How Long Do Marine Batteries Last & How To Maintain
November 14, 2023 7 min read
Battery technology has progressed by leaps and bounds over the past 15 years. The electric vehicle revolution has funneled millions of dollars into battery storage research and development. This technology has spilled into boating, benefiting boats of all sizes and propulsion methods, creating longer lasting marine batteries. But how long do marine deep cycle batteries really last?
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Marine Batteries 101
A couple decades ago, there was only one kind of marine deep-cycle battery — the flooded lead-acid battery. Then came absorbed glass mat (AGM) batteries, gel cell batteries, and, most recently, lithium-ion batteries.
That has made the question of how long you can expect your marine batteries to last even harder to answer since the different chemistries have different expected lifespans.
Still, there are some general guidelines to help you understand how long your batteries should last and to extend that lifespan as much as possible.
Deep Cycle vs. Starting vs. Dual Purpose Boat Batteries
First, we need to address the different designation of marine batteries, since they have very different purposes and different expected lifespans. There are two main types of boat batteries: the starting boat battery and the deep cycle battery.
Starting Batteries
Starting batteries are designed to provide a large burst of energy for a short duration of time. This short burst of energy is designed primarily to start your main propulsion system, hence the name.
Normally, they aren’t used to power electrical accessories, since as long as the engine is running, the alternator supplies enough current to power electronics, pumps, etc. That means they stay almost fully charged and are only discharged for a few seconds at a time. When used as intended, starting batteries can be expected to last quite a bit longer than deep cycle batteries — five years or more.
When used as intended, starting batteries can be expected to last quite a bit longer than deep cycle batteries.
Deep Cycle
The next type of battery is the deep cycle battery. The deep-cycle battery provides a smaller amount of power over an extended period of time. Deep cycles are most commonly used to run radios, audio systems, trolling motors, and other auxiliary equipment.
They are designed to be discharged deeply — by powering your trolling motor all day, for example — and then recharged.
This discharge-recharge process is referred to as a cycle, so a deep cycle refers to a battery meant to be deeply discharged and recharged many times. This article will deal mainly with the expected service life of deep-cycle batteries, which is much more variable than that of starting batteries.
This discharge-recharge process is referred to as a cycle, so deep cycle refers to a battery meant to be deeply discharged and recharged many times.
Dual-Purpose
Another type of battery worth mentioning is the dual-purpose battery. This type of battery can serve both purposes, but isn’t perfect for either. Since the dual purpose battery can be used to both run auxiliary systems and start main propulsion systems, if relied on to power auxiliary systems, it may not be able to start the main propulsion system.
For greater protection, zinc anodes are often attached directly to aluminum hulls as well. Any pontoon billed as a “saltwater edition” or similar should already be equipped with zinc anodes. Even on pontoons not factory-equipped with anodes, they can usually be easily attached to the same brackets/pads that are used for mounting transducers, water pickups, etc.
Boat Battery Chemistry: Flooded, AGM, Gel and Lithium
Before talking about how long you should expect your deep cycle batteries to last, we also need to address the different battery chemistries available currently.
Wet Cell
The oldest and most widely used battery across the boating and automotive world is the wet cell battery. Wet cell batteries are commonly referred to as flooded batteries. These batteries use lead plates, a sulfuric acid electrolyte, and plate separators to store energy. They’re the cheapest type of battery on the market, and they are not sensitive to overcharging. Wet cell batteries have some drawbacks as well. They’re heavy batteries, with some weighing well over 50 pounds.
They’re also fairly bulky, requiring dedicated storage to house them. Some wet cell batteries must also be maintained, requiring additional fluid to be added to maintain levels. They can also leak fluids and are sensitive to vibrations. Although wet cell batteries have their disadvantages, they have provided millions of great days on the water at a great price point.
Absorbed Glass Mat
The absorbed glass mat battery, commonly referred to as AGM, is a type of battery that has a fiberglass mat separating the lead plates within.
These glass mats hold an electrolyte solution between the lead plates. Unlike the wet cell battery, which can leak and spill, the AGM is completely sealed and maintenance-free. It’s also designed to handle vibration better than a wet cell. It does have two drawbacks. It is heavier than a wet cell battery, and it is much more sensitive to damage if overcharged.
Gel Cell
The gel cell battery uses a thick paste to separate its battery plates. It’s similar to the AGM in many ways, such as being maintenance-free, spill proof, and leak-proof. It also has drawbacks. It’s expensive, prone to damage when overcharged, and does not have the same capacity as AGM batteries of similar size.
Lithium Ion
The lithium ion battery is the newest type of battery to hit the boat market. They’re gaining a ton of popularity and for good reasons. They offer impressive size, capacity, and longevity. They’re roughly 60 percent lighter than similar capacity conventional batteries. They also take up much less space, leaving more room for storage. Lithium ion batteries can also be discharged completely with no loss of performance.
Lithium ion batteries only have one major drawback — their price.
For example, a lithium ion battery powering your trolling motor would deliver consistent performance for the duration of its battery life. Lithium ion batteries only have one major drawback — their price. A lithium ion battery can cost up to 5 times the cost of AGM, gel cell, or wet cell batteries; however, they can last up to 20 years, with some brands providing an 11-year warranty. In addition to a lifespan of years, lithium batteries are designed to last up to 7,000 charging cycles.
While the upfront cost may be staggering, their lifespan may end up making them more economical in the long run.
How Long Should Your Marine Battery Last?
Let’s get this out of the way up front: there are simply too many variables to give a definitive answer about the expected lifespan of a deep cycle marine battery. We can, though, make some informative generalizations about what factors affect lifespan and the range of how long certain types of batteries will last.
Relative Lifespans of Different Battery Chemistries
All else being equal, wet cell batteries and gel cell batteries in typical marine applications will have the shortest lifespans — around 2 to 5 years. AGM batteries can be expected to last a bit longer — say 4 to 7 years. Lithium batteries last far longer than other types — upwards of 10 years and sometimes as long as 20. Typically, lithium batteries should last around five times as many cycles as other batteries.
For most non-lithium deep cycle batteries, 300 to 500 cycles is a realistic expectation, assuming batteries are well maintained.
A more accurate way to measure the lifespan of deep cycle batteries is cycles, not years. Consider a wet cell battery that can be cycled 400 times under optimal conditions. If that battery is used by a fishing guide to run a trolling motor for five hours a day, six days a week, its lifespan is years will be much shorter than if it gets discharged half a dozen times a year and sits in climate-controlled storage the rest of the time. For most non-lithium deep cycle batteries, 300 to 500 cycles is a realistic expectation, assuming batteries are well maintained.
Boat Battery Maintenance: Factors in Lifespan
Many factors contribute to the real-world lifespan of any given deep-cycle battery. Here are some of the most important:
Depth of Discharge
The depth of discharge is the amount of power the battery has been discharged relative to its overall capacity. For instance, using half of the available power in a battery would be a 50% depth of discharge. The more deeply a battery is discharged, and the more often, the shorter it will last. For maximum service life, try not to regularly discharge conventional batteries more than 50%. Lithium batteries can tolerate much deeper discharges without shortening their lifespan. In other words, they have more usable capacity.
Ambient Temperature
Temperature is a factor in a battery's performance and lifespan. Extreme temperatures increase the internal chemical reaction inside the battery, resulting in sulfation, and other permanent damage to the battery. Studies have shown that for each 10-degree rise in temperature, the life of a sealed battery is cut in half.
Cold temperatures can impact battery performance as well. Cold batteries hold a charge longer than normal temperature or warm batteries; however, they discharge faster than warm batteries.
The bottom line is that all else being equal the same battery will last longer in California than Florida.
The bottom line is that all else being equal the same battery will last longer in California than Florida. In cold climates, batteries should be taken out of boats and stored at room temperature during winter.
State of Charge
Leaving a battery discharged shortens its lifespan. Always fully recharge batteries after a day on the water. For extended periods of non-use, use a battery maintainer or trickle charger to maintain a full charge. Remember that different battery chemistries require different charging profiles. Be sure to set your battery charger to the correct setting.
Condition of Terminals
Keep battery terminals clean and free of corrosion! Just because you still get power from your batteries doesn’t mean the terminals are in perfect condition. Over time, corroded terminals can shorten battery life.
Electrolyte Level
Not long ago, most wet cell batteries had removable caps for adding distilled water to keep the electrolyte level above the tops of the plates. Most newer batteries — even conventional wet cells — are “maintenance-free,” meaning you can’t add electrolytes to them. If you do have batteries that require maintenance, though, keeping them topped up is critical.
Installation Quality
Marine batteries should be fully secured in a dry, ventilated space. Wet cell batteries must be installed upright and are more susceptible to damage from shocks and vibration than other battery types. AGM, gel, and lithium batteries can be installed at any angle but should still be tied down tightly to prevent movement.
Loose wires or accessories can short your battery out, creating a significant fire hazard.
Even if the battery itself isn’t damaged, loose batteries can damage terminals. Make sure your battery storage is clean and tidy. Loose wires or accessories can short your battery out, creating a significant fire hazard.
When to Replace Boat Batteries
With car batteries and boat starting batteries, when to replace them is obvious. With deep cycle batteries, the decline is slower, but it’s usually apparent that their capacity is declining. The simple answer is that when they no longer last as long as you want them to, it’s time to replace them. Physical signs of failure like warping, cracking, or bulging also tell you it's time for a replacement.
When you do, always replace all batteries in a bank together. For example, if you’re running a 24V trolling motor with a pair of 12V batteries, always replace them together and always use identical batteries. Or you can just buy a 24V trolling motor battery for easier replacement!
Sixty to 80% of the material in a lead-acid battery is recyclable while more than 90% of material in a lithium battery can be recycled.
Also, be sure to dispose of your old batteries properly. Deep cycle batteries — regardless of their chemistry — contain lots of nasty stuff. Fortunately, 60 to 80% of the material in a lead-acid battery is recyclable while more than 90% of the material in a lithium battery can be recycled. Many automotive retail stores will pay a core charge for your battery, and properly recycle it.
