We deliver the truth behind one of the most questionable manufacturer specs in the latest MP3 Insider.

One of the key specs you mustn't ignore before purchasing an MP3 player is its battery life. This number, whether it's 14 hours (the 30GB 5G iPod) or up to 35 hours of audio playback (the 30GB Cowon iAudio X5L), gives you an estimate of how long your gadget will play tunes on a single charge--in the best-case scenario.

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. Here at CNET, we drain MP3 players using a methodology that's similar to those found in the preceding paragraph. We know which players and brands outperform or underperform. Apple iPods typically outperform the company's ratings. In our tests, the 30GB 5G iPod lasted 14.5 hours, 30 minutes longer than what Apple claims. The Cowon iAudio X5L, on the other hand, tested at 27 hours when it was rated for 35. 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.