i have been using AA rechargables for about two years in my M8.
i now have 2000mah. when fully charged i get 11.4 volts. are you using a wall charger?? if you are using the airtronics wall charger , those are for nicad batteries. i use the wall chargers that come with the batteries. or my novak millenium in receiver pack mode at 1 amp. i would not use the plug on the radio to fully charge batteries. it should have a diode in it that will not let the charger know it has reached its peak. you can also just buy some AA battery holders at radio shack and wire it to charge your batteries. if your charger can charge 8 batteries.
either you are not getting a full charge or you got some cheap batteries.