While OS X does a decent job of telling you to plug in your computer before the black screen descends, it doesn't allow a lot of flexibility in how it provides you with alerts. That's where Low Battery Saver comes in: it warns you when your battery hits a certain amount of time remaining, and then shuts down your computer. Choose Preferences from its menu-bar icon, and you can configure at what point the program's Safe Sleep feature will turn off the screen.
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