Its important to keep an eye on your battery usage over it's lifetime. In my case, it happens to be a macbook pro. Knowing its capacity, charge cycles and more will help you maintain it, and know if you are having issues that are battery related.
I like to use an application called Battery Health Montior from Sonora Graphics. It's free, and works on Macbooks, Powerbooks and iBooks. Battery Health Monitor provides information on:
- Current Capacity
- Original Capacity
- Current Charge
- Charge Cycles
- Voltage
Checking up on these bar graph values will indicated changes in the lifespan of your current battery. A very important part of extending your battery's life, is to let it run down completely and then fully re-charge it at least once a month. I try to do it once a week, since my macbook is on AC power most of the time.
Coconut Battery is another free application I've used in the past worth noting.
Download - Battery Health Montior