Probably don't need 15 amp hrs. Can't remember what I use offhand, but weight is the issue. I like having the battery in a dry bag inside the hull - every ounce that is kept low help with your center of gravity
My first ducer I gooped to the inside, same with the second. On the third time I mounted it I went for the best reading possible - link is here rather than repost all the pix
http://www.kayaksportfishing.com/php...pic.php?t=8407
NOT for teh faint of heart nor those that shy away from a workbench. Not implying anythin, just saying.....
There are also those that did wet mounts where it sits in a container of water inside the hull. I have not done that before so can't comment.