# How to determine what size battery is right for your setup.



## Bigkat650 (Jan 20, 2012)

I'm sure most of you have wondered how much battery do I really need? Of course the ideal situation is to get the largest capacity battery as possible--but this is not always applicable. What if a large battery is too much to haul around for either you or your boat? Maybe you have a size restriction for the battery? Well then you may want to know what kind of power you will need for an average day's use.

Let's first start with the basics:

Typically, marine batteries are lead-acid or sealed lead acid. Lithium Polymer batteries (or LiPo batteries for short) are coming on the market now--which are somewhat smaller and lighter, but usually about 4 times the price of lead acid batteries.

There are two types of batteries found on boats--a *starting battery*, or a *deep cycle 'trolling motor' battery*. 

*A starting battery* is designed to give maximum amperage pull for a shorter period of time. The common measure is Cold Cranking Amps (CCA), which is defined as the amount of amps the battery can sustain for a time of 30 seconds at 0 degrees before the voltage drops to 7.2volts. Starting batteries are designed to be connected to an alternator to keep the reserve capacity from dropping too low, and are not ideal for running electronics for extended periods of time without a constant charge.

*A deep cycle battery* is designed to give a lower, more constant draw for a longer period of time. They are also designed to allow the consumer to use up to 95% of the battery capacity between charging. Unlike starting batteries, deep cycle batteries measure the capacity in Amp Hours, or Ah. Amp Hours refers to the reserve capacity of amps the battery has. For example, if you have a 100Ah battery, and you are using 10 amps per hour--you will get approximately 10 hours of use if the rate remains the same. If you are using 100 amps per hour, you get 1 hour of usage. Likewise, if you are only using 2 amps per hour, you can get 50 hours of use out of the battery before charging.

*So how many amps does my equipment use?* Well, that depends on what you are running. For music, your receiver and amplifier are measured in wattage. We can calculate our amp usage if we know the wattage of the product. WATTS divided by VOLTS equals AMPS. So a 120 watt system divided by 12 volts, means we are using about 10 amps at full volume. There are other factors to consider in this equation--first, no electronic is 100% efficient, so a 120 watt amp, may use 150 watts to operate. This means you would be using about 12.5 amps her hour. Obviously since you will not operate the stereo of full volume most of the time, you would use far less wattage then the full capacity of the amplifier.

Another popular electronic in most of your boats is an electric trolling motor. Instead of watts a trolling motor's "power" is measured in foot pounds of thrust. Most modern trolling motors general use approximately 1.1 to 1.3 amps per foot pound of thrust at full throttle. So, if your trolling motor has 30ft-lbs of thrust, it will use approximately 35amps per hour if ran on the highest setting. This means that on a 100Ah battery, we would have around 2 hours and 45 minutes of continuous full power usage before the reserve is depleted. At half throttle, you would have over 5 hours of continuous usage. This means if you use your trolling motor 50% of the time, at 50% throttle, you would get over 10 hours of on-the-water usage.

*So whats the bottom line?* My suggestion is to determine what electronics you use, or would like to use, on an average trip on the water. Figure out how many amps per hour these electronics use, and how many hours you will operate them. This will give you a rough estimate of how many amp hours you need in a battery. Typically, if the situations permits it--I would add at least another 25% to what you determine an 'average' amp hour usage day would be. This will give you some leeway to make sure you don't run out of power when you need it! Using a trolling motor to fight current or windy days can really drain a battery!

I hope this helps some people out there, and please feel free to correct me on any of these points if I screwed something up hahaha.

**note, if you don't like math, its pretty easy to determine amperage draw using an inexpensive multi-meter available at almost any hardware store, including Walmart for less then $15


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## Brine (Feb 4, 2012)

Good post =D> 

Just found it.


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## dryfly24 (Mar 25, 2012)

That's great info. How practical is it to use a deep cycle battery to both start an electric outboard and run a trolling motor? I would like to avoid having 2 batteris if possible.
Thanks


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