SlimBatteryMonitor is a free battery status monitor for the Mac. It can display information about your built-in battery and some UPS devices right on your Menubar. It doesn't replace the Apple battery monitor, but this one can be greatly customized to show more information or to take less space, if that is what you need. This app only runs on the menubar.
By default, the application is configured to show a green icon when the battery is completely charged and you are running on AC power. When the Mac is running on battery, there will be a black icon, and the time remaining until the battery is depleted. The same goes for when the battery is charging, but the icon will be orange instead of black. You can change this to show the percentage remaining or the percentage charge. You can also omit the icon and change the colors.
SlimBatteryMonitor can warn you when the charge of your battery goes under a set value. By default, it is 15%.
All in all, this is a very nice app that lets you customize how you are informed of the battery level. You can change every little aspect, and the app is really easy to set up.