-check the battery condition in System Information.app:
About This Mac>
On the left hand side select
Power and look for
Article OS X Mountain Lion: Battery conditions lists the possible conditions:
Normal: The battery is functioning normally.
Replace Soon: The battery is functioning normally but holds less charge than it did when it was new.
Replace Now: The battery is functioning normally but holds significantly less charge than it did when it was new. You can
continue to use the battery until you replace it without harming your
Service Battery: The battery isn’t functioning normally, and you may or may not notice a change in its behavior or the amount of charge
it holds. Take your computer in for service. You can continue to use
your battery before it’s checked without harming your computer.