Check out a power cord tracker for Mac devices. Integrate the module to work in the background mode and view notifications and alarm messages to see that your cord is plugged or unplugged. Monitor the capacity and the current charge level of your battery.
UnPlugged keeps you apprised of the state of your Mac's power intake. This is an application that sits in the background and monitors whether your Mac is running on AC power or on battery power. It displays a Growl notification when you disconnect your Mac from AC or when you connect it back on. It can also display notifications when you are running on battery power and the charge of your battery drops or increases. This can be changed from the app's preferences. By default, the charge notification pops-up when the battery charge either drops or increases by increments of 10%.
From the preferences window, you can choose whether you want to display notifications when the battery is fully charged or when the charge changes. An interesting way in which you can customize this app is by telling it not to bother you with notifications when a certain application is running. All you have to do is add an application to the list. This is useful for when you are watching a movie and don't want the notifications to disturbe you.
UnPlugged can be downloaded for free from the Mac App Store. If you are running an earlier version of Mac OS X, you can download the old version from the developer's website.