How can I fix the "communication with battery" at 6D?

first i got the err 20 message. after some time i didn't get that but i started getting the "can not communicate with battery" message when I insert the battery. if i say OK it works fine but the battery sign on the screen does not appear and the battery info also does not appear in the window. i use a second battery, still same message. i used a battery grip still the same issue. this is annoying! how can i fix this? thanks Najwa