won't charge until the battery is dead
I own a 1 1/2 year old iPod touch 4th gen, and it will not charge until battery has completely died. Then, once it dies, I plug it in to a wall charger. It only chages until it has enough juice to power on, then I have about 5 minutes of life before it dies again. I have gotten it to be full battery, if I turn it off while it first starts charging, and leave it for the night. In the morning it is full battery.
iPod touch, iOS 6.1.6