Where is the battery cycle count kept?

Hi all;

I have a battery which does the job but is about to die shortly. I saw some guy on eBay selling a second hand battery for a somewhat reasonable price, I know it's stupid but I'm thinking of giving it a shot. What bothers me is;

Where is the battery cycle count kept?

Does the system keep it or is there something like a chip on the battery that stores such information. I mean if I install this battery for a quick test I might see the actual capacity via Coconut Battery or iStat menus but will it give me an accurate cycle count or will it think it's brand new and show zero?

Thanks in advance.

 

Thanks a lot.