How to size a 12 volt battery

As we evolve as kayak anglers it becomes our nature to want to upgrade.  We buy the new kayak and we want to trick it out with a bigger depth finder, lights, USB ports, etc.  All of which are great products to give you more functionality while out on the water, but with all of those upgrades our power demand also increases.  In this post I hope to give you an idea of the simple calculations you will need to size your battery as well as which types of batteries are the most commonly used on kayaks today.

The first step in sizing your battery is understanding your TOTAL power demand.  In order to keep this as simple as possible we will work solely with the units Amps and Amp-hours.  Amps will be used to signify the current required by an electrical product, and Amp-hours will be used as the rate of the current draw from the electrical source.

In my case for example, I have two products that will be drawing current from my battery on my kayak.  One is my depth finder which is a Lowrance Elite 9 TI and the other is my LED light kit from SuperNova Fishing Lights.  If I look up the power consumption numbers for both of these products I find the following…

Lowrance Elite 9TI: 0.9 Amps

Extreme LED light kit: 2.0 Amps

So, adding these two together I have a total of 2.9 amps of current that I need to supply to these two products every hour.  Now if you look at a typical 12-volt battery they usually have the total amount of Amp-hours listed for it.  One would think then if I needed the battery to last a typical 7-hour long fishing trip that I would multiply 2.9 Amps by 7 hours giving me roughly 20.3 Amp-hours…then I would go out and buy a 12-volt 20 Amp-hour battery and I would be done, right?  The answer is unfortunately no as we live in an imperfect world and we are limited in some cases by the technology of the battery.  So, let’s go over the two typical styles of batteries that are most commonly used by kayak anglers and what equation to use in each case.

1)  SLA Batteries

SLA batteries or sealed lead acid batteries are the most common style batteries that kayak anglers can purchase.  They are cheap and easy to find which are great characteristics, but like anything else in life it has its drawbacks…

  • In order to get the longest life out of your SLA battery it is recommended that you do not discharge the battery past 50%. As an example, if you have a 20 Amp-hour battery…you should not consume more than 10 Amp-hours as it can slowly degrade the battery which causes it to lose charging capacity and overall life.  Realistically we don’t always have a way of knowing how much we use on an average trip so the equation below will help us take this into account.
  • As we consume current from an SLA battery we start to experience a slow drop in voltage. Once the supply voltage gets low enough we eventually lose power to some of our electronics as they usually need a certain voltage to operate.  As an example, a depth finder’s transducer signal slowly becomes weaker as we experience a voltage drop which also causes a decrease in performance of certain products.
  • SLA batteries do not respond well to extreme cold or hot temperatures which also degrades the amount of battery life.
  • SLA battery amp-hour ratings are based on a 20-hour discharge time frame. So, if you have a 20 amp-hour battery discharging over 20 hours that comes to 1 amp per hour being taken from the battery.  If the electronics on your kayak take more than that in an hour then you also lose battery life during a fishing trip.

Long story short, you can see that a lot of factors act against us using SLA batteries…but at the end of the day they are still the most cost-effective battery and should last you a full season if you take care of them.  In order to size your SLA battery, I recommend using the following equation below. It takes into account the issues we will run into above and should give you a conservative number to use when you purchase your battery.

So, if I want to have my battery last a minimum 7 hours I need to use the following equation…

Total fishing time (hours) x current consumed by electronics (amps) ÷ efficiency factor (0.7) = Battery current draw rating (amp-hours)

7 hours (total fishing time) x 2.9 amps (current consumed by all electronics over 1 hour) ÷ 0.7 (efficiency factor) = **29.0 amp-hours

**Note that this calculation does not take into account if you turn off electronics part way through a trip…I would recommend doing a calculation for each electronic component and their typical run time and at the end add up all of the separate amp-hours. Also, this equation does not take into account voltage drop which may cause your electronics to turn off…but overall this equation should be conservative enough to give you a ball park battery size**

2)  Lithium Batteries

Let me just preface this by saying that not all Lithium batteries are the same across the board so your results may vary, however, Lithium Ion and LiFePo4 batteries are a much higher caliber battery and come at a premium price.  The reason they come at a higher price is because of their advantages over the typical SLA batteries.

  • Lithium batteries can be discharged near 100% without causing damage to the battery or affecting the overall lifespan.
  • Lithium batteries are voltage stable and do not drop in voltage much if any when current is pulled from the battery.
  • Lithium batteries can tolerate extreme cold much better than SLA batteries but they do get hotter much more easily than an SLA battery causing stress on the individual cells inside of the battery which could degrade overall life of the battery…so keep them cool below 85 degrees if at all possible.
  • Lithium batteries are current stable and are not rated on a 20-hour scale like SLA batteries. The amp-hour rating is the amp-hour rating no matter the amount of current your electronics pull.
  • Lithium batteries can take 10 times more charging cycles than a normal SLA battery…but they do require specific chargers as to not overcharge the cells which can put stress to the battery and also degrade life just like heat will.
  • Lithium batteries of the same amp-hour rating as an SLA battery are much lighter offering you some weight savings on a kayak.

Now, if I want to size a Lithium battery to power my electronics I will most likely use the following equation…

Total fishing time (hours) x current consumed by electronics (amps) ÷ efficiency factor (0.97) = Battery current draw rating (amp-hours)

7 hours (total fishing time) x 2.9 amps (current consumed by electronics) ÷ 0.97 (efficiency factor) = **20.92 amp-hours

**Note that this calculation does not take into account if you turn off electronics part way through a trip…I would recommend doing a calculation for each electronic component and their typical run time and at the end add up all of the separate amp-hours**

There is a lot more information that could have been covered in this post, but I figured if you understood the equations for both an SLA or Lithium battery above that it would give you a straight forward answer in how to size your battery.  I also hope that the points I have made help you decide what battery makes the most sense to use on your kayak based on your power needs and budget.

Like always, if you have any questions don’t hesitate to email me at arw.fishing@gmail.com

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