Make sure you’re using a 30 pin dock connector to FireWire cable to charge it, not a USB cable. While this iPod has a dock connector, it can’t charge through a USB to dock connector cable like the other iPods. You will need the cable and the power adapter.
Cable: https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=30+pin+dock+connector+to+firewire+cable&_sacat=0
Power adapter: https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=iPod+firewire+power+adapter&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=firewire+power+adapter&LH_TitleDesc=0
Note that some eBay listings might contain both the cable and power adapter.