lithium battery
*Charge often*.
charge every day if possible. when your battery reaches 70% you can charge already. if you need to use it for longer, no problem. but if you're not using your ipod, just plug in the charger when it reaches 70% or less.
try not to let it reach 50%. this is why lithium batteries are chosen for phones and ipods.  reasons:
a. you don't have to completely discharge your phone battery daily.(discharge daily means you need at least one battery. like maybe 14 years ago)
B. they are light/not heavy. they have high energy (mah) per gram.
c. they are the only battery that you can use that allows to be charged two or three times a day or whenever it reaches two or three bars of power.
Nickle metal hydrate nimh:
charge when almost flat. it will self discharge 1.5% per day.
*normally you have to charge them before you use them.*
they don't like being charged every day.

nickle cadmium
not common any more.
you MUST finish them before you charge them. the chargers usually have a discharge function. if not large crystals will form INSIDE the battery and it won't hold so much charge (memory effect).
if you charge and full discharge a few times (reconditioning) you can make the battery better again because the crystals get smaller.

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Added on November 30, 2008, 12:09 amnickle cadmium battery lasts maybe 8 years max
nickle cadmium battery lasts maybe 5 years max
lithium battery lasts 2 years max.

oh yeah, if you hammer nail into a lithium battery it will burst into red fiery flames. see youtube.