Well, Ni-cads actually don't do so well in constantly charged scenarios. Unlike lead-acid they do better if they are fully cycled (last about 1000-2000 cycles) or if they are being stored, fully discharged.
This is why car batteries are supplied charged and cell phone batteries are supplied DISCHARGED.
Nowadays, with the availability of super capacitors in small factor sizes, these are the best option for cmos backup. They never leak, they don't self discharge, they are not damaged by constant charging and they cycle millions of times.
Having said this I paid Analogic £100 last year to replace my A4K battery! 8-)
