I had a old color ipod 4th gen and a cell phone battery that fit it well so I converted it to all fit together. These models are great old school ipods that run rockbox as well.
separating isn't that hard, just use a sharp spatula type tool and under one side should pop out easy.
you have to unscrew the board to get the batter out since the wire runs under it and when prying up the battery careful of the ribbon cable underneath it.
Old batter from a old smart phone that was broken I found fits great. 2400mah
cf to ide adapter, easy to find on ebay and dirt cheap.
This was a lucky find in a storage auction, its actually worth over $200 but will work great in the ipod.
Ipod fire wire and usb split cable is needed for this older model to charge it and sync it unless your pc had firewire support.
Can snip or desolder the ends but leave as much of the metal as possible and if snipping do it fast and dont let the metal of the snips touch anything else while its cutting. Can skip this step if you managed to find a batter that fits with a circuit protection with a positive and negative wire that fits into the case still then just wire that to the new board.
Need to remove the circuit board from the old battery.
Ghetto wiring also I had to remove a second board since I think desoldering it damaged it so try cutting the metal close to the board and soldering to that instead. I made the wires long so I could drop the board into the old battery slot for more room.
Cf is just a drop in and snip off the 2 tabs on the side to make it fit.
Ok to have a few pins on the left over hanging.
I put some electrical tape just to prevent any shorting.
Plug it in and can drop the board into the old battery slot
Bend this little jumper down so the battery can fit. I flipped mine around after as well just so the jumper is connecting them still also can just snip and solder.
Then best to turn on and test everything and reset in itunes to test everything first and then carfully squeeze it together. I installed rockbox and it gives me 24 hours battery life but rockbox tends to drain more battery then the stock firmware anyway but it has a run ttimer I wanted to use.