What are lithium deep cycle marine batteries?

Deep cycle batteries, are designed to have their entire capacity removed from the battery very slowly over time. In an SLA battery, a deep discharge can be seen at the 10- or 20-hour rates. In a lithium iron phosphate (lithium, for the purpose of this blog) battery, energy cells are used for deep cycling purposes. Now that you have a basic understanding of deep cycle batteries, let’s dive into deep cycle lithium marine batteries.

Marine Batteries

Marine batteries can be broken up into two groups – starting batteries and deep cycle batteries. Starter batteries are designed to – you guessed it – start the boat’s engine. Deep cycle batteries can be further split into two groups – house batteries, which are used to power anything from the radio to the fish finder to house lights, and trolling batteries, which are used to power your trolling motor. To optimize your battery’s life and performance, it is often common to have a separate deep cycle battery bank in addition to your starter battery. In this blog, we will be covering marine deep cycle batteries.

Deep Cycle Marine Battery Sizes

Marine batteries typically come in a range of BCI (Battery Council International) group sizes – and these group sizes equate to the physical size of the battery. The most common are 24, 27 and 31. When looking at traditional sealed lead acid batteries, it is usually safe to assume that the larger the case size, the more capacity. This is because the bigger case allows more room for electrolyte and lead plates, which translates into higher capacity.

However, with lithium, because of the higher energy density of the cells, overall construction of the battery, and smaller footprint, we can manufacture group sizes in different amp-hour (AH) ranges. For example, Power Sonic offers the PSL-SC-121000 in both a group 31 (PSL-SC-121000-G31) and a group 27 (PSL-SC-121000-G27).

The batteries, regardless of size, are offered in a 12-volt (V) platform, usually connected in series to bring the total voltage up to 36V (3 in series) or 48V (4 in series). The most common amp-hours (AH) fall between 70AH and 125AH but depending on the size of the boat and power draw, may be as low as 20AH.

How to Size a Deep Cycle Battery for your Boat

Before picking a battery for your boat, you’ll need to understand its power requirements, how long and how often you are using it, and how big the battery cabinet is. Items that might be pulling power on your boat include: trolling motor and house power items like lights, radio/stereo, pumps, and fish finders, among others. Often times, fishermen will use a separate battery bank for their trolling motor than they will for the rest of the house power items.

For example, let’s say the boat has a trolling motor that will use 25 amperes and will be in use for 4 hours, you would need a 100Ah (amp-hour) battery to power the trolling motor for that long. Depending on the trolling motor’s requirement, you will also need 24V to 36V, meaning you would need two to three 12V batteries in series to meet the requirement.

If you were to have any additional accessories needing battery power, like house lights or a stereo, you would also need to take those amperage needs into account of how large to size your house power battery bank.

A great formula to use would be:

Estimated run time (hours) x estimated amperage needed = total amp-hours required

Once you have the total amp hours required, you need to check your system’s requirements for voltage. It is important to always match the voltage of your system to the voltage of your battery bank. For example, trolling motors can require 12, 24, or 36 volts. This means you may need up to three batteries in series to meet the voltage requirements for your trolling batteries. In the photo below, two PSL-SC-12750 batteries have been placed in series to bring the kayak’s power up to 25.6V (24V SLA equivalent).

Power sonic deep cycle marine batteries

For house power, you would use the same formula to estimate amp-hours and refer to system requirements for voltage.

Benefits of Lithium Deep Cycle Marine Batteries

While you can use the SLA house batteries that your marine vehicle likely came with, lithium iron phosphate is considered a great drop-in replacement upgrade for SLA marine batteries. This is because lithium batteries are designed to last longer and weigh less than the comparable SLA batteries.

Lithium batteries outlast SLA batteries – in both run time and cycle life. The average SLA battery will see 300-500 cycles before it has reached the end of its useable life. The average lithium battery will do 4000 cycles before it has reached the end of its life – about 4 to 6 times longer than the SLA battery. Additionally, a lithium battery will provide consistent power delivery across each cycle for the entire life of the battery. Whereas an SLA battery will slowly start to deliver less power and shorter cycle lengths as time goes on.

Think of a trolling motor using SLA batteries. If it is a windy day, the spot lock trolling motor would gradually lose power. Even worse, as the day wears on when using SLA batteries, the spot lock function of the trolling motor may stop holding it’s lock, and the batteries may even die. With a lithium battery bank, even on windy days, the batteries can hold up for 10 hours or more, without ever losing power and therefore never losing the spot lock. 

In marine applications, it is ideal to lighten the load as much as possible to reduce resistance through the water. If you were to have three 12V 100AH SLA batteries on board the boat for the trolling motor, for example, they would weigh approximately 60 pounds each, or 180 pounds total. The same lithium batteries would weigh less than 30 pounds each, and less than 90 pounds total – less than half of the SLA batteries. This is a significant weight savings

How is the drop-in replacement possible?

This is possible because the form factor and voltage of lithium batteries are closely matched to that of SLA, making for an easy drop-in replacement between chemistries.

Lithium batteries, because they are composed of cells and circuitry, can fit varying amp-hour ranges in cases of different sizes. Cylindrical cells are a smaller form factor, usually with lower amp-hours, but can allow for many in series and parallel. Prismatic cells are larger form factors, but with much higher amp-hours – meaning less can be used to make up a battery pack. However, in most instances, a lithium pack is still smaller than the SLA equivalent battery – meaning you can use a smaller case or a larger case to encapsulate the pack. This allows for lithium to be a physical fit drop-in replacement for SLA.

Likewise, lithium and SLA are similar in voltages. Each lithium iron phosphate (LiFePO4) cell is 3.2 volts. To make a battery pack that is close to a 12v SLA battery, we use 4 lithium cells in series – making 12.8V.

This 12.8V is close enough to a 12V SLA battery, meaning the lithium battery can be charged using an SLA charging system with ease. The bulk charging voltages of a 12V SLA battery are 14.4-14.7V. The charging voltage of a 12.8V lithium battery is 14.6V. As you can see, the charging voltage for lithium is within the range for SLA, meaning you can safely charge the lithium battery with an SLA charger. You can read more about charging lithium on an SLA charger here.

In conclusion, lithium marine deep cycle batteries make for a wonderful drop-in replacement of SLA batteries into your boat. From trolling motors to house lights, the lightweight SLA alternative can power it all. To speak with a specialist about your battery needs, please contact us.

Categories: Blog, Batteries, Lithium

PSL-SC Series - Lithium Battery


The PSL-SC series by Power Sonic is ideal for marine applications due to their ability to be connected in series or parallel.


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