Mine is an older model with 4 aa batteries. A company called grand cell makes rechargeable aa and aaa batteries, not sure about the 9v ones, These give a pretty good life, around the equivilant to standard alkeline batteries.
I know when using nicad batteries in things such as pagers life is very short and this is because of the lower voltage in Nicad batteries. (on a 1.5 v battery it is around 1.2 v fully charged, not sure on the figures with the 9 v one)