I bought a 21" iMac in 2012 and it obviously came with a new battery set which I didn't have to change even ONCE for the next 20 months! I don't know what kind of batteries they put, but I've been going through a set of 2 in 4-5 weeks ever since. If anybody gave me a pointer as to what exactly I should check on my computer to see what is draining the batteries so fast. Muchas appreciado!