What error do you get when the memory card battery dies?
I'm getting a card no good error when card is in @ boot up and a unable to format card error when trying to save a game to it. Card was working fine when I last used it.
I think I may gave a dead battery SNK mem card..The last I turned on the cab with it in it said Memory Card Error....I have to try it again..
My bad card said it needed to be formatted and when I tried to format the card it said it was copy protected. Replacing the battery was easy enough & no problems since.