You can take a six-hour flight from Pennsylvania to California and listen to your iPod the entire time. All iPod models use the same type of built-in rechargeable lithium ion battery; the first-, second-, and third-generation iPods offer up to eight hours of battery power, and the fourth generation offers up to 12 hours.

However, keep in mind that playback battery time varies with the type of encoder that you use for the music files in iTunes. It also varies depending on how you use your iPod controls and settings.

The iPod battery recharges automatically when you connect the iPod to a power source — for example, it starts charging immediately when you insert it into a dock that’s connected to a power source (or to a computer with a powered FireWire connection). It takes only four hours to recharge the battery fully.

Need power when you’re on the run? Look for a power outlet in the airport terminal or hotel lobby — you can fast-charge the battery to 80-percent capacity in one hour. If you can wait four hours, your iPod is fully charged. Don’t fry your iPod with some generic power adapter — use only the power adapter supplied with the iPod from Apple.

A battery icon in the top-right corner of the iPod display indicates with a progress bar how much power is left. When you charge the battery, the icon turns into a lightning bolt inside a battery. If the icon doesn’t animate, the battery is fully charged. You can also use your iPod while the battery is charging, or disconnect it and use it before the battery is fully charged.

The iPod’s built-in rechargeable lithium ion battery is, essentially, a life-or death proposition. After it’s dead, it can be replaced, but the replacement may cost more than $50 (some services may charge less for older models). If your warranty is still active, you can have Apple replace it — but you don’t want to do that yourself because opening the iPod invalidates the warranty.

Fortunately the battery is easy to maintain. I recommend calibrating the battery once soon after you get your iPod — that is, run it all the way down (a full discharge) and then charge it all the way up (which takes four hours). Although this doesn’t actually change battery performance, it does improve the battery gauge so that the iPod displays a more accurate indicator.

Unlike batteries that require you to fully discharge and then recharge in order to get a fuller capacity, the iPod lithium ion battery prefers a partial rather than a full discharge, so avoid frequent full discharges after the initial calibration (frequent full discharges can lower battery life).

Lithium ion batteries typically last three years or more and are vulnerable to high temperatures, which decrease their life spans considerably. Don’t leave your iPod in a hot place, such as on a sunny car dashboard, for very long. For an excellent technical description of how to maintain rechargeable lithium ion batteries, see the BatteryUniversity.com Web site.

The bottom of the iPod warms up when it’s powered on. The bottom functions as a cooling surface that transfers heat from inside the unit to the cooler air outside. The iPod’s carrying case acts as an insulator, so be sure to remove the iPod from its carrying case before you recharge it.

Keeping the iPod encased in its carrying case when charging is tempting but also potentially disastrous. The iPod needs to dissipate its heat, and you could damage the unit by overheating it and frying its circuits, rendering it as useful as a paperweight.

To get around this problem, you can purchase one of the heat-dissipating carrying cases available in the Apple Store. For example, Marware offers the SportSuit Convertible case ($39.95). Even when not in use, your iPod drinks the juice. If your iPod is inactive for 14 days, you must recharge its battery — perhaps the iPod gets depressed from being left alone too long.

Power Saving on iPod

Your iPod is a hard drive, and whatever causes the hard drive to spin causes a drain on power. Your iPod also has a cache — a memory chip holding the section of music to play next. The iPod uses the cache not only to eliminate skipping when something jostles the hard drive but also to conserve power because the drive doesn’t have to spin as much.

If you use the AIFF or WAV formats for importing music into iTunes (or MusicMatch Jukebox), don’t use these formats with your iPod — convert the music first. These formats take up way too much space on the iPod hard drive and fill up the iPod cache too quickly, causing skips when you play them and using too much battery power because the drive spins more often.

The following are tips on power saving while using your iPod:

• Pause: Pause playback when you’re not listening. Pausing (stopping) playback is the easiest way to conserve power.

• Back away from the light: Use the iPod backlight sparingly. Press and hold the Menu button to turn off the backlight, or select Backlight from the iPod main menu to turn it on or off. For a solution that lasts longer, turn the Backlight Timer setting to Off on the Settings menu. Don’t use the backlight in daylight.

• Hold it: Flip the Hold switch to the lock position (with the orange bar showing) to make sure that controls aren’t accidentally activated. You don’t want your iPod playing music in your pocket and draining the battery when you’re not listening.

• You may continue: Play songs continuously without using the iPod controls. Selecting songs and using Previous/Rewind and Next/Fast-Forward requires precious energy. Not only that, but the hard drive has to spin more often when searching for songs, using more power than during continuous playback.

Always use the latest iPod software and update your software when updates come out. Apple is always trying to improve the way your iPod works, and many of these advancements relate to power usage.

Replacing iPod battery

Apple customers aren’t always happy campers. Early iPods came with batteries that couldn’t be replaced, but all it took were a few premature battery failures and quite a few customer complaints for Apple to institute a battery replacement service. Apple also offers a special AppleCare warranty for iPods.

You can’t remove or replace the iPod internal battery yourself — you need Apple to replace it if it dies. If your iPod isn’t responding after a reset. If these steps don’t restore your iPod to working condition, you might have a battery problem. Go to the Apple support page for the iPod and click the iPod service FAQ link to read frequently asked questions and answers about iPod support.

Then click the iPod service request form link on the support page and follow the instructions to request service and return your iPod for a replacement. The only time I had to do this (with a 30GB iPod), Apple required us to send just the iPod unit itself, without the power adapter or any other accessories, to Apple’s service facility. Within a week, Apple sent back a brand new iPod (same model).