After lying on the shelf for a couple of months, the iPhone 3Gs did not boot up any more. Charging had no effect, it just wouldn't start up --> Battery exhausted!
Since I might still use it or want to sell it, the battery had to be replaced.
Just following the guide got my through the repair in no time, 30 minutes max!
Identifying all the parts mentioned in the guide was not always clear at first, but a second look at the full scale images was all it took to clarify.
One thing that I wasn't sure about was step 6: The fastening mechanism of the cable labeled "3". How would it work? A "before" and "after" image in the guide might help.
Plus it often felt like breaking something when bending the display upwards. It feels sketchy but in the end it worked well, even though I sometimes bent it further upwards than advised in the guide.
These things are TINY! I was afraid to break something, but with caution and patience, it all went well.
The connectors are actually sturdier than they appear.