How to Choose a Battery for an Electric Outboard or an Electric Boat

Marine Battery

All batteries are not suitable for use with an electric outboard or electric boat. The risk in using a wrong type of battery is that they wear out prematurely and that your electric trolling motor is not performing well. The more powerful the electric outboard needs of the more quality and suitable batteries.As part of the use an electric outboard, generally, the quality of a lead battery is related to its weight.

Battery for an Electric Outboard

How a lead battery is made up:

  • A 12V lead-acid battery consists of 6x “batteries” of 2.1V each connected in series.
  • The “accumulators” are composed of positive and negative electrodes (plates) isolated by a separator to avoid short circuits while allowing the ions to circulate.
  • The “Planes” plates are either thin (2.5 mm) or thick (5 mm) or tubular.
  • The electrodes are formed of a lead alloy grid, the cells of which are filled with a porous paste:
  • The electrodes are immersed in the electrolyte (water and sulfuric acid solution) either in liquid form (open lead) or in the form of a gel (GEL) or absorbed in fiberglass felts (AGM).
  • A well-charged battery has a voltage higher than 12.6 V (6x 2.1V)
  • A discharged or in poor condition battery has a voltage lower than 10.8 V (6x 1.8V)

The various “lead” battery technologies

Liquid lead batteries:

Liquid electrolyte batteries are the first batteries. This technology is typically used in starter batteries. There are liquid electrolyte traction batteries which require maintenance (liquid addition). Before the arrival of the GEL and AGM batteries, they were used in electric boats. For various reasons, they are less resistant to the high current requirements of Torpedo Cruise engines. Moreover, they can work only flat.

GEL batteries:

GEL batteries

  • Battery GEL 140 Ah in C20 (51 kg)
  • The electrolyte is gelled with silica which avoids problems of flow of the electrolyte, which is more suitable for boats. The cycle number is better than for a liquid battery (or open lead). Perfect for use with an electric outboard. Depending on the quality of the battery, you can use 80% of its capacity. The cycle number is approximately 400 cycles (depending on the brand)
  • Be careful to choose the suitable charger.

The AGM (Absorbed Glass Mat) Batteries:

  • AGM batteries are relatively new. They use fiberglass separators between the plates which make it possible to compress the plates in them. The electrolyte is absorbed and therefore immobilized in fiberglass blotters placed between the electrodes. Thanks to this system AGM battery plates can withstand shocks and vibrations better than conventional battery packs.
  • Battery Rolls AGM 12V – 145 Ah in C20 (44 kg)
  • AGM Batteries have all the advantages of gelled electrolyte batteries by being more resistant.
  • The self-discharge rate of an AGM battery is very low. It is usually between 1 and 3% per month. So you can store them longer without having to recharge them.
  • AGM Batteries can work in any direction, as no risk of liquid flow.
  • You can use 80% of its capacity. The cycle number is about 500 cycles (depending on the brand).
  • AGM batteries offer many advantages over gelled electrolyte batteries and open lead batteries without being expensive.

Pure lead battery:

The unique technology of fine lead plates. The electrolyte is absorbed in a high-quality micro porous fiberglass separator providing high absorption and stability, designed to improve cycling performance.

These batteries are recently used in the world of electric boats by the rental companies. In the form of a suitcase, they are easily lodged in a boat. They can work in any direction.

Some advantages:

  • Very short recharge time (less than 3 hours at 60% depth of discharge, with approved charger)
  • Totally maintenance-free, no water supply
  • Reduction of CO2 emissions for recharging thanks to the very low load factor
  • Space saving: an XFC FLEX battery usually occupies 30% less space compared to its equivalent in calcium lead = more power and less space.
  • Excellent cycling life (up to 1200 cycles at 60% discharge depth)
  • Minimal gaseous release: ideal for use in shops, public areas and sensitive manufacturing sites
  • Model used in electric boats: 12XFC177

How to choose the right battery and why?

Starting batteries:

Used in the world of automobiles or to start the thermal engines of boats, they are composed of thin plates (2.5mm). They allow high discharge currents, but for a short time.

Start-up batteries should not be used to supply electric outboard or electric boats. They are not adapted to this type of use because their plates are thin and will rapidly deform and irreversibly. Their discharge must not fall below 50%, see 20%.

By using this type of battery, your engine will be less and less performing in speed and autonomy.

Service batteries:

These batteries are designed for a deep discharge of about 80% of their capacity and with a large number of charge and discharge (up to 1200 times) but not with high intensity as a powerful electric motor can demand.

There are also mixed start / service batteries. They allow a discharge around 50% of their capacity with a larger cycle number than a starter battery. It is an intermediary but not really adapted to the electric motorization for a boat, especially for high powers.

Traction Batteries:

This is the battery for the electric motorization of a boat.

They are capable of providing high intensity for a long time.

Try to choose them either AGM technology or GEL, because more resistant.

Marine Battery:

Marine Battery

It is not necessarily traction, and may be starting.

It must be designed to accept all the constraints of a boat. That is to say sealed, accept the deposit so have a sufficient electrolyte level so that the electrodes are always immersed and be designed to limit the gaseous releases to the recharge.

Some landmarks for determining the capacity of a battery depending on the use you want:

Know the consumption of its electric outboard at full throttle.

Determine a speed for a time (3 hours at 10 km / h) and determine the electric outboard power consumption at this speed.

To know the autonomy of a boat a quick calculation can be carried out. To divide the capacity in A / h (C20) of the battery by the consumption of the motor (given in Amperes), one then obtains the number of hours of autonomy (without reserve) at maximum speed.

The value supplied in C20 (usually given by the manufacturer) or C5 is the discharge capacity provided by a battery for use over a period of 20 hours (C20) or 5 hours (C5). The C5 value is preferable but rarely given by the manufacturers.


The choice of the battery is decisive in the performance of an electric outboard or an electric boat.

Prefer the AGM or GEL traction batteries to the starter batteries. The choice of a charger is also important in the life of the battery. And do not completely discharge (100%) a battery and never let it discharge. To learn details about this topic you can visit this helpful site.

Some recommendations:

  • Interconnect only identical batteries (manufacturer, capacity, and age).
  • Interconnect only batteries with the same charging status.
  • Remove metal jewelry before working on a battery or near a battery.
  • Always remove tools and metal objects without touching the battery.
  • When connecting the battery, make sure that the correct polarities and connections are in place.
  • The battery terminals must be clean and free from corrosion.
  • Do not store batteries in a box or drawer, p. ex. In a badly ventilated trunk, which would be dangerous.
  • Damage to the battery or other consumers due to a short circuit! This can cause property damage.
  • When disconnecting the batteries, first remove the black negative cable and then the red positive cable.
  • When connecting the batteries, first connect the positive red cable and then the black negative cable.
  • If there is a problem with the battery, switch off the system with the main switch.
  • As soon as a battery fails, it is recommended to replace the other batteries as well.
  • Never reverse polarity.

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