Battery Selection

The Battery System

The battery system consists of a set of batteries that provide the essential energy storage. Typically, this is a set of 6V or 12V lead acid batteries connected in series to form a 48, 72 or 96Vdc battery pack. This battery pack powers the electric propulsion (EP) motor and can also function as the energy storage for other on board (house) power needs. It can be charged by multiple sources, including shore power, regeneration under sail, solar panels, wind generators and/or an on board fossil fuel powered generator.

The right solution depends on your individual desires and needs.

Battery Sizing

Next to selecting the right propulsion motor, battery sizing is the most important factor in designing an EP system that will meet your needs. Let’s face it, energy storage density of current battery technology is the greatest factor limiting the adoption of EP. If not for this limitation, the reduced maintenance, compact size and quiet operation of an electric motor would render the ICE obsolete.

Because the storage density of batteries is substantially less than an equivalent weight/volume of fossil fuel, a compromise must be made. However, an adequately sized battery can supply the needs of most weekend sailors. Combine a small ICE powered generator with renewable sources and the needs of many cruising sailors can be met with greatly reduced dependence on fossil fuels.

A good starting point is to choose a size that will provide an hour of runtime at hull speed. Typically, this will provide 3-4 hours of runtime at 5 knots and many hours of runtime at reduced speeds or for motor sailing in light air.

This chart shows a typical example in calm seas. It is an estimated runtime for a typical 35ft 12000 pound sailboat with a 48Vdc x 300Ah AGM battery and QuieTorque™ 10.0 motor. Typically, the motor and batteries will be similar in weight and fit in the space formally occupied by the ICE, tankage, starting battery and other removed components.

This chart provides a general guideline for choosing the best battery bank size for your boat. Racers will typically want to use minimum sizes to save weight, often reducing weight compared to replaced ICE.

Others will want as much capacity as space and budget allow. Contact Electric Yacht, or one of our dealers for further assistance.

Battery Type

There are three batteries types often selected or electric propulsion. Following is a short description of these with advantages and disadvantages.

Flooded Deep Cycle Lead Acid

This type is the most cost effective. The most common arrangement would be a set of eight 6V T-105 “golf car” batteries. Cost is the main advantage. Disadvantages include the need for periodic watering, less efficient charging and discharging, release of hydrogen gas when charging and shorter life span when deeply discharged on a regular basis. This type is most appropriate when budget is tight and for boats that are not heavily used. We do not recommend flooded batteries for most of our clients.

Absorbed Glass Matt (AGM)

This type is the most commonly used for EP applications. AGM batteries will last longer than flooded batteries if regularly deeply discharged. They charge and discharge more efficiently than flooded batteries, don’t out gas (unless over charged) and don’t need watering. On the downside, quality AGM batteries cost more than quality flooded batteries.


Lithium-Iron-Phosphate (LiFeP04 or “Life-Po-Four”) are the Li-ion chemistry most appropriate for marine EP usage. This type is not as energy dense as some Li-ion chemistries, but is long lasting and not much safer than some of the other chemistries. These batteries offer five times the life of AGM batteries and provide twice the energy density of lead acid chemistries. They also charge and discharge very efficiently. The negative side is cost. Prices are coming down, but they are roughly five times the cost of quality AGM batteries. They make the most sense for in situations of daily usage or where maximum capacity are key.

Depth of Discharge (DOD)

A popular misconception is that batteries should not be discharged below 50%. There is a belief among some that doing so will greatly reduce the life of your batteries. In fact, the average depth of discharge (DOD) plays a major role in battery life. It is a very common practice to size batteries such that the AVERAGE DOD does not exceed 50%. We support this practice. However, do not be concerned with discharging more deeply when necessary.

As a guideline, properly maintained deep cycle flooded batteries will last roughly 1000 charge/discharge cycles to 50% DOD. This drops to 200-300 cycles to 80% DOD. Quality AGM batteries will also last 1000 cycles to 50% DOD, but can be expected to last 400-600 cycles to 80% DOD.

The most common cause of early failure of lead acid batteries is not deep discharge. Chronic under charging or leaving them in a deeply discharged state is much more likely to lead to failure than discharging beyond 50% DOD. This is due to a process known as “sulfation”, which is a build up of crystals on the lead plates that blocks the flow of electrons, reducing capacity. To some extent, there are ways to reverse this process. However, the best practice is to always bring the batteries to full charge as soon as possible after usage.

LiFePO4 batteries are typically rated at something like 5000 cycles to 50% DOD and 2000 cycles to 80% DOD. They do not suffer from “sulfation” and are not affected by being left in a partial state of charge.

As such, they are the best choice for high usage applications.

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