BatteryBar is a free utility that can show you the level of "battery wear" on your laptop, tablet, or netbook. BatteryBar won't show battery wear for every type of laptop, but it's free to try. I've found that it's fairly accurate in telling me that my one-year-old battery has roughly 5% wear while my three-year-old battery has over 20% wear.
Lithium ion batteries have a limited life span. They can be charged and discharged a finite number of times. Plus, age will wear them down too, even if you're not using the battery. Some of my older laptops have batteries that won't even last 30 minutes. The batteries will still charge to 100%, but they discharge at irregular rates.
Want to check the level of battery wear on your laptop? You can get BatteryBar here.