Deep Cycle vs Starter Batteries: Why Deep Cycle is the Right Choice

In a Nutshell

Choosing the right battery is crucial for your boat's performance and reliability. While a deep cycle marine battery is designed to provide a steady, long-lasting power source for your boat's domestic systems, a starter battery delivers a quick, powerful burst of energy to fire up the engine. For sustained loads and peace of mind on the water, the deep cycle option is the clear winner for your house bank.

What Makes a Deep Cycle Marine Battery the Best Choice?
The Anatomy of a Marine Battery
Understanding Starter Batteries
The Benefits of a Deep Cycle Marine Battery
Different Types of Marine Batteries
Charging, Monitoring & Maintenance
Summing Up
Frequently Asked Questions

What Makes a Deep Cycle Marine Battery the Best Choice?

When you’re out on the water, the last thing you want to worry about is whether your fridge, lights, or navigation instruments will give out on you. This is precisely why a deep cycle marine battery is the best choice for everything other than engine starting.

An engine starter battery is built for one purpose: to deliver a huge jolt of power for a very short period of time. Once the engine roars to life, the alternator takes over and quickly recharges the small amount of power that was used. But if you’re like me, you’ve probably had that sinking feeling of trying to start a stubborn engine. If you keep hitting the starter button, you’ll drain that battery in no time.

A deep cycle battery, on the other hand, works completely differently. Its design is all about conservation and endurance. It delivers a slow, steady flow of power over a much longer period. This makes it perfect for the constant, sustained demands of a boat’s domestic electrical system.

The Anatomy of a Marine Battery

At its core, a battery is a simple device. It works by generating a voltage across two different metal plates submerged in an acidic electrolyte. A standard 12-volt battery is made up of six individual cells, each one producing about 2.1 volts, all wired together in a series.

Each cell is filled with a sulphuric acid electrolyte and contains a series of positive and negative plates. The positive plate is made of lead dioxide and the negative one is made of lead. It's the design of these plates that makes all the difference between a starter battery and a deep cycle battery.

Understanding Starter Batteries

A starter battery cell is constructed with a large number of thin plates to provide a maximum surface area for the electrolyte to react with. This design is what allows it to release that powerful surge of current needed to turn over a high-compression diesel engine.

Sketch showing the construction of a marine engine starter battery

Internals of an engine starter battery

However, this design comes with a major drawback. If you repeatedly discharge a starter battery down to around 50% of its capacity—which is what you would do when running your domestic electrical systems—it will quickly become damaged and die a premature death. This is why you must have a separate deep cycle marine battery (or several of them linked in parallel) to handle the demands of your house bank.

On the Water...

My engine starter battery used to be identical to the other AGM batteries that made up my house bank. That was before I heard about Red Flash starter batteries.
These, apparently were developed to spin up jet engines in military aircraft. Worth a look I thought - and I liked what I saw...

Small size.
Impressive cold cranking amp capacity.
AGM technology, rapidly charges back up.
Can be fixed in any orientation - on its side, on its end, upside down even.
Leakproof.
Maintenance free.

So I bought one, a Red Flash 750.

Its small size - 165mm (6.5") x 176mm (7") x 125mm (5") - allowed me to locate it alongside the existing engine starter battery, which I then connected to my house bank - a real win/win situation!

I've been relying on my Red Flash battery for 4 seasons now. It spins my Yanmar 3YM30 over very nicely and has never let me down.

They're marketed by DMS Technology in the UK who will happily ship one to you whichever side of the Atlantic you're on.

The Benefits of a Deep Cycle Marine Battery

In contrast to the starter battery’s thin plates, a deep cycle battery's plates are much thicker and more robust. There are also fewer of them, which means less surface area to react with the electrolyte. This results in a much slower release of power, making it perfectly suited for the steady draw of your boat’s domestic systems, such as your chartplotter, interior lights, or even a small electric winch. While they're built to be tough and durable, they simply can't provide the quick, high-amp kick needed to start an engine.

Sketch showing the construction of a marine deep cycle battery

Internals of a deep-cycle marine battery

Different Types of Marine Batteries

The world of marine batteries extends far beyond the basic flooded lead-acid type. Here's a breakdown of the most common types you’ll encounter:

Flooded Lead-Acid Batteries

These are the traditional, liquid-filled batteries. They're reliable and often the most budget-friendly option. However, they're not sealed, so they can lose water from the electrolyte over time due to evaporation. This means you'll need to check the fluid levels and top them up with distilled or de-ionized water occasionally. Never use tap water, as the minerals and impurities in it can damage the cells. One thing to be mindful of is the gases they produce—a mixture of oxygen and hydrogen. A properly vented battery box is essential to safely dissipate these gases.

Valve-Regulated Lead-Acid (VRLA) Batteries

VRLA batteries are a significant step up from their flooded counterparts. Unlike flooded batteries, which can leak if tipped over, VRLAs are sealed and won't spill, even if they get knocked around during a rough passage. They also require less maintenance and have a lower self-discharge rate.

Gel Batteries

Gel batteries use a thick, jelly-like gel to suspend the electrolyte. They are sealed, maintenance-free, and can be installed in almost any orientation without the risk of leaking. They also have minimal gassing and a very low self-discharge rate, making them a great "install and forget" option. However, they are sensitive to overcharging, so you need a charger with the right settings, and they can be more expensive than flooded batteries.

AGM Batteries

AGM (Absorbed Glass Mat) batteries are another type of VRLA battery. Their name comes from the fine fibrous glass mat that holds the electrolyte in place through capillary action. Like gel batteries, AGMs are sealed, leak-proof, and can be mounted in any position. They have a very low internal resistance, which means they can accept a high charge current, allowing them to recharge much faster than other battery types. This makes them ideal for use with solar panels and wind chargers. AGMs can also deliver a high current, making them suitable for dual-purpose use as both house and starting batteries. They are generally more expensive but can be a cost-effective solution in the long run due to their long service life.

Lithium Batteries

Many sailors are now turning to lithium batteries for their house banks, and for good reason. They are incredibly lightweight, about half the weight of a comparable lead-acid battery, which can be a real plus on a cruising boat where every kilo counts. They also have an impressive cycle life, lasting up to 10-12 years. They provide consistent power throughout their discharge cycle and are virtually maintenance-free. The main downside is the high initial cost, which can be a significant investment, but their longevity and performance often make up for it.

Lead Carbon Batteries

Lead carbon batteries are a newer development that blends the best of both worlds. They are designed to reduce sulfation, which is the leading cause of battery failure, thereby extending their lifespan. They also require a lower charge voltage, making them more efficient to charge. However, they are heavier and bulkier than lithium batteries, and they cannot be used for engine starting, so their application is limited to a dedicated house bank.

Charging, Monitoring & Maintenance

A battery is only as good as the system that supports it. Here's what you need to know about keeping your batteries healthy and your electrical systems reliable.Battery Charging

There are several ways to charge your boat's batteries, and a good electrical system often uses a combination of them:

Engine Alternator: When the engine is running, the alternator charges the batteries. On many boats, this is the primary source of charging while underway.

Shore Power Charger: When you're connected to the marina's electrical supply, a dedicated battery charger converts the AC shore power to DC to replenish your batteries.

Renewable Energy: Solar panels and wind generators are fantastic for keeping your batteries topped up while at anchor, providing a quiet, eco-friendly source of power.

Always use a smart charger that has a three-stage charging profile: bulk, absorption, and float. This ensures your batteries are charged correctly without over-stressing them, which is especially important for gel and AGM batteries.Battery Monitoring

Relying on a simple voltmeter is a bit like driving a car and only looking at the fuel gauge once you're on 'E'. A proper battery monitor is essential for understanding your battery bank’s health and preventing premature failure. The best monitors, like those from Victron Energy or NASA Marine, use a shunt to measure the actual amps going in and out of the battery bank. This gives you a clear picture of:

Routine Maintenance

State of Charge (SoC): The percentage of charge remaining in the battery.

Time Remaining: An estimate of how long your batteries will last at the current rate of power consumption.

Amps In/Out: A live view of your power consumption or generation.

Even with the best batteries, a little care goes a long way. Here are some basic maintenance tips:

Clean Terminals: Keep battery terminals and connectors clean and free of corrosion. A wire brush and a mixture of baking soda and water work wonders.

Check Fluid Levels: For flooded lead-acid batteries, check the electrolyte levels regularly, especially during warmer weather or after heavy use, and top up with distilled water if needed.

Secure Installation: Ensure your batteries are firmly secured in a dedicated, well-ventilated battery box or locker. A battery that's rattling around during a rough passage can suffer internal damage.

Summing Up

Choosing the right battery is a critical decision for your boat's electrical systems. While there are many options available, from traditional flooded lead-acid batteries to cutting-edge lithium and lead carbon batteries, understanding the fundamental difference between a starter battery and a deep cycle battery is the most important takeaway. The specific design of a deep cycle marine battery—with its fewer, thicker plates—is what makes it the reliable, long-lasting workhorse for your onboard electrical needs, allowing you to run your systems with confidence, whether you’re at anchor for a night or sailing across an ocean.

For a complete understanding of how batteries fit into the bigger picture of a boat's power system, you can explore our comprehensive guide on Marine Electrical Systems for Sailboats.

This article was written by Dick McClary, RYA Yachtmaster and author of 'Offshore Sailing' published by the Royal Yachting Association ( RYA) and member of The Yachting Journalists Association (YJA).

Frequently Asked Questions

Can I use a deep cycle battery to start my engine?

While it's technically possible, it’s not recommended. A deep cycle battery is designed for slow, steady power delivery, not the huge, quick surge needed to turn over an engine. Using it for this purpose can significantly shorten its life.

What's the difference between a marine battery and a car battery?

A marine battery is specifically designed to handle the vibrations and constant movement of a boat. While car batteries are primarily starter batteries, many marine batteries are designed for deep cycling or a dual-purpose role.

How do I know when to replace my deep cycle marine battery?

Signs of a failing battery include a shortened run time for your electronics, a battery that doesn't hold a charge for as long as it used to, or a voltage drop under load. On average, a deep cycle lead-acid battery lasts 3-5 years, while AGMs and Gels can last 5-8 years or more. Lithium batteries can last up to 10-12 years.

Do I need a special charger for my deep cycle marine battery?

Yes, it is very important to use a charger that is compatible with your battery type, especially for gel, AGM, and lithium batteries. These batteries have specific charging voltage requirements to prevent damage and ensure a long service life.

Is a "leisure" battery the same as a deep cycle marine battery?

'Leisure' batteries are often a hybrid or dual-purpose battery, offering a compromise between a starter and a deep cycle battery. While they can handle some deep cycling, they are not as robust as a true deep cycle battery and are best suited for boats with modest electrical demands.