What Size Battery Do I Need for MY Boat?

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Whether you just purchased a new boat, need an upgrade, or added equipment to your sailboat, the question always comes up as to what size battery I need for my boat. We cover everything you need to know about selecting the correct battery size for your boat.

What Size Battery Do I Need for My Boat?  To determine the battery size for a boat, you need to

  • Measure the amount of amperage that you need
  • Measure the amount of cranking amps you need
  • Figure out the types of batteries needed
  • Calculate the number of the cells required to run in parallel

Putting in the extra effort to understand your vessel’s electrical requirements not only helps prevent damage to your boat, but it can also help prolong the equipment of your ship. If you changed out a fish finder or added a GPS, you most likely changed the electrical requirements for the boat. Upgrading equipment on your sailboat may require an upgrade to your battery size also.

Picking the Correct Size

Having an underpowered or overpowered battery for your marine vessel can damage your electronic equipment. Choosing the correct size based on the manufacturer’s recommendations is always the go-to route.

First, you need to determine the exact amount of power requirements for your boat. There are two different types of batteries for your marine vessel, the starting battery, and the deep cycle battery. The standard is one engine that takes one starting battery and one deep cycle battery; if you have duel engines, then it adds one deep cycle battery.

To figure out your boat’s Amp Hour requirement, you are going to need to create a spreadsheet using the formula W=IV or Watts is equal to amperage multiplied by voltage.  To figure out the amp hours required, you take the average amps per hour used.   A 10-watt appliance on a 12-volt system would need 1.2 Amps and, if turned on for 10 hours, would require 12 amp hours to run.

The Battery Council International has provided a class chart for battery sizes to pick the correct one for your boat.  You need to select the battery setup based on the maximum estimated amount of AH that you will need to operate your boat. Totaling your AH that need replacing, will help you figure out how many deep cycle batteries you need to be wired in parallel to run the sailboat.

Types of Marine Batteries

Aside From choosing the correct size, you also need to select the right kind of marine battery for you. Different types of batteries are going to have distinct advantages and disadvantages of using them. In this section, we cover the four types of cells and their pros and cons.


The Lead-acid or flooded batteries are the most common batteries used in all industries. These batteries are the most cost-efficient to use but may require frequent replacement. They work by mixing sulfuric acid with distilled water to create an electric reaction between two lead alloy threaded posts.

Cost-effectiveNo fast charge
Easily rechargeableHigh Maintenance
High power outputProne to corrosion
 Low power density


The Absorbed Glass Mat battery works by storing the sulfuric acid and distilled water mix in a fiberglass mat separating the plates. This system weaves throughout the entire casing of the cell.

Charges up to 5 times faster than Lead-AcidLess specific measurements
Easily rechargeableCan only discharge 50% of the battery
High power outputOvercharging/undercharging lower life expectancy
Electrolyte will not spill with a damaged case 


The Gel Battery works by placing a mix of distilled water, Sulfuric acid, and Silica gel fumes in a tube. The chemical reaction occurs in a one-way cell turning the water fumes back into the water.

Can be used in places with limited ventilationHot Temperatures can damage the battery
Maintenance-freeNeed regulators to prevent overcharging
Lowest Cost per CycleOvercharging/undercharging lower life expectancy
Electrolyte will not spill with a damaged case 


The Lithium-Ion Battery works by mixing the ions between the positive and negative plates.

Can be used in places with limited ventilationWill Catch Fire if overcharged
Maintenance-freeNeed regulators to prevent overcharging
Does not contain cadmiumOvercharging/undercharging lower life expectancy
Electrolyte will not spill with a damaged caseMost Expensive
Longest Life expectancy 
Drop-in replacement 

Battery Storage & Maintenance

Proper storage of the marine battery will prolong its life cycle. Winterizing your boat also consists of adequately storing your battery. Trickle chargers can cause damage to your marine boat unless it is set up in a way not to overcharge the battery.

Picking an excellent automatic marine battery charger will save you money in replacement costs. You also want to store your battery in a manner that allows the battery no to stay at a constant temperature.

The Battery cannot be set on concrete as it may expand and leak, or completely discharge depending on the moisture and temperature of the concrete. Always set your battery on a dry wooden surface.

Getting your battery ready for the spring season is not as simple as picking up and going.  Here is an excellent video that explains the entire process.

Cranking Batteries

Cranking batteries are different from your deep cycle batteries. Cranking batteries are going to be used to start your boat engine and do not require a deep cycle discharge. Most cranking cells, except for a dual-purpose marine battery, which will not slow release as a deep cycle battery will.

You need to know how many amps it takes to start your boat engine. The cold-cranking amps is a measurement of the maximum number of amps a battery can deliver in 0 degrees Fahrenheit for 30 seconds without dropping below its voltage requirements, either 10.5 or 7.5 Volts depending on the battery.

If you are unsure what your boat requirement is for cold cranking amps, you can take a clamp meter and clamp it to the lead into the starter to measure how many amps it is receiving.

Mixing Batteries

Marine batteries are the most significant maintenance cost in owning a boat. It may be tempting to mix battery types based on price. Mixing new cells with older ones lowers the performance of the new batteries down the performance level of the old batteries.

Mixing marine batteries can lead to over/undercharging the cells of the batteries, causing replacement sooner than Should be required. Sticking with the same battery types is recommended to avoid over/undercharging of battery cells.

Since each type of battery has different chemical makeup, each type will all charge differently, as well. Stick with the same battery types to get the maximum charge out of each cell.  If possible, we advise you to stick with the same brand of battery as well.

Final Thoughts/Conclusion

Your sailboat is one of the most prized vessels. Caring for your boat doesn’t have to be overwhelming or even excessively expensive. Avoiding costly mistakes like overcharging or undercharging your marine batteries will help with cost reduction in maintaining your sailboat. Before you install that new fish finder, or marine GPS, double-check the amperage and amp-hours required for that device, you may need a battery upgrade too.

It also helps to carry some extra tools onboard for emergencies.  A battery jump box could be the difference between making it back to the shore or calling emergency services to tug you back to shore.  As John Rousmaniere said:

“ The Goal is Not to Sail the Boat, but Rather to Help the Boat Sail Herself”

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