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.
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.