OS X: Battery Diag is a battery monitor for your MacBook that looks great and provides useful information about the battery in your Apple laptop. It runs from the menu bar and takes up minimal resources, so you can let it run in the background.
Clicking on Battery Diag from the menu bar will show the battery's capacity and remaining charge as a percentage. It also displays the source, battery health, number of charge cycles, and estimated time remaining on the current charge. Clicking on the "i" will show you even more details, like the temperature of the battery and power usage in watts. Battery Diag also supports two additional themes: Rainbow and Dark.
Battery Diag is a good looking, useful little app for your MacBook and if you run it in the background, only uses about 8MB of RAM and hardly any CPU time.
Battery Diag | Mac App Store via AddictiveTips