How do i know what kind of laptop battery i need?

My laptop is getting kind of old and the battery is dead or something it used to be it would only work when the charger was plugged in, but now it wont work at all. ive been told i need a new battery. It's an HP computer so i went to their site. it said something about whether my batter was 3 6 8 9 12 cell,... show more My laptop is getting kind of old and the battery is dead or something it used to be it would only work when the charger was plugged in, but now it wont work at all. ive been told i need a new battery. It's an HP computer so i went to their site. it said something about whether my batter was 3 6 8 9 12 cell, what does that mean? How do i know what kind of battery to buy?
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