I plug my ipod in, and it says it needs to be charged and never charges or shows up on my computer. But it works in the charger

I plug my ipod in, and it says it needs to be charged and never charges or shows up on my computer. But it works in the charger. I cannot sync it, and the computer doesn't detect it. It charges in the charger, but as soon as you unplug it, the ipod shuts off. Doesn't charge in the computer at all.