While a graph does not draw that many amps it is usually a good idea to run them on their own battery. When running a graph on the same battery as your t-motor there is a strong possibility that you will get interference on your graph while running your t-motor. A lot of the guys run their graph off of small 12 volt batteries like this one from Cabelas, it is what I use on my boat.



http://www.calfishing.com/dc/user_files/6175-bat.jpg