Fish finders are often used on kayaks and small one- or two-man boats and choosing right battery with maximum performance and minimum weight at acceptable price can be confusing thing to do.
Powering fish finders, sonars, GPS and other electronics on larger fishing boats is not a problem - just plug the devices in and few Amps of current will hardly deplete the main deep cycle (dual purpose) marine battery, which is often connected to solar panels, wind turbine or at least, to the main engine's alternator.
Required Battey Capacity
First step is to determine required capacity of the fish finder battery. Capacity is obtained by multiplying fish finder current (Amperes) and desired operating time (hours) and is given in Ampere-Hours (Ah).
Most fish finders consume around or less than 1 Amp of current, when powered from 12V sources. Smaller, portable models draw even less than that, but larger ones, especially fish finder/GPS combos can draw more than that.
Just to be sure, let us assume that our hypothetical fish finder draws 1.5 Amps at 12V.
When going fishing from kayak or similar small boats, fishing trips rarely last more than 10-12h and often are much shorter, for example 3-4 hours. Upon return to shore or larger boat, fish finder battery is recharged.
So, if one prefers longer trips lasting 10-12h, required theoretical battery capacity is 15 to 18Ah, while for shorter trips (3-4h), required theoretical capacity is 4.5 - 6 Ah.
In real life, battery capacity is lower than theoretical and hence, larger batteries must be used, depending mostly on the age and condition of the battery.
When buying new battery for fish finder, increase capacity by 20-25% - instead of 18Ah (theoretical) for longer trips, consider 21-24Ah batteries and instead of 6Ah batteries for shorter trips, consider 7-9Ah batteries.
Type of Battery
There are several types of batteries which can be used for powering fish finders.
Sealed lead acid (SLA) batteries are commonly used as deep cycle or dual purpose marine batteries. They operate regardless of their position, they are leek proof, come in various sizes and shapes and they are rather cheap. Unfortunately, they are also the heaviest batteries used on boats and kayaks.
Good 7Ah/12V SLA (absorbed glass mat - AGM, or gel cell) battery weighs around 5.5-6 pounds (2.5-3 kg), while 21Ah/12V battery easily weighs 14-15 pounds (6.5-7 kg).
Personally, I would never recommend flooded lead-acid batteries for use on kayaks and small boats.
Nickel Cadmium (NiCd) batteries are lighter than SLA batteries, can provide insane high currents, but they are not environment friendly, have strong memory effect and other issues. If you need lighter batteries, choose Nickel Metal Hydride (NiMH) or lithium batteries.
Nickel Metal Hydride (NiMH) or lithium batteries provide same capacity for much lower weight. NiMH batteries can be up to 3-4 times lighter, while lithium batteries 5-6 times (or even more) lighter than SLA batteries of the same capacity.
However, such batteries also cost more money and require special chargers.
Fish Finder Battery Box
Manufacturers of cordless tools like Makita, DeWalt and similar, have NiMH and lithium batteries in use in their 'power centers'. Such power centers are designed to be very robust, water- and dust proof, and resilient to mechanical damage. But, such power centers, in combination with several high quality NiMh or lithium batteries can cost up to or even more than $1000 - rather high initial costs, really not worth it if you fish from your kayak occasionally.
But, if you like to spend every possible free moment fishing from your kayak, put NiMH or lithium batteries in the power center box (with proper electronic controller), connect small solar panel and small wind turbine and power your your fish finder, GPS and even small electric trolling motor.
SLA batteries should also be protected from open water, sun, cold wind etc. Store them in the watertight compartment, low in the boat (to improve stability, especially if you choose larger battery).
Charging Fish Finder Batteries
Batteries should be charged using chargers designed for specific battery type. Using wrong chargers can lead to damaged and destroyed batteries and chargers and even to injuries or worse due to possible heat, fire, smoke and even explosion. And we are serious about this.
SLA batteries are cheap and can be fully charged overnight or even faster, using 'fast' chargers. Note that such fast charging tend to slightly reduce battery's lifetime. Using 'fast' chargers, NiMH and lithium batteries can be charged even under hour, although charging them for 4-6 hours prolongs their lifetime.
Again - using wrong chargers can lead to physical damage and injuries caused by high temperature, smoke, fire, explosion ...
Long story short - if you are not sure what to do, go for cheap and reliable SLA battery, dimensioned for your fish finder. Even 2-3 small, 7Ah/12V batteries with proper charger, cost under $60-$70. Over time, you will figure out your most common practices and you will be able to more accurately define your needs regarding fish finder battery.