Sure you can. There's a couple of different routes you can take:
Check out this MacOSXHints [link|http://forums.macosxhints.com/showthread.php?t=101474|forum post] on how to set up an Applescript that gets triggered at a certain battery percentage.
There are also 3rd party battery warning applications available, such as [link|http://www.orange-carb.org/SBM/|SlimBatteryMonitor], that lets you set the low battery warning level to whatever percentage you want.