iPod will not charge

I recently borrowed a iPod Classic from my father while I work to get enough money to replace the hard drive to an iFlash board in my iPod Video. The iPod Classic charged fine until about an hour ago when the lightning icon stopped appearing. This happened when I kept plugging in and unplugging the cable as my stereo wont let me play next tracks unless I do it while its disconnected. The device does have a little battery but its at red. After plugging it in for a while, it doesn't SAY its charging but goes up to a green color around halfway but once I disconnect it, it goes back to red. This is really worrisome as the device was working fine until I borrowed it. I tried using 2 cables and connected through both DC and AC power. If anybody knows what the issue is please notify me. I want to know if either it was the charging port, a visual glitch or the battery finally dying. I really don't want to take apart the iPod even though its already scratched up as I want to keep it in the condition it was handed to me in.