Been running SI since 2009. I am totally spoiled by it.
Depending on the draw of the particular unit, You will probably need a 12ah (+-) if you are fishing all day keeping SI on. When I put my first HB997 on my standard 7AH would give out after about 6-7 hours. For a while I just linked two batteries paralell and got full/long day. When I got the 998 my draw went down in spite of the fact the specs did not change. I run a 10 or 12 AH and get a decent full day from it still.
If I am night fishing and sharing batteries with my lights, I have a 20AH.