Hi
If you have an iPod which is a 3G or older [as yours is] you should use the included iPod FireWire 6-pin cable that it came with it. Avoid cheaper cables...they honestly can ruin your day when ya least suspect it 😠
Cheap cables can definitely be an issue. It may look just fine and may work for some things however won't cut it for others. I learned too with things in the past.
I don't mean to sound insulting here, however, did you ensure the new battery was indeed connected to the circuit board or harness? I have done things like that and was kind of embarrassed to admit it lol.
Also; if you have access to a DMM or DVM (digital multimeter) you can make sure the proper voltage is indeed present. MOST new batteries come partially charged if not fully and checking it should give you the indicated voltage as stated on the label or packaging.
Make sure iTunes is updated to the latest version and also make sure your connecting the cable to the Firewire port itself (preferably on the back of the PC if using one) and not an extension cable or muti port adapter.
It's possible your Firewire port has bit the bullet as well. I'm assuming you only have one (1) port for FW. You can always rule that in or out by trying to find another source [PC/laptop] with a 1394 port and plug it in and see.
If it is a Firewire port and it's a PC you can grab a 1394 PCI-E Firewire card to plug into your motherboard OR a USB to Firewire adapter cable for a laptop.
If it is a bad port, be sure to disable that port [if that option exists] from your BIOS/UEFI so it plays nice with a new one and disable it from within the Device Manager in Windows.
Let me know how ya make out. I have the schematics for your device on hand if need be - that is the very first model released back in 2001.
Sincerely,
Tim Dickerson
ARS N9NU
RF Engineer, FutureNETSystems