Battery set up question

Hello everyone, I have a 16’ extra wide Monarch with a pull start 40hp Yamaha , and a 50# Minkota on the bow. Deep cycle battery sits under the front deck which I’m thinking of running wires for the led navigation lights to. I’m putting my Lowrance Mark X5 fish finder about mid way in the boat on a swivel base so I can see it while running either motor. I’m not going to use the deep cycle for the ff so it needs its own battery. I’ve been looking at 12v 12ah sla batteries which takes up very little room or a lithium iron phosphate which is also small but last longer if I want to spend the extra money . Am I correct in thinking this should run the ff several hours?

Look up the current draw for your FF. It is probably on the back of it, or else in the owner's manual. If it draws 1 amp.
12 amp-hour / 1 amp = 12 hours of energy. Nothing is 100% efficient, so de-rate that maybe 50% & call it 6 hours of use.

Why not run some wire back to the FF? Should be cheaper & easier than mounting a separate battery & having to recharge it all the time.

I too would run it off the battery to have ... 1 amp is nothing ...

I get interference on the ff when connected to the same battery as the trolling motor. Right now I do have an old cranking battery in the back (it’s big and heavy) with my navigation lights wired to that area. Since it’s hardly ever used I wanted to loose the extra weight. I maybe better off keeping it and running the ff wiring to it. I liked the idea of the small battery because I have to basically remove everything that’s not attached when it’s parked and I’m not getting any younger. Lol Also I would have to get a charger just for the small battery. Let me ask y’all this. If I moved the rear battery to the front would it make much difference helping the boat plan out faster? It’s easier for me the remove the batteries from the front than lifting it out of the transom area as well. That cranking battery is big and weighs more then my deep cycle.

Best bet is keep the one in the back and run your accessories off of that. Some fish finders are more sensitive at lower battery voltage than others. I run hummingbirds and if the battery voltage is low my birds start doing weird things. Helix 9 and a 10 plus a 360 and a hub to link it all .I have no motor to charge the battery as I am electric only so I run a group 31 sla battery . Overkill ? Probably but it guarantees I don’t take more than 50 % of the battery on a 13 hour day which is my limit. Lead acid don’t like discharge below 50%. Lithium are a different animal you can use more than 50 % without hurting the battery so you should be ok for 75% discharge. It you ever upgrade your finder to bigger screen 12 amp hours won’t be enough.