Ditto on wiring the graph on the trolling motor battery. I don't know how the newer trolling motors / graphs behave together but every time I've tried it in the past I got interference. I use a small Cabelas 12v battery to power my graph instead. You can get the battery and charger for $30.

http://www.cabelas.com/cabelas/en/content/Item/01/80/49/i018049sq01.jpg