I just added a third battery into my 1990 Catalina 30 MKII. If anyone is interested in doing this project I can give you some info on it. It was actually an easy job and it didn't cost too terribly much. I put the third battery into the storage area under the rear quarterberth. I ran the negative cable from the battery box under the starbard part of the salon to the rear storage area by running it under the nav station. I just pulled the hot lead for battery one out of the battery box and ran it to the rear storage area. The third battery, in the quaterberth area, is a dedicated starting battery and it is on #1. #2 is 2 group 27 deep cycles run in parrellel. I like the setup and it seems to be working great.