Replace your iPhone 3G battery for $6

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It's a sad fact of life: iPhone batteries wear out. Most users learn this about 18 months after they buy one, when the phone can barely last a day without running out of juice. Sadder still: With iPhones sealed up tighter than an astronaut's flight suit, there's no easy way to replace a worn-out battery.
But there is a way. In fact, if your iPhone is out of warranty anyway (and at 18 months it undoubtedly is, unless you extended it) and you're reasonably handy with small tools, you can swap in a fresh, longer-lasting new battery. Your cost: As little as $6.
Start with the battery itself. Meritline sells a iPhone 3G-compatible 1600mAh battery for $5.99 shipped. The standard 3G battery has a capacity of 1150mAh, so in theory the replacement should give you about 50 percent more runtime.
Now it's time for some surgery. The iPhone may look airtight, but a little careful prying with an X-Acto knife (or similar tool) will get the case open.
To see how, check the nicely illustrated iPhone 3G Repair Guide at Rapid Repair. Once you hit Step 10, just swap in the new battery and close everything up. That's really all there is to it.
Well, almost. I've got a few caveats, starting with that warranty thing again: Opening up your iPhone will void your warranty. So will installing a third-party battery. This isn't a big deal if the warranty's already expired, but if it hasn't, take your iPhone to Apple. I understand they're good about dealing with battery issues.
The real caveat is that unless you're comfortable with tiny tools and electronics, you may want to hire a professional for this. It just so happens that Rapid Repair can do the job.
In fact, they have a pretty nice deal: $39.99 buys you a "lifetime battery," meaning Rapid Repair will replace it for you every 12 months. You'll also have to pay a one-time installation fee of $29.99 and cover shipping costs. Still, it's way cheaper than a new iPhone--and a bum battery is a chief reason people buy replacements.
Given that my 3G hasn't had its first birthday yet (it's a mere seven months old), I've yet to attempt a battery replacement myself. But when the time comes, I'll almost certainly rise to the challenge.
I've replaced iPod and Zune batteries before, and both times were a piece of cake. I've got to assume this isn't much tougher. If you've tried this option, let me know how it went!