The iPhone 3g battery, which was in the phone had been used by my friend next door since it first came out, well as you would expect it wasn't holding a charge very long.
I searched the web for a replacement iPhone 3g battery, then I was went to YouTube and looked for replacement proceedures. I found an iFixit video and the link. Well I'm a Linux guy, cost effectiveness is my top of the list item. First and foremost this is a great deal, I've been a computer network engineer and repair guy for 33 years and if I've Got Parts I'll fix it. However, even though everything went well i.e. disassembly, reassembly, it wasn't until I went to replace that last screw in the bottom (of the first two that were removed), that I lost one office my desk. First procedure was to grab a magnet and scan the carpet with the magnet, no results, scanned my tennis shoes, nada, disappointed leaned back and there it was on my office chair between my legs. Other than that everything went well, but please consider doing on a bigger table so as not to loose any of the tiny screws.
Well I must say that what I've been doing for that 6 years since I retired is to repair old computers, re-install OS'es, add memory from other old computers that are just not repairable and once they're up and running I give them to those whose computers have died. Well with that in mind, since everyone is upgrading to the iPhone 4s from 3g/3gs/and 4 I've put out the word instead of tossing them away give them to me and I'll restore to original state (wiping any info off pertaining to the past user) and give them away if the next person will do the following: purchase the Battery Kit from iFixit, and then purchase the One (1) year GoPhone sim from AT&T which has 1,000 min for $100.00 and I'll give you a free iPhone, and start a iTunes account of your own, of course if you would like to donate any funds I would have not problem accepting it.