Problem charging iPod Black 60GB VIDEO

I put my iPod to charge from the wall as always. After a while as usual the charge green icon appear in my iPod, but when I try to play my music my iPod said LOW POWER BATTERY and something else that I couldn't read 'cause the iPod shut off. I put it to charge [for 6 hrs] in the PC [Windows XP] and the 'Very Low battery' monochrome icon appears. You can browser through the iPod content but as soon as I push the play button it turn off saying the low battery messege. The thing is that when you are browsering through the battery icon shows fully charge. If I have the iPod charging from the wall, I can listen to the music, if I have the iPod charging from the PC the monochrome icon will appear and I can't update, sync ect. The charger works because my sister charged her iPod [Black 30GB VIDEO] and the sync cable works 'cause she updated her iPod software, since she uses the laptop to sync.

Compaq Windows XP iPod with video Black 60GB