Outboard Motor Battery: What Size Do I Need & What’s Best for My Boat?
New to boating and trying to figure out what outboard motor battery you need?
However, you might get overwhelmed by the vast choice of outboard motor batteries in the market.
To name just a few: marine batteries, lithium batteries, lead-acid batteries, AGM batteries, deep cycle batteries, starting or cranking batteries…
And all outboard motor batteries are available in various volts, sizes, brands, etc.
The more you search online, the more confused you might be.
You are not alone! Here we will explain all your confusion in a simple guide.
If you are one of those who are trying to figure out the battery needs for your boat (types, sizes, etc), read this post to find quick answers.
Besides that, you will also learn about outboard motor battery wiring and charging in minutes.
Table of contents:
Outboard Motor Battery Types
What kind of battery do I need for an outboard motor?
You might have seen outboard motor batteries with confusing labels. In fact, they are categorized according to their purposes and chemistry.
2 Types Based on Purposes
Basically, all outboard motor marine batteries serve two purposes: starting the engine or running the electrical load onboard. This divides outboard motor batteries into two types.
- Starting Batteries
As the name implies, a starting battery (or cranking battery) is ultimately designed to start the engine. This kind of outboard motor battery comes with high CCA values, providing high current surges and cranking amps to start your engine.
If using the wrong battery that’s not designed for a starting (or high-power) application, you will have a high chance of starting a fire in the voltage regulator and killing the battery.
- Deep Cycle Batteries
By contrast, a deep cycle battery is made for discharge and supports intensive cycling. It provides rechargeable power for many cycles, ideal for running electric outboard motors, trolling motors, and electronics onboard.
Compared with starting batteries, deep cycle batteries are more expensive and heavy, but they are perfect for powering stuff without an engine running.
Key takeaway:
Basically, the kind of battery you need for an outboard ultimately depends on your purpose.
If you’re looking for a battery for an electric start outboard motor (vs pull start), the starting battery is what you want. However, if you own an electric outboard or trolling motor, you should use deep cycle batteries (LiFePO4 is recommended).
4 Types Based on Chemistry
You will find four kinds of outboard motor batteries if you look at their chemistry inside the cells, namely flooded batteries, gel batteries, AGM batteries (advanced lead-acid batteries), and lithium batteries.
Among them, lithium is a HUGE upgrade in battery technology, and there are a great number of advantages that people run to lithium (especially LiFePO4) for their outboard motors:
You can fully discharge the battery cells every day (almost 100% vs 50% for AGM) without killing them. Moreover, they can recharge 2k-10k times vs 300-800 AGM.
These mean significantly more power capacity, so it can last twice as long on the water.
Not to mention they’re also a lot smaller and lighter than the comparable lead-acid battery, and charge much faster. And you can see the battery state on your phone or via a digital screen.
That’s why all reputable electric outboard motor brands, including ePropulsion, use lithium (or even LiFePO4) batteries as their accompanying batteries.
Further Reading: Why You Should Choose LiFePO4 Batteries
Electric Boat Motor Battery Sizes
What size battery do I need for an outboard motor? What amp-hour rating should I get?
The electric outboard motor batteries are available in a variety of sizes and power levels.
Here is how to calculate the outboard battery sizes in simple steps:
Method #1. Start with Amps.
Step 1. Get the outboard motor electric consumption in Amps (usually available in the manual). Or calculate with Amps = Watts / Volts.
Step 2. Calculate the Amp hour rating of the outboard motor (Ah = Amps x Time).
This Ah rating indicates the battery size that will be sufficient for your requirements.
For example, if your motor consumes about 60 Amps and you want an outboard motor battery that will last 3 hours, you will need a 180 Ah ( = 60 Amps x 3 hrs) battery.
Method #2. Start with Wattage.
(Use this method if you know the wattage rating of your outboard motor.)
Step 1. Calculate the total watt required for the outboard motor battery (Wh = Watt x Time).
Step 2. Calculate the Ah rating with the given voltage (Ah = Wh / Volts).
For example, if you own a 1 kW outboard motor ePropulsion Spirit Evo and you want to use it for six hours on a 48V boat system (on a single charge), you will need a 125 Ah battery.
Note that the calculation is under full load and max amp draw (meaning you’re running the motor wide open the whole time).
In real life, the amp will be much less. That’s to say, you may get away with a smaller battery if you run at part throttle with intermittent use.
However, always go for a larger battery if you go with AGM/lead-acid deep cycle batteries.
Generally, you don’t want to discharge the battery more than 50% to prevent damage. So half the rated capacity.
Alternatively, lithium batteries can be more fully discharged without damage, so you can use more of the rated capacity, which is a big plus for outboard motor batteries.
Best Outboard Motor Battery
Reading this far, I believe you should be able to decide the best outboard motor battery for your boat. If you are still not sure, here is a simple guide to help you make a quick decision.
Essentially, three things to consider:
#1. Weight
If you are looking for a small outboard motor battery, the weight should be a top priority.
You will never want an outboard motor battery to sink your small boat and break your back every time you carry it from your car and to the dock.
In that case, the lightweight lithium battery would better fit your needs.
Typically, a 12V 100Ah lead acid battery weighs over 60 lbs. Comparatively, a 100Ah LiFePO4 battery weighs only 25 lbs. So you can easily shave about 35 lbs off the weight in your boat.
Not to mention its larger usable battery capacity and smaller lighter battery cell.
As a Zodiac boat owner reported, his 12V LiFePO4 outboard motor battery lasts 2 times as long as a 120 Ah lead acid that weighs 3 times what the LiFePO4 does.
#2. Battery Life
Outboard motor battery life should be a deciding factor in purchase decisions. You need to make sure the outboard motor battery gets enough juice to do the job.
So choose an outboard motor battery that comes with a larger battery capacity and longer life cycles. And the lithium battery goes a long way in these two aspects, with at least 80% DoD (depth of charge) and 5 times more charge cycles than lead-acid batteries.
However, how long your outboard motor battery can run actually depends on a lot more factors.
One of the key factors is outboard power. The best battery for an 25 HP outboard motor is definitely different from the one for 40 HP, since they are running at different watts.
For example, if your motor runs at 300W and you use it with a 230Wh LiFePO4 battery, you will get 40 to 50 minutes’ continuous usage out of this battery.
Note that throttle power settings also make a great difference in your outboard motor battery life:
When moving a boat through water in displacement mode, to go twice as fast you must increase the power about fourfold. That’s to say, if it theoretically takes 200W to go at 2 knots, it would take 800 watts to go at 4 knots.
At the same time, going at half throttle may quadruple your range, even though you’ll get there half as fast. Many electric boat owners simply go quite slowly to make the cost and physics add up.
Also, you can use a solar panel to help keep your outboard motor battery topped up.
Further Reading:Solar Powered Boat Motors: Easiest Way to Set up Your Solar Boat
#3. Cost
The last step to get the best bang for your buck is to compare costs.
And that’s not all about the price tags.
Although the lithium outboard motor battery is more expensive, it can last 10+ years if taken care of properly.
Actually, it becomes cheaper if you plan to use it for many years. By this I mean that it may cost 2 times more upfront, but it will survive roughly 5 times as many cycles.
All in all, a lithium battery would be the best outboard motor battery option nowadays if you want to minimize size and weight and don’t mind a higher upfront cost.
Other than that, lead acid is also a reliable technology, proven and improved over hundreds of years. If you have a tight budget, go for a larger lead-acid battery.
Outboard Battery Wiring
How to wire an outboard motor to its battery?
In fact, it’s easy to hook up your outboard motor battery if you follow the manual. You don’t need to fumble with the outboard motor battery cables to get it plugged in.
Basically, you just need to connect the outboard motor and battery with the provided switch cable and communication cable. Also, the control system should be connected to the battery with communication cables.
Some small outboards come with an integrated battery, making it easy for the battery wiring.
For example, the 3 HP ePropulsion Spirit 1.0 Evo only requires you to place the battery in place (secured by two side slots), and then connect the power cable to the battery.
Outboard Motor Battery Charging
How to charge an outboard motor battery?
Generally speaking, an outboard motor battery needs to be charged with a specialized charger. And the charger decides the charging efficiency.
Fast chargers are available for some models, or you can also use two or more chargers in parallel connection to speed up the charging.
Does an outboard motor charge the battery itself?
Yes, most outboard motors can charge a battery directly while running just like your car’s motor can charge its battery:
The engines come with an alternator or generator. When you start the engine up, the engine will charge the starting outboard motor battery until full. If there is an aux battery, it will then switch over and charge the aux battery.
More About Outboard Motor Batteries
1. Does an outboard motor need a battery?
Yes, your outboard motor will need a battery to work unless it’s a pull start.
2. Can a lithium battery start an outboard motor?
Standard deep-cycle lithium batteries are not designed for starter uses.
However, lithium batteries for starting outboard motors are now available, which are specially designed to meet the strict specifications for marine cranking use.
In fact, some outboard manufacturers, such as Mercury Marine, encourage users to use qualified lithium batteries to start the engine for optimal performance.
3. Does an outboard motor charge the battery when running?
Yes, you can charge a battery from an outboard by burning fuel. For electric outboards, some models (including ePropulsion Navy Evo) can collect electricity from wind and water and charge themselves via a hydrogeneration function.
4. Can an outboard motor charge a lithium battery?
Unfortunately, most outboard motors can not charge a lithium battery directly. You will need a DC/DC charger to make it possible.
Other than that, you can invest in an electric outboard motor with hydrogeneration function which allows the motor to charge itself while sailing.
Hopefully, this post can help you clear your confusion about outboard motor batteries. If you have other concerns, please leave your question in the comment below and I will get back to you ASAP and add it to the list.