Can an Alternator Charge a Deep Cycle Battery?

When it comes to deep cycle batteries, things can get confusing pretty quickly. Volts, amps, voltage regulators, gel-cell batteries, and the list goes on. People want to understand the best way to charge them and get the most out of their battery and their boating setup. Today, we are taking a look at alternators and the idea that they can charge a deep cycle battery.

Can an Alternator Charge a Deep Cycle Battery? 1

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Can An Alternator Charge a Deep Cycle Battery?

Alternators are used on a boat to charge deep cycle batteries or any marine battery on your boat. If you have an outboard engine produced since 2000, your engine has an alternator designed for charging the electrical system and batteries.

The best way to know you have an alternator is if you have an electric start motor. If you have an inboard or I/O (inboard/outboard) engine, then you, too are likely to have an alternator to charge your boat batteries.

While an alternator is a good way to charge your boat batteries while on the water, there is a lot more to know about maintaining your marine batteries.

Let’s dive into some additional details about the batteries on your boat and how to care for them.

Charging A Deep Cycle Battery With An Alternator

If you are operating a modern fishing boat; you are going to depend on electrical power from your batteries for most of the functions that you are using.

If you are using pumps, monitoring your sonar unit, or any number of other functions, you’ll need battery power. Without battery power, you are basically floating uselessly.

Unfortunately, a lot of boaters find that poor battery performance or even poor charging of their batteries can lead to a lot of significant problems when they are out on the water.

You shouldn’t let these common reasons for having a bad day stop you though, you should learn about how to charge your batteries correctly so you can get the most out of them.

Even premium batteries need to be taken care of the right way, or their performance will suffer greatly or even get ruined if you completely mess them up.

What is a Deep Cycle Battery?

When dealing with boat batteries, you will actually see a few that function in different ways.

A marine starter battery, which is also known as a cranking battery, is used to start the boat’s motor. You can almost think of these as operating in a similar way to a car or golf cart battery because they give a short burst of high current and they are used to simply turn the outboard’s starter so you can get moving.

Once the motor is running; the boat engine’s alternator will recharge the battery to compensate for starting it as well as any power that is being drawn for things like pumps or lights.

Trolling motors are usually used along side true deep cycle batteries which can provide a low to medium amount of current over a longer period of time and give off significantly more output over time than cranking batteries do.

They should last a long time and you need to select one based on their reserve capacity which means how long your batteries can last while you are out on the water.

Having a deep cycle battery that doesn’t last as long as you need it to for your boating trip kind of defeats the whole purpose of having a deep cycle battery.

There are also hybrid batteries which can also be called dual-purpose batteries. These batteries make a great option because they have the capacity to function as a starting battery and a deep cycle battery.

Some boating enthusiasts don’t prefer to use these batteries, as it can be a kind of compromise and you may get better performance from two separate batteries, however that probably depends on the brand and quality of each system.

Charging and Maintenance

There are various conditions that can affect your deep cycle battery and how it charges. If you store your boat in an extremely cold climate, your battery will have a much lower capacity during that time. This is similar to car’s batteries and how they have trouble getting started in the cold.

If the condition of your boat battery is in an extremely hot climate, you will have a higher capacity of your battery but the lifetime falls as the temperature rises.

Generally speaking, a battery will simply need more charging in terms of actual voltage as the temperature drops. This means that you should give a battery charger that uses automatic temperature compensation and why you want your batteries to have proper maintenance at all times of the year.

Charging Cycles

When using a deep cycle battery, you should be aware that the battery is designed to reach a significant level of self-discharge and then to get charged all the way back up to 100%. The battery is designed to complete that cycle many times over the lifetime of its use.

One example would be to have a battery starting with 100% of its capacity, be used all the way down to 25%, and then be charged back up to 100% again before its next use. There is a balancing act that comes along with your deep cycle battery in this sense.

If you use the entire depth of the discharge every time you use a battery, let’s say you use 90% of its capacity each time. That could drain your battery a bit faster because you are using so much of the battery; however if you use a very small amount of your battery before charging it, you will also probably be shortening the battery life.

Basically, these batteries are designed to be used. Don’t try to only use half of them or less than half, go all out on the water and then make sure they get charged all the way up after use.

Charging Duration

Deep cycle batteries will vary when it comes to charging duration. If you have a small boat that has a cranking battery and a deep cycle battery, you would be best served by getting a 2-bank battery charger.

If your charger has a higher amps per bank, the charge rate will be faster and you’ll achieve a full charge more quickly.

For someone who has a larger boat; let’s say they have three deep cycle batteries and a cranking battery; they would probably want to get a charger with 4 banks that has some extra power. You want to find a battery charger that can give you the appropriate power to get your batteries charged conveniently.

Onboard Battery Chargers

When looking for an onboard charger, you should try to find one that gives you the best amenities for the best price.

Some of the things you should be looking for are chargers that allow for digital control of the battery charging. You should also look for automatic chargers that help you compensate for temperature and the different stages of charging. If you are going to be onboard a boat; you probably want to get waterproof chargers so that nothing gets in your way.

Warranties are always nice to see because that means that you are getting a product that the manufacturer is completely confident in.

At a bare minimum, a quality battery charger is going to offer all of these features so that you can get a great battery charge from onboard your boat. This is an area where you need a reliable product, so it doesn’t make much sense to try and save your budget here.

Portable Chargers

If you don’t have room on your boat for an onboard motor, but are taking a long trip or boat away from home quite often, you may want to get yourself a portable marine battery charger. These can help you if you are on an adventure or have limited maximum storage.

Many portable chargers are growing in popularity and offer as many features as some other models do.


There are a ton of capable marine deep cycle battery chargers on the marketplace. You should really get a system that works for you because there is so much variety.

While it is technically possible to rig something up, the growth of the marine battery industry means that different chargers are available for different settings.

You should always use precaution to take care of your marine batteries in the right way. Getting out onto the water with a battery that isn’t working can be an extremely inconvenient and potentially dangerous situation.

When you are in the off-season, you should be sure to perform regular maintenance on your battery so that you know your boat will have plenty of charge when you’re back on the water.

At the end of the day, your deep cycle battery is responsible for keeping all of your boat’s functions operating once you already have the motor running and you’re well into your day.

You should have regular systems in place as much as possible to prevent the need for emergency charging.

Trickle Charger (4.5 Stars – ⭐️⭐️⭐️⭐️)

This is used to maintain the battery when stored for the off-season or between uses. It is designed to be left hooked up to the battery terminals and will maintain a proper charge. This way you know your battery should be good to go the next time you want to take your boat out for a day on the water. It’s amazing how inexpensive these are today. I picked up one like this from Amazon for cheap.

Jumper Box (4.5 Stars – ⭐️⭐️⭐️⭐️)

A Jumper box is used like jumper cables except you do not have to have another vehicle, boat or battery nearby to get your boat engine cranked. These are great for automotive or marine use. I keep one in the glovebox of my SUV just in case.

Jumper boxes are more expensive than a trickle-charge, but when you have to have power to crank your boat or car up, this is the tool to do the job. I first had a jumper box I bought at a local auto part store and it was junk. My friends and I have had good luck with a jump box like this one. Just make sure to get one big enough for your size of engine.

If you have a big diesel engine to jump start in your tow vehicle or boat, you will want the jumper box to be adequate in getting the engine started.

Bench Battery Charger (4 Stars – ⭐️⭐️⭐️⭐️)

If you have access to power right beside your boat slip or lift, this might be a good all-around option. I have one of these units sitting on my work bench inside of my garage.

I use it for keeping my old Jeep’s battery charged up and I use it to maintain my boat batteries during the winter months.

In general it’s good to take your batteries off of your boat during the winter so you can keep them properly maintained.

This bench battery charger is great because it does so many things. It can recondition a battery, it can act as a jump starter when your battery is dead and it is used to maintain batteries when they are in storage.

This specific model has several other useful features, but these are the three things I use mine for all of the time.