the battery seem to be the problem. restarts don't work.

OK to disconnect means that nothing on your computer (iTunes if that's what you're using, or Finder if you have Mac OS Catalina or later) is currently trying to put anything onto the iPod, so it is safe to unplug (disconnect) the iPod from the computer.

If you unplug (disconnect) the iPod from the computer while iTunes or Finder is writing to the iPod, you will corrupt the information on the iPod. So at that time, it is not okay to disconnect it.

You haven't actually told us what your iPod is doing that you're not happy about. Until you do, there's no point in suggesting battery replacement or anything else.

Forcing a restart and replacing the battery do not appear to have any relationship to one another.