thats a sort of a huge battery just for a fish finder.
the 1280 is much smaller at under 6 lbs, is 8ah, and fits perfectly in the hobie tray. they are all over ebay for 15-20 bucks, free shipping.
when we upgraded to the Elite-7 DSI, i wanted to upgrade power options along with the units. but, after searching high and low, i have found nothing that fits the bill any better than the UB1280. during the day i run the Elite-7 all day, sometimes 10 hours. at night i run the FF and two LED lights. rarely more than 6 hours (last night from 2:30 to about 7:30 this morning), and i have yet to run this battery down. during the Vero Beach Boat show, i had the unit on the simulator program and ran it both saturday and sunday without charging and it never missed a beat. that was probably 14 hours total?
i know you have been looking for power for your new E-5. dont overthink the AH. anything over about 6ah should be plenty for even the largest screens on kayaks, and the most powerful transducers.
i think (hope) the new Hobie kit will have everything everyone needs as far as power, storage and connections, but for just a battery, i have found no better option over the UB1280.
drewhttp://www.ebay.com/itm/12V-7AH-12-Volt ... 5d3cb4da2c