[Complete] Guide to Batteries For Electric Outboard Motors

In the new(ish) world of electric boats, batteries are an expensive necessity. Ensuring that you choose the most appropriate battery for your motor is essential.

The first decision you must make is the battery type that suits your budget and circumstances. Lithium-ion is the winner in almost every metric, except that all-important one of cost.

Once you have decided on the battery technology, you will have to select one that will allow you to spend as much time on the water as possible. For this, you need to understand the term amp/hours.

Finally, once you have installed the new unit on your pride and joy, you must ensure that it is maintained properly. It can make a substantial difference to the battery’s life expectancy and ensure that you get the best return possible on your hard-earned dollars.

Getting the battery selection right is a pivotal part, and I’ll cover it in detail here as part of our buyer’s guide to electric outboard motors.

Key Takeaways

  • A lithium-ion battery is most suited to power an electric boat motor if the budget is sufficient.
  • The range available from an electric boat battery is directly related to the throttle settings.
  • Actively maintain the battery, and it will last longer.

🔋 Types Of Batteries For Electric Outboard Motors

Battery technology is the biggest potential roadblock to the mass rollout of solar systems and electric transportation.

The difficulties with batteries are due to their weight; they require a significant portion of an electric motor’s energy for transportation.

Compared to their gas-powered counterparts, electric boats possess a more limited range. And it often takes an extended period to fully recharge batteries.

The lifespan of batteries tends to be relatively brief, and acquiring batteries can be quite costly.

Because of this, substantial efforts are being expended by technology companies worldwide to continually develop battery technology.

The devices that may be suited to electric mobility (cars, aircraft, and boats) are listed below.

Battery TechnologySuited For Electric Boats
Lead Acid Batteries✅ Yes
Absorbent Glass Matt (AGM)✅ Yes
Gel Cell Batteries Hydrogen batteries✅ Yes
Lithium-Ion Battery✅ Yes
Nickel-Cadmium (NICAD)❌ (short life expectancy)
Red-ox flow batteries❌ (less efficient than lithium)
Sodium Batteries (Saltwater)❌ (not yet commercially viable)

Given this, our discussion will be based on:

  • Lead Acid Batteries
  • Absorbent Glass Matt (AGM)
  • Gel Cell Batteries Hydrogen batteries
  • Lithium-Ion Battery

Lead Acid Batteries

A lead acid battery consists of alternating lead and oxide sheets placed, in sequence, in a pool of sulfuric acid and water (electrolytic solution) within the battery casing.

When the battery is being discharged (powering an electrical device), the current moves from the lead sheet – anode (-)  to the oxide sheet-  cathode (+). 

The acid makes the lead sheet give up units of electricity (electrons), which are accepted by the lead oxide sheets. 

A 12 v battery consists of 6 cells (made up of a lead sheet and an oxide sheet). A cell produces 2 volts, and when connected in series, the battery produces 12 volts.

When the batteries power an electrical component and so discharge, lead sulfate forms, and cakes onto each sheet. When the battery is recharged, this process is reversed,

If the battery is discharged below its recommended depth of discharge level, the lead sulfate layer becomes very thick and finally won’t accept being able to recharge.

This is one of the weaknesses of a lead acid battery, which causes the potential life expectancy to be very short.

Absorbent Glass Matt (AGM)

AGM batteries use the same chemical process as lead-acid batteries, but they have an absorbent glass mat installed between the positive and negative matts.

This reduces the movement of acid within the battery, making it safer and preventing acid spills. These batteries are advertised as maintenance-free, and there is no way to top up acid levels.

Gel Cell Batteries Hydrogen Batteries (Gel Cell)

Another name for Gel Cell batteries is “valve-regulated lead acid batteries (VRLA).” 

Gel Cells differ from lead acid batteries in the following respects.

The sulphuric acid is converted into a gel by mixing it with silica (sand), which prevents battery acid spills.

The design of the internal structure differs, and the battery gasses are recombined within the battery and not vented outside. This makes the battery more efficient and also more eco-friendly.

The depth of discharge of a gel battery is 50%.

Lithium-Ion Battery

Lithium-ion batteries are the most efficient and have the longest potential lifespan of current commercially available battery technologies.

A lithium-ion battery contains the following components:

  • Anode (-)
  • Cathode (+)
  • Plastic separator
  • Electrolyte
  • 2 current collectors (positive and negative)

The lithium is stored by the anode and cathode. 

The liquid electrolyte moves lithium ions (positively charged)  through the plastic separator between the anode to the cathode.

In the process of moving, free electrons are created in the anode. In turn, this creates a current at the (-) current collector.

The current flows from the negative collector through the powered electrical device (cell phone, computer, etc.) to the positive current collector. 

🎭 Advantages And Disadvantages Of Each Type Of Battery

The different battery types have advantages and disadvantages, as described below.

Lead-Acid Batteries

They are commonly used in internal combustion engines to provide the initial surge of power to turn the engine’s starter motor over. After that, the engine’s alternator keeps it charged.

Conventional lead acid batteries are not used to power electric motors because they are not designed as “deep cycle” units. This means that they are not designed to be consistently fully discharged and then repeatedly charged many times.

Lead acid batteries with deep cycle capabilities have the following modifications incorporated:

  • A larger battery case is used, and the cells are spaced further apart.
  • Thicker electrodes are used.
  • They are installed higher in the case. They are lifted above the battery’s base to allow waste sulfate to drop.
Feature/AspectLead Acid Batteries
CostConsiderably cheaper than Lithium-ion batteries
Life ExpectancyShorter; lasts approximately 1/5th of the time as lithium-ion units
Weight and DesignHeavy; redesign to increase charge cycles makes them big and ungainly
Weight per kWhDeep-cycle batteries: ~70 lbs. (32 kg) per kWh (high for small boat usage)
Lifespan ComparisonLasts only 1/6th the time of an equivalent lithium-ion battery

Lithium-Ion Batteries

Lithium-ion batteries are the current state-of-the-art devices used commercially.

Although new and exciting battery technologies are on the horizon, these are not commercially available and are still in development.

Compared to other commercially available battery technologies, lithium offers the following benefits.

Feature/AspectLithium-ion Batteries
Depth Of DischargeCan be discharged to 15% of their capacity; provides twice the range between charges as a lead acid battery
LifespanCan be discharged/recharged 3,000 times before damage; compared to 500 deep cycles for lead acid batteries
Recharge Time1 – 2 hours; compared to 8 hours for an equivalently sized lead acid battery
Weight per kWhOnly 13 lbs. (6 kg) per kWh
CostExpected to cost 10 times the price of an equivalently sized lead-acid battery

Before you write Lithium-ion batteries off, you should consider the following because lithium boat batteries are worth it.

☑️ Lithium batteries last at least six times longer than lead-acid batteries.
☑️ Lithium-ion batteries generally have higher power ratings than lead acid batteries, meaning fewer are required.
☑️ Lithium-ion batteries charge 6 – 8 times faster, meaning you can get back on the water faster.
☑️ Lithium-ion batteries require less maintenance.

🎯 Factors To Consider When Choosing A Battery For An Electric Outboard Motor

When selecting an electric outboard motor and the most suitable battery, the following must be considered.

Boat Size And Weight

It goes without saying that the larger the boat, the more powerful an engine that is required. The rule many boaters use is that the motor should have 1hp of thrust for every 25-40 pounds of laden weight in the boat.

This means that a boat that weighs 2,000 pounds will need an engine capable of 50hp (39kW.)

This high electrical draw engine will need substantial batteries to cope.

The Elco EP-50 Electric Outboard, which is one of the more powerful electric outboards, uses 96 Volts, and the suggested battery is 275 amp hours DC.

Duration Of Use

The expected range provided by the battery will be determined by the size of the electric motor and the speed at which it is being run.

If the battery is rated at 200-amp hours, it can produce 200 amps for one hour.

If the motor consumes 300 amps at top speed, it will have the following endurance.

Throttle settingEndurance
Top speed40 minutes
Half speed1 hour and 20 minutes
10% speed6 hours 49 minutes

Available Charging Options

Depending on the available space in the boat, there may be an option to use solar power or a solar generator to charge the battery.

If not, the battery will have to be charged from shore power. 

🔧 Maintenance And Care Of Electric Outboard Motor Batteries

How the battery is stored will significantly affect how long it lasts. The battery should always be maintained and stored as follows:

  • A dry area that has an even temperature (not too hot or cold).
  • Placed on a constant trickle charge.
  • The battery is kept as clean as possible, ensuring that there is no corrosion on the terminals.
  • Ensure there are no leaks.
  • If it is a lead acid battery, ensure sufficient water in the cells.

Frequently Asked

Do You Need A Special Charger For Lithium Boat Batteries? 

A standard battery charger will successfully charge a lithium boat battery. The ideal charger matches the charging profile recommended by the manufacturer; however, if this is not available, a standard charger will suffice.

Dual Pro 10 Amp/Bank Sportsman Series 3 Bank Charger

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The fastest charge rate for lithium-ion batteries is .5C, while a lead-acid battery is .1C. 

The “C” rate is the maximum charge factor. This means a 100Ah lithium battery should charge at 50A maximum = .5(C) x 100(Ah) = 50A.

Similarly, a 100Ah lead-acid would charge at 10A max = .1(C) X 100(Ah) = 10A. On this basis, the following recharge times would apply.

Lithium-ion Battery2 hours to charge (🏆)
Lead Acid Battery10 hours to charge

Is A Deep Cycle Battery Necessary For My Outboard Motor? 

A conventional battery in a motor car is only expected to produce a short burst of current (starter motor) infrequently; once the engine is started, the battery is immediately recharged.

Electric boat motors only consume current and cannot replace used current.

A deep-cycle battery will provide the best performance and last the longest. If a deep cycle battery is not chosen and is regularly discharged to its maximum discharge level, its life expectancy will be severely compromised.

Nice to e-meet you. I’m Justin, a seasoned sailing journalist and communications pro with more than 25 years of extensive industry experience. And a track record of successfully promoting teams and events on the global stage.