Basic Electrical Terms
There are some basic electrical terms you will need to understand. We’ll start off with voltage; in layman’s terms think of voltage as the force behind electricity. The more voltage you have, the more power you have available: you can equate this to water pressure. For this article we will limit ourselves to talking about 12VDC (Volts Direct Current), since that is what most fishfinders are designed to run on. However, most units are able to accept a range of voltages as wide as 10VDC to 20VDC without damage. We also won’t get into the difference between DC (Direct Current) (what powers your car) and AC (Alternating Current) (what powers your house) in this article, but please understand they are different and AC current is much more dangerous. The next terms you need to know are Amperes (A) (“Amps” for short) and milli-Amps (mA) (1/1000 of an Amp). Amperes is the measurement of the current or flow of electricity.In a water system you could equate Amps to GPM (Gallons per Minute). The last term you need to know is what an Amp/hour rating is. This is how a battery’s size is rated - the capacity of a battery. For example a 5 Amp/Hour battery will produce 1 A worth of power for 5 hours, like a 5 gallon bucket will produce 1 GPM for 5 minutes.
Sizing Your Battery
To figure out what size of battery you need, you have to first figure out how much power your fishfinder will use. This is one place where bigger is not necessarily better. As you can imagine, the more bells and whistles you have on your fishfinder the more power it will use. So that color, wide screen, HD, Down Imaging, GPS, sonar unit will use a lot more power than the small, black and white, sonar-only unit. The good news is most electronics companies will tell you what the mA draw of their units are. This is normally listed in the specifications section either on the website or in the owner’s manual. If you can’t find it, don’t be afraid to call the Tech Support line - they should be happy to help you out.
For this example we will use the Humminbird “598ci HD SI Combo” unit, which is rated at 615 mA with the lights off. Also please note this is normally a peak current rating, and your actual current draw may be less. If you have an appropriately-sized mA meter and know how to use it, you can place it in line and measure the actual current draw. When I measured my unit with the back light fully on, my peak measured current draw was 430 mA - over 25% less power than the rated draw with the light off. For calculation purposes, though, we will use the 615mA to add a safety factor, since most batteries will not match their rated performance more than a few times.
Now that we have an idea of how much power we will be using, we need to figure out how big a “bucket” we need. Since I regularly do long and multi-day fishing trips, I will use 12 hours for the minimum time I want my battery to last. If I take 12 hours X 615 mA = 7380 mAmp/hours. Divide that by 1000, and you get a 7.38 Amp/hour battery.
Choosing Your Battery
There are several different types of batteries to choose from. AGM are the most common batteries people use in kayaks. They are type of lead acid battery (like the one you have in your car) where the acid has been gelled and then the battery sealed so it can’t spill. The advantages are they are inexpensive, can be recharged often, and are easy to find. The disadvantage is that they are heavy.
Next there are alkaline batteries; these are your typical AAA, AA, C, or D cell batteries. Each alkaline cell will produce 1.5VDC so you will need eight of them to get to 12VDC. You can get eight-cell holders for the different cells, but the AA is the most common. Eight AA cells will give you about 2 amp/hours, C cells are around 6 amp/hours, and D cells are about 12 amp/hours. The advantages of alkaline batteries are they are easy to get and have a good power to weight ratio. The disadvantages are they cannot be recharged so they end up in landfills, and they are expensive if you go out often.
Next you have your newer types of rechargeable batteries such as NiMH,lithium, and others. The best way to get these is in a hobby battery pack like the ones that you would use in a radio-controlled car. These batteries are normally lighter than traditional batteries, and can be recharged hundreds if not a thousand times. The downsides are they can be expensive, may need special chargers and or procedures, and can be more difficult to obtain.
Back to our example: I want a 7.38 Amp/hour battery. I don’t want to deal with special chargers or procedures, and I don’t want to buy new batteries every time I go fishing, so my choice is the AGM battery. The most common sizes for AGM batteries are 7-8 Amp/hours, so I have a good selection to choose from.
Charging Your Battery
I chose the Cabela’s 12V 8 Amp/hour Battery, which comes with a charger rated at 500 mA. Calculating the charge time of your battery is the reverse of calculating the capacity. For example, take 8 Amp hour X 1000 = 8000 mA/hours, then divide that by your charging rate of 500mA. That gives you a charge time of 16 hours. In actuality, the likelihood of your charger ever coming close to the rated capacity is unlikely; normal performance for chargers I have tested is normally 50% of the rating. I would leave this battery on the charger for at least 32 hours,but no more than 48, because you do not want to overcharge it. For batteries in this size range I would not recommend charges larger than 1 Amp.
Solar chargers are another option that can even be mounted on your kayak to charge the battery while you are fishing. The cost of these is generally more than buying an additional battery, so unless you are planning on doing multi-day treks where it is infeasible to swap out batteries they probably are not worth the cost. However, if you want to look into this option here is some basic information.. Solar chargers are rated in Watts at peak output - this means a clear summer day at noon. To convert Watts into the units we were using above, you divide by the voltage. At peak output a 5 Watt solar panel will charge at 0.416 A, or 416 mA (5 Watts / 12 Volts = 0.416A). At best I would calculate that on a sunny day you might average 50% of that while the sun is in the sky. You should also consider using a charge controller to prevent discharge at night and overcharging.