I have an HB 899 with a 7 inch screen and it draws 780 mA. I had an Werker 9.5 AH battery which would give me 12 hours on the water. The Lowrance Elite 5dsi draws 630mA so in theory you could run your unit for 15 hours. I don't know how long you plan on being out but a 9.5 AH should be plenty.
Regards