Apple menu > About this Mac > more information > Power and post for your Battery Cycle count and status.
Mine is 18 months old, the battery charges to 96% and the cycle count is a mere 13.
I use power most of the time, rarely on battery, only if the power goes off or the Magsafe is not fully connected/accidentially disconnected somehow.
You could also reset the SMC
The computer doesn't respond to the power button when pressed.
A portable Mac doesn't appear to respond properly when you close or open the lid.
The computer sleeps or shuts down unexpectedly.
The battery does not appear to be charging properly.
The MagSafe power adaptor LED doesn't appear to indicate the correct activity.