Why does charging kill the battery?

Why does charging kill the battery?

Installed a new battery and the iPod started up - plugged it in to charge and it was dead again when I returned later.

answers:

"Hi,

please check your charger and your cable. Try to use a wall charger, or another usb port a.s.o. An iPod that gets unchard during beeing plugged in, is, to me, a complete new issue."