We ditched a Sony cordless after 11yrs cause two or three buttons on the keypad stopped working consistently. After the 2nd NiCd batt pac cratered at less than 3months, I pulled one apart (replacement from Sony was ~$60) and realizing it was AA sized, I made my own using 3 2300mah Powerex rechargeables. That pac was still working when we upgraded to the Panasonic more than 9yrs later.
The Panasonic paperwork for the one we got goes on about how much more efficient they've made the device in the name of going green. It comes with Panasonic NiMh AAAs and after charging, I've not not seen any handset battery guage say anything other than full even after one of the wife's 2-3hr marathon gab sessions. The important thing batt wise is that its not NiCd, those don't work due to charge/discharge they see in use and their memory issue.