dell c810 power mgmt prob - batt to ac prob

I have a Dell Latitude C810 & with both the batt & ac connected works well.
when I unplugg my ac it immediately goes to batt power w/o shutting down,
but when the batt indicates its getting low I plug the AC back in and the
icon on the bottom will not switch back to AC power and eventually when the
batt goes dead the computer shuts down. I thne reboot and its all ok under
AC power in the lower right screen area. Why can I go from ac to batt w/o
shutting down but not from Batt to AC? I am using Windows XP and hv 512MB
sdram.

Do I need a new patch or what?