When the battery is low, Mac OS alerts me to plug it into the power supply.
But, how to setup to alert me when the battery is full?
You can use the MiniBatteryStatus widget.
It will growl when the battery is full.
But you can also set it up to growl when you switch between battery and AC power. Or when you need to be warned when the battery charge percentage goes lower some value.
You can add the "state of the battery" to the menu bar (this can be set in the prefs panels - energy saver / second tab). Then an icon (and/or some text) will display the % of his power (on the top right corner of the main menu bar, next to the date). When it's totaly recharged, the icon will change. Another way is to (phicaly) push on the button on the battery itself. There are 5 led's that will or won't blink. If all the five leds are lit, that means that your battery in totaly "fully" recharged.