What is a deep cycle battery plus answers to your boat battery questions

As long as it seems to be doing its job, you might not give your boat battery much thought. In fact, many boat owners just use the batteries that came with their boats when they bought them.

But what if your battery needs replacing? Or what if you’re buying your first boat? Do you need a deep cycle marine battery? (Yes!) Or will a cranking battery do the trick? (Also yes!) It’s time to go boat battery shopping….and we’re here to help.

As technology advances, more and better battery options become available. But like a 10 page menu, more options can make things more confusing. It can be harder to choose!

Marine batteries have different chemistries, sizes and uses. Before you make your pick, it’s good to know what a deep cycle marine battery is, how it works, and what type is best.

To begin, we’ve provided a short, not-too-technical explanation of the most common terms on the boat battery “menu”:

Boat Battery Terms

Think of this basic terminology as an “appetizer”. Being familiar with boat battery vocab will help you understand the confusing specs and jargon you might come across when it’s time to shop.

Amp Hour (AH)

This refers to the amount of energy a boat battery can store. The higher the AH number, the higher the capacity.


One cycle is a complete discharge and a full recharge. Deep cycle batteries typically have a cycle life or cycle rating. This rating denotes the amount of cycles that you can discharge and recharge a battery before its capacity reaches 80% of what it once was. At that point, most batteries need to be replaced.

C rate (or Capacity Hour Rate)

This is a measurement of the rate at which your battery is charged or discharged. You could also say it’s a ratio between current drawn, battery capacity, and time. Most deep cycle marine batteries follow a standard 20hr rate. The number “20” refers to a discharge over a 20 hour period. For example, a battery with a 100AH capacity with a C20 rating has an output of 5 amps for 20 hours until it is discharged.

Depth of Discharge (DOD)

This is the amount of your battery’s total capacity that you drain before recharging. For example, if your battery has reached 60% DOD, it means that you have 40% capacity left. A battery with a capacity of 100AH would have 60% DOD if you drained it to 40AH.

Internal resistance

This term refers to how much a battery resists energy while it is charged and discharged. The lower the internal resistance, the better. A battery with higher internal resistance won’t charge as efficiently, and may get hot while charging because the lost energy converts to heat.

State of charge

This is how much charge your battery has. For example, a battery that is fully charged has a 100% state of charge.

What is a Deep Cycle Marine Battery?

Now it’s time for the main course: what is a deep cycle marine battery? This type of battery allows you to power a trolling motor, radio, GPS, fish finder, and other gadgets on your boat (and also RV stuff, but that’s another topic). It’s different from a cranking or starting battery, which we’ll discuss more in depth below.

A deep cycle marine battery can withstand many charges and discharges. This is why (in most battery types) it has thick, heavy plates. It can last through rigorous charging, and hundreds of cycles.

Much like the tortoise in the old fable “The Tortoise and the Hare”, you might consider deep cycle batteries a “slow and steady” source of power. They have a limited amount of instant power. This is why you can’t use them to start your motor. For that, you’ll need a starting/cranking battery, which we’ll discuss below.

Cranking/Starting vs. Deep Cycle Marine Battery

The design of a cranking/starting battery allows you to rapidly start your main motor. These batteries have thinner plates to supply large amounts of energy–perfect for getting your motor going.

But try using a cranking battery for a “tortoise” job, like your trolling motor and other electrical accessories, and you’ll soon find yourself back at the battery store. Your battery won’t be able to withstand the rigorous, constant discharging. It will overheat and probably quit for good.

Dual Purpose Batteries: The Best of Both Worlds?

At this point you might be wondering, “Isn’t there a battery that can do both jobs?” There is, and it’s called a dual purpose battery.

But while dual purpose batteries have some characteristics of cranking batteries and deep cycle batteries, they don’t quite measure up to either one. Most won’t have as much “oomph” for starting motors as a true cranking battery, or withstand as many cycles as a deep cycle marine battery.

But unlike other battery types, our lithium batteries are designed to handle both starting and deep cycling! The best of both worlds.

Types of Deep Cycle Marine Batteries

So you’re in the market for a deep cycle marine battery. But which type should you choose? That depends on several key points:

  • How much you’re willing to spend upfront
  • How long you want your battery to last before replacing
  • How much weight you can allow on your boat
  • Where/in what position you’ll install your battery
  • Whether you are willing to maintain your battery, or if you prefer maintenance-free

Below, we’ll discuss the most common options for deep cycle marine batteries on the market right now:

Lead Acid Deep Cycle Marine Batteries

The main components of a lead acid battery are lead plates and acid. The kind you would use in your boat are called flooded lead acid (SLA) batteries. These contain cells with a mixture of sulfuric acid and distilled water. To maintain them, you have to replenish the distilled water from time to time.

Advantages: These batteries are the cheapest upfront option. Since they are popular, they are easy to find. If you take care of them properly, they can last around 4-8 years (Keep in mind that the number of years varies based on how much use you give them. FLA batteries last about 300-400 cycles.

Disadvantages: Lead acid deep cycle marine batteries are very heavy– about 80 lbs of extra weight on your boat per 12v battery. Hauling them on and off can be quite a chore, not to mention the constant maintenance they require, and increased cost of gas.

Gel Deep Cycle Boat Batteries

Like lead acid batteries, gel batteries contain liquid electrolytes. However, they have silicates that gel these electrolytes together. This gel eliminates the risk of damage from vibrations and rough seas. Gel batteries are an improvement over FLA batteries, but more advanced technology in marine batteries is available.

Advantages: The main advantage of gel batteries is that they can tolerate long periods without being charged. They have a self-discharge rate of 1% per month. You can also install them in any position in your boat (except upside down).

Disadvantages: They’re twice as expensive as FLA batteries, but have lower capacity for the same amount of space. They also require a special type of charger and don’t handle high discharge rates as well as other boat battery types.

AGM Deep Cycle Marine Batteries

AGM stands for Absorbed Glass Mat. These batteries have thin fiberglass matting between their lead acid plates. These plates are saturated with acid. Because AGM batteries are so tightly packed, they’re highly shock resistant.

Advantages: AGM batteries are an improvement on FLA batteries. You don’t have to replenish them with water, and they recharge faster. They self-discharge at a rate of about 3% per month. They won’t spill acid, which means you can install them sideways on your boat if need be.

Disadvantages: These batteries are susceptible to overcharging, which can ruin them. They cost more than FLA batteries of similar capacity. They also have a shorter lifetime for their cost among all boat battery types.

Lithium/LiFePO4 Deep Cycle Marine Batteries

If you’re looking for an all-around good investment in a deep cycle marine battery, LiFePO4 is for you. Instead of lead plates and acid, it’s built from lithium iron phosphate.

The latest in boat battery tech, LiFePO4 offers a host of improvements over other battery types, especially SLA batteries.

Advantages: They are the lightest of all deep cycle marine battery types. You can reduce up to 70% of the weight in your boat by switching to lithium. They also recharge faster, and are leak and maintenance free. 

Disadvantages: The lone disadvantage of a LiFePO4 boat battery is its higher upfront cost. But when you consider its lifespan (2-4x that of other batteries), you’re saving money in the long run.

What Deep Cycle Marine Battery Should I Choose?

If you have the dough to purchase the tastiest item on the menu, why wouldn’t you? Right now, lithium boat batteries are that highly coveted item. They’re already popular among fishing competition enthusiasts. They also present many advantages for avid boaters, or even occasional fisherman. Their higher upfront cost levels out when you end up replacing your boat battery less.

So, unless you find the price tag too steep, there’s no reason not to choose a LifeP04 deep cycle marine battery over other types. If you’re not sure exactly what boat battery setup you need?

Leave a Reply

Your email address will not be published. Required fields are marked *