I use a sealed gel cel battery for my fishfinder and lights on my kayak. Most of the guys fishing here in Texas use the kind that are for deer feeders, or emergency lighting. Those batteries probably weigh 7 or 8 pounds and last a long time. I also have 2 small (2 1/2 pounds) sealed gel cel batteries that were surplus medical grade batteries. I've used them all day with out ever going down. I found them by searching the internet with google for sealed gel cel batteries. You can get them in 6 or 12 volt. The cost was $9 + $5 shipping for both of them. Gel cel batteries like to keep a charge and it increases the life of the battery by keeping them charged with a small trickle charger.