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Boat Batteries 101: AGM, Lithium-Ion, Lead-Acid, and Gel Explained!

Jun. 15 2022 Technology By Mason Strother

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Choosing the right battery for your boat is never an easy choice. This blog will go over each major marine battery type (Lead-Acid, Gel, AGM, and Lithium-Ion) and go over their pros and cons.




 Marine batteries are responsible for any electrical process on your boat, including your: trolling motor, fish-finders, GPS, radios, motor, and more! Boat batteries are different from the ones in your car, as they are designed to supply higher bursts of energy and withstand the forces of the water. However, the chemistry and processes of each type of battery are the same. 


First, it’s important to discern another difference between battery types: starting & deep-cycle. As the name suggests, starting batteries provide quick bursts of amperage to start your vehicle, while deep-cycle batteries are great for more prolonged periods of energy usage. Deep-cycle batteries allow more room for drainage, letting you discharge about 80% energy before additional drainage may damage the battery, where starting batteries sit around 50%. Most batteries on your boat are designed to accommodate both functions and combine the benefits of each! 


There are three main types of batteries for boats: Flooded, Sealed (which includes AGM & Gel), and Lithium. They each have their own differences and uses for various applications. Let’s start by going over the different types of batteries:


Flooded Lead-Acid

Flooded lead-acid batteries (often referred to as “wet cell” batteries) are the most common type of deep cycle batteries and are the most cost-effective upfront. These batteries get their electrical current from chemical reactions between lead dioxide and sulfuric acid. 


The greatest benefit of a flooded battery is its price point. It is the most economical and well-researched battery with a variety of options to choose from. 


Most of the cons of the flooded battery arise from them containing free liquid and gas that requires periodic inspection and maintenance. They emit hydrogen gas, which can be explosive and corrosive in extreme cases. Because of this, maintenance can be more difficult and dangerous. Flooded batteries can also be damaged in rapid, forceful movements. Flooded batteries are more susceptible to extreme climates due to the battery having the ability to evaporate or freeze. 


Absorbed Glass Mat (AGM)

AGM, or Absorbed Glass Mat batteries, provide superior power for the industry’s most demanding boats. AGM batteries have a dense filling of ultra-fine absorptive fiberglass matting. This fiberglass is saturated with electrolytes and tightly packed between battery plates, leading to an efficient, high-output battery.


AGM batteries can handle high electrical loads, are resistant to vibration, can be installed at any angle, are sealed, non-spillable, and are maintenance-free. Some AGM batteries last up to 2x longer than standard flooded batteries, which makes them an efficient and strong choice on your boat. Most AGM batteries have a lifespan of around 500 cycles. One charge cycle is completed after you’ve discharged 100% of your battery’s capacity. AGM batteries also charge well in harsher weather conditions, such as freezing temperatures. 


However, AGM batteries are generally more expensive due to high manufacturing costs, and they are sensitive to overcharging and high voltages. Given this sensitivity, AGM batteries require specific chargers to monitor their volts, amperage, and temperatures. The benefits strongly outweigh the downsides and make AGM batteries a great choice for boaters.


Gel Cell

Gel cell batteries have many similar traits to AGM batteries, including non-spill-ability, deep-cycle, multi-angle mounting, low self-discharge, maintenance-free, and are sealed. Gel batteries are also sensitive to correct charging, or else the battery will prematurely become inoperable. Their difference lies in their usage of conductive materials. Gel batteries use a special type of silica gel that holds electrolytes together and allows for the flow of electrons through each interior plate. On the other hand, AGM batteries use the fiberglass matting we talked about to conduct electricity. 


When compared to AGM batteries, Gel batteries are often less durable, have a shorter lifespan, and are more expensive. As we’ll discuss, more and more boat owners are making the switch away from gel cell batteries. 


Lithium-Ion (Li-Ion)

Lithium batteries are a relatively new entry to the marine market, but are a great - and rising choice - for boaters around the world. It’s important to note, motor brands like Mercury and Yamaha do not support the use of lithium batteries with their motors. Lithium batteries are a great choice for your boat’s electronics. 


When compared to lead-acid batteries, lithium batteries often perform better and last longer. Lithium batteries often have lifespans of 2,000 cycles, many times more than AGM batteries. They also have multiple voltage output options. For example, think of a small bay boat with very limited storage, instead of needing 4 batteries to run a 36 volt trolling motor and boat functions you can do the same job with 2.


With lithium batteries, you do not have to worry as much about the depth of discharge rates (as they sit around 80%-90%, compared to AGM’s 50%-60%), so it boasts a much longer life cycle. Because of this, lithium batteries are often smaller than lead-acid batteries. This leads to perhaps the most important benefit of lithium batteries: their weight. Boat performance is greatly impacted by weight, and lithium batteries weigh significantly less than other alternatives. 


Their charging times are also much faster and are less sensitive to charging levels. In other words, you can be more at ease knowing that your lithium battery effortlessly charges. 


The largest con to address is their price. Lithium batteries often cost four or five times more than their lead-acid counterparts, due to limited resources like lithium. Lithium batteries also require temperature regulation when exposed to freezing temperatures, reducing their ideal range of usage. Lithium batteries have made moves in the marine market, being incredibly efficient at their task and are definitely an option worth considering.


Recap

So which battery is right for you and your boat? Well, flooded and gel batteries are quickly being replaced by AGM and lithium batteries as technology advances. Their efficiency, low maintenance, and reliability are all great reasons to make the switch. AGM batteries are much more economical than lithium and have a proven track record of research and success. However, lithium batteries are a strong and efficient choice that are getting stronger each day. Let’s break them down again:


AGM batteries are still a great choice for many. Here’s why:

  • They can be used as starting batteries for your motor (most lithium batteries are not recommended by the engine manufacture and may void Mercury/Yamaha warranty coverage)

  • AGM batteries perform better in colder conditions

  • They have time-tested technology

  • They can be wired in series, or a continuous loop of battery connections

  • AGM is less expensive initially

  • They are a good starting point for most new boaters


Lithium – While new to the marine battery market compared to AGM, the lithium battery is an incredible choice. Its benefits are as follows:

  • They have up to 15% higher charging efficiency

  • Lithium batteries have up to 50% lighter than AGM

  • They offer multiple voltage output in a single battery platform

  • Have a longer lifespan

  • Deeper depth of discharge

  • While more expensive upfront, they make up for the difference over time


The future of batteries is particularly interesting. Could we see marine batteries be packed into something as tiny as a smartphone? Only time will tell. 

Thank you for reading & we’ll see you in the next Foothills Marine Learning Hub blog.