iPhone 3G and iPhone 3GS have stay for almost 4 years, and some of them are no longer holding a charge, and always running out of power, yeah 4 years long really is the life-time a battery can live, if you are holding the iPhone 3G or 3GS, meanwhile have the same problem like I mentioned, Follow Tipb to replace a new battery in the old deveice:

Disclaimer: As with any repair, like TiPb said we can't be held responsible for any damage you may do to your device. It’s also worth considering that opening up your device to perform any repair or modification can and will void your Apple warranty. If you don’t feel comfortable opening your device, don’t. Use extreme care and caution when performing a repair on any device.

What you need for a DIY iPhone 3G/3GS battery swap

PXLFIX recommends using only quality and genuine parts from a reputable supplier like eTech Parts. They have quality parts, tools, and much more for all your repair needs.

*A sad iPhone 3G or 3GS
*A new battery
*A #00 Phillips screwdriver
*A suction cup
*A pry tool or spudger
*Sim removal tool or paperclip

Power off your iPhone

Before performing any repair, always power off your device before removing any screws or parts.

Remove the dock connector screws

Using your #00 Phillips screwdriver, remove the 2 screws located on each side of the dock connector.

Remove the digitizer & LCD assembly

This part can be a bit tricky so please do this with extreme care so you don’t tear any cables during the process. I highly recommend using the suction cup method unless your screen is cracked above the home button for some reason and you can’t get a proper seal with the suction cup.

Suction cup method (recommended):

 

The easiest way to remove the digitizer assembly is by using a suction cup.

1.Position the suction cup on the screen directly above the home button.
2.Using your other hand, hold down the frame while gently pulling up on the suction cup.
3.The goal is to get the bottom of the digitizer loose. Do not pull directly up. There are 3 cables attached in the top right of your iPhone. You do not want to tear these. Just pull up until the bottom of the assembly pops out.
4.After you have the bottom up you can remove the suction cup.

Spudger/razor blade method (only use this method as a secondary method):

 

ONLY use this method if your screen is cracked above the home button. If this is the case, you’ll need to use a razor blade or spudger in order to remove it. And while you’re at it, replace that broken screen!

1.Starting in the bottom left hand corner, insert a plastic spudger and gently pry up. Once the first side is starting to pop up, do the same on the bottom right side.
Run your pry tool along the bottom of the device in order to pop up the bottom of the assembly. Take special care around the home button area.

2.Remove the cables that attach the digitizer assembly to the logic board:

Once you have the bottom of your digitizer assembly popped up we can remove the cables attaching it to the logic board.

1.Gently lift up your digitizer enough to have a clear view of the 3 cables in the top right of your iPhone. 

2.With your plastic spudger or pry tool, you’ll need to pry up the first two cables that attach the digitizer and LCD. They should be labeled with orange stickers numbered 1 and 2. Simply pop them off the logic board gently. 

3.Underneath these cables you’ll see one other labeled as cable 3. This one is a bit trickier as it is clipped down. You’ll need to gently pop up the clasp holding it into the fastener. It is a tiny white clip. Simply pop it upwards and the number 3 cable should then slide right out. 

4.Remove the digitizer & LCD assembly and set it aside.