For most manufacturers, the perfect scenario requires that you play only MP3s encoded at 128Kbps; you're wearing bundled earbuds; your volume level is at about 50 to 75 percent; the backlight of your screen turns off automatically within 5 to 10 seconds; your equalizer setting is flat or normal; there are no DSP settings (such as the iPod's Sound Check) enabled; you listen to your music in one, maybe two sessions; and if applicable, you don't view any photos or videos. Given that these conditions are rarely ever met in the real world, you'll never achieve the number x in "up to x hours."

That's OK. If battery life is your primary concern -and it may be for world travelers- then you'll naturally go with a player that has at least 20 hours of rated battery life per charge or AA (or AAA) battery, as stated in the player specifications. This figure is rarely ever left out of specs, by the way. The best hard drive-based players last more than 20 hours-such as the Sony NW-HD5 and the Cowon iAudio X5L; the best flash players- such as anything from Sony, Samsung's YP-T6, and iRiver's T10- last more than 40 hours.

You may read MP3 player reviews to verify battery life. Factors such as sound quality, features, format compatibility, and looks may overshadow battery life, but when your player runs out of juice, it doesn't really matter which features it has or how good it sounds.

In the real world, there are plenty of factors that will help drain your battery much quicker than you'd like. For example, while the iPod's 14-hour audio-only rating is acceptable (the first iPods had 8 to 9 hours per charge), you’d ever get that many hours, and in fact, the average is probably less than 8 hours. It’s not that the battery is dying prematurely; rather, most people use it with the screen on and browse photos, as well as watch an occasional video and they crank the volume up, all of which drains juice faster than you can say ‘recharge’.

Adding to the battery drain is users’ tendency to use big headphones, which draw serious juice and therefore increase noise and distortion.

Those who belong to subscription services such as Napster or Rhapsody have it worse. Music rented from these services arrive in the WMA DRM 10 format, and it takes extra processing power to ensure that the licenses making the tracks work are still valid and match up to the device itself. Heavy DRM not only slows down an MP3 player but also sucks the very life out of them. Take, for instance, the critically acclaimed Creative Zen Vision:M, with a rated battery life of up to 14 hours for audio and 4 hours for video. CNET tested it at nearly 16 hours, with MP3s - impressive indeed. Upon playing back only WMA subscription tracks, the Vision:M scored at just more than 12 hours. That's a loss of almost 4 hours, and you haven't even turned the backlight on yet.

So when you go for your MP3 check out not just what the functions are but how long the battery is likely to last.