I personally prefer having a second battery I'm one of those better safe than sorry kind of guys.I also wouldn't own manual downriggers.If you don't have enough space in the stern you might be able to mount the battery in the bow or elsewhere you can run some pretty long cables and still have it work fine. Have you thought (if same fuel mix) of hooking your kicker up to the main tank? Kinda tough to comment on without being there hope everything works out the way you want it.