What Is A Deep Cycle Marine Battery?

A deep-cycle marine battery is designed for prolonged discharge at a steady rate, enabling significant power delivery over extended periods.

Unlike regular car batteries that provide short, high-power bursts for starting engines, deep cycle batteries support the continuous operation of marine equipment by enduring multiple charge and discharge cycles.

This capability makes them ideal for powering electronics, trolling motors, and other accessories on boats.

How Deep Cycle Marine Batteries Work

Deep-cycle marine batteries function by utilizing a specific chemical process (that varies by battery technology) that allows them to provide a consistent and reliable source of electrical power for extended periods. 

Unlike traditional automotive batteries that are designed for short bursts of high power to turn a starter motor over, deep cycle batteries are built to endure deep discharges without sustaining damage.

The basic principle behind deep-cycle batteries involves a reversible chemical reaction that occurs within the battery cells. 

During discharge, the battery releases electrical energy as it converts chemical energy stored in the battery’s plates and electrolytes. 

The converse is also true, and when charging, electrical energy from an external source is used to reverse the chemical reaction, restoring the battery’s stored energy for future use.

Types Of Deep Cycle Marine Batteries

Deep-cycle marine batteries come in various types, each with its own set of advantages and disadvantages

The main types of deep-cycle marine batteries are lead-acid, AGM (Absorbent Glass Mat), and lithium-ion.

Lead-acid batteries are the most traditional and widely used type of deep-cycle marine battery. They are known for their reliability and affordability. 

The chemistry of Lead-Acid Deep Cycle Marine Batteries relies on a liquid electrolyte. Two battery sub-types are derived from Lead-Acid Deep Cycle Marine Batteries:

  • Flooded lead-acid batteries
  • Gel batteries

Flooded Lead-Acid Batteries

These batteries contain a liquid electrolyte, which requires periodic maintenance to ensure proper water levels. They are known for their robustness and ability to handle deep discharges.

Gel Batteries

Gel batteries gel-like substance (hence the name) to immobilize the electrolyte (stop it from splashing around in the battery casing.) This makes them maintenance-free and less prone to spillage. They are a safer option for certain marine applications.

AGM batteries are a development of the traditional lead-acid battery. They utilize an absorbent glass mat separator to hold the electrolyte in place, eliminating the risk of spillage. 

AGM batteries are maintenance-free, offer high reliability, and are resistant to vibration and shock.

Lithium-ion batteries are relatively new to the marine industry but have gained popularity due to their high energy density and lightweight design. 

Lithium-ion batteries, although more expensive, offer significantly more benefits than other technologies:

  • They are capable of providing more power in a smaller package, making them suitable for boats with limited space. 
  • They can be discharged to a greater percentage before incurring any damage.
  • They will last for many more discharge and recharge cycles than all the other battery technologies discussed in this article.

Lithium-ion batteries offer longer lifecycles and faster charging compared to traditional lead-acid batteries.

Performance Metrics Of Deep Cycle Marine Batteries

To choose the right deep-cycle marine battery for your specific needs, it’s crucial to understand the various performance metrics associated with these batteries. 

The following table details the technical performance parameters of each battery technology. Please note that these values are generalized, and individual battery models may vary considerably.

Performance MetricLead-Acid Deep Cycle Marine BatteriesFlooded Lead-Acid BatteriesGel BatteriesAGM (Absorbent Glass Mat)Lithium-Ion Deep Cycle Marine Batteries
Capacity (Ah)36 to 110 Ah36 to 110 Ah36 to 110 Ah36 to 110 Ah70 AH to 125 AH
Voltage (V)12 volts12 volts12 volts12 volts12 volts
Cycle Life2001,500500 – 1,000300-5002,000 – 5,000
Peukert’s Exponent (n)1.1 to 1.31.2 to 1.61.1 to 1.251.05 to 1.15The formula cannot be applied to Lithium-ion batteries
Self-Discharge Rate3% – 5% per month3% – 5% per month3% – 5% per month1% – 2% per month< 1% a month
Depth of Discharge (DOD)50%50%50%50%85%
Charging Efficiency (%)80%80%80%80%+95%
Weight (lbs./kg)40 and 50 pounds40 and 50 pounds50 – 60 pounds25 – 35 pounds25 – 35 pounds

Note 1: A lead–acid battery capacity is not a static value. It varies according to how quickly the battery is discharged. 

Note 2: Peukert’s Exponent (n) is a measure of how a battery’s capacity changes with the rate of discharge. The lower the number, the better.

Applications Of Deep Cycle Marine Batteries

Deep-cycle marine batteries find extensive use in various marine applications, ensuring the smooth operation of electrical systems on boats and vessels. Here are some key applications:

Trolling motor batteries are generally deep-cycle marine batteries as they provide the necessary thrust for extended periods, often at low speeds. 

The ability of deep cycle batteries to handle frequent and deep discharges makes them perfect for this application.

Electric outboard motors are gaining popularity as a more environmentally friendly alternative to traditional gasoline outboards. 

Deep-cycle marine batteries, especially lithium-ion variants, play a crucial role in powering electric outboard motors, offering sufficient range and performance for various boat sizes.

With the current state of battery technology, the battery required to power an electric motor is a significant part of the overall unit cost. Because of this, electric outboard motors are still a significantly more expensive option than the equivalent gas-powered motor.

As battery technology continues to develop and expand, the costs will reduce, and as a consequence, electric outboard motors will become substantially more cost-effective. 

Fishing BoatsThese boats often rely on trolling motors and additional electrical equipment for angling purposes.
SailboatsDeep cycle batteries provide power for lighting, navigation equipment, and electric winches on sailboats, enabling safe and efficient sailing.
Pontoon BoatsPopular for leisure cruising, pontoon boats use deep cycle batteries to run lights, sound systems, and other onboard amenities.
Cruisers & YachtsDeep cycle batteries are the most appropriate energy storage devices for use in renewable systems (solar or wind power)
Larger vessels like cruisers and yachts generally meet the requirement. They are often large enough to have sufficient spare space to install solar panels, which generate electrical energy. 

I’m the founder and chief editor here at Kite Ship. The electrification of boating is the most exciting thing to happen to the marine industry in a generation! Welcome, and I hope that we can provide the portal you need to dive into the world of electric propulsion and power.