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Battery options for boating

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What is the issue?
Once a vessel is cast off from the dock all its electrical requirements have to be self-generated and stored in batteries. But there are many different types of batteries with slightly varying properties and widely varying costs that can make this area somewhat confusing.

Why address this?
Electricity plays a pivotal role aboard a seagoing vessel, not only for our personal comforts but also running key sailing equipment such as the vessel's instruments, radar, self-steering, navigation lights etc. Indeed the vessel itself and its crew may be put in jeopardy if there is not enough power to start the engine. It is therefore essential that batteries are optimally selected and maintained.

This article aims to provide a good overview of the primary types of battery that may be deployed aboard a seagoing vessel, their benefits and trade-offs, and how they may be fed in.

How to address this?

An electric battery is a device consisting of one or more electrochemical cells that convert stored chemical energy into electrical energy. Each cell contains a positive terminal, or cathode, and a negative terminal, or anode. Electrolytes allow ions to move between the electrodes and terminals, which allows current to flow out of the battery to perform work.

Most boats are equipped with rechargeable batteries of some kind which can be discharged and recharged multiple times. Charging a battery typically involves running a current in reverse through the charge, i.e. applying a voltage across the battery to create a current stronger than the battery current, so that the current flows from - to + instead of from + to - inside the battery. Batteries come in many different types:

Wet Cell (flooded) Lead Acid Batteries

Varta 12 Volt 170 Ah sealed deep cycle battery
Photo: Michael Harpur
The lead-acid battery was invented in 1859 by French physicist Gaston Planté and is the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, its ability to supply high surge currents means that the cells have a relatively large power-to-weight ratio. These features, along with their low cost, makes them ideal to supply the high current required by starter motors.

The main advantage that they have is that they are inexpensive compared to newer technologies. Lead-acid batteries are widely used even when surge current is not important and other designs could provide higher energy densities. Lead–acid battery sales account for 40–45% of the value from batteries sold worldwide.

Modified versions of the standard cell may be used to improve storage times and reduce maintenance requirements. Gel-cells and absorbed glass-mat batteries are common in these roles, collectively known as VRLA (valve-regulated lead-acid) batteries.

Deep Cycle vs Starting Batteries

A typical starting battery
Photo: Public Domain
A deep-cycle battery is a lead-acid battery designed to be regularly deeply discharged using most of its capacity. In contrast, starter batteries are designed to deliver short, high-current bursts for cranking the engine, thus frequently discharging only a small part of their capacity. While a deep-cycle battery can be used as a starting battery, the lower "cranking amps" implies that an oversized battery may be required.

A deep-cycle battery is designed to discharge between 45% and 75% of its capacity, depending on the manufacturer and the construction of the battery. Although these batteries can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 45% discharge.

Deep cycle batteries are manufactured with thicker plates that hold a higher charge for longer. The downside of the thicker plates is that they do not easily release a large current. Starter batteries, on the other hand, are manufactured with thinner plates that are capable of delivering a higher current, but in turn, do not hold as high a charge as deep cycle batteries and are more easily damaged by deep discharge.

Absorbed Glass Mat Batteries

CENTURY 12V 120Ah AGM Battery
Photo: Public Domain
AGM batteries, also known as A valve-regulated lead-acid battery (VRLA battery) sometimes called sealed lead-acid (SLA), gel cell, or maintenance free batteries, differ from flooded lead acid batteries in that the electrolyte is held in the glass mats, as opposed to freely flooding the plates. Very thin glass fibres are woven into a mat to increase surface area enough to hold sufficient electrolyte on the cells for their lifetime. The fibres that compose the fine glass mat do not absorb nor are they affected by the acidic electrolyte.

Gel Cell Batteries

A modern gel battery (also known as a "gel cell") is a VRLA battery with a gelified electrolyte; the sulphuric acid is mixed with fumed silica, which makes the resulting mass gel-like and immobile.

VRLA battery
Photo: Public Domain
Unlike a flooded wet-cell lead-acid battery, these batteries do not need to be kept upright. Gel batteries reduce the electrolyte evaporation, spillage (and subsequent corrosion problems) common to the wet-cell battery, and boast greater resistance to shock, and vibration, making them attractive in marine environments. Chemically they are almost the same as wet (non-sealed) batteries except that the antimony in the lead plates is replaced by calcium, and gas recombination can take place. Gel cell batteries are typically only used in deep cycle applications.

Lithium Batteries

Lithium-ion batteries are common in consumer electronics. They are one of the most popular types of rechargeable batteries for portable electronics, with a high energy density, no memory effect, and only a slow loss of charge when not in use. Beyond consumer electronics, Lithium batteries are also growing in popularity for marine applications.

Lithium Battery
Photo: Public Domain
Lithium batteries offer much better energy density than lead-acid batteries, as well as significant weight advantages, typically ¼ of the weight, however at a much higher cost.

Newer boats fitted with electric motors in place of the traditional diesel-powered inboard are increasingly moving to Lithium battery technologies. Catamaran owners are often attracted to Lithium batteries due to their much lower weight for the same charge. There are an increasing number of types of Lithium battery, for example:

Comparison with flooded lead–acid cells

VRLA Gel and AGM batteries offer several advantages compared with conventional standard lead-acid batteries. The battery can be mounted in any position since the valves only operate on over-pressure faults. Since the battery system is designed to be recombinant and eliminate the emission of gases on overcharge, room ventilation requirements are reduced, and no acid fume is emitted during normal operation. Flooded cell gas emissions are of little consequence in all but the smallest confined areas, and pose very little threat to a domestic user, so a wet cell battery designed for longevity gives lower costs per kWh. In a gel battery, the volume of free electrolyte that could be released on damage to the case or venting is very small. There is no need (or ability) to check the level of electrolyte or to top up water lost due to electrolysis, reducing inspection and maintenance requirements.

Wet cell batteries can be maintained by a self-watering system or by topping up every three months. The requirement to add distilled water is normally caused by overcharging. A well-regulated system should not require top-up more often than every three months.

The underlying problem with all lead-acid based technology batteries is the requirement for an excessively long charge time arising from a two-stage process: 'bulk charge' and 'float charge'. All lead-acid batteries, irrespective of type, are quick to charge to 70% of capacity within 2 or 3 hours, but require another 9 to 10 hours to "float charge" after the initial charge. If users fail to float charge, battery capacity is dramatically reduced.

Because of calcium added to its plates to reduce water loss, a sealed AGM or gel battery recharges more quickly than a flooded lead-acid battery. From a standard car, 4WD or truck alternator they will recharge quickly from full use in about 2 to 3 hours. A deep cycle wet cell battery can take 8-12 hours to achieve only 70% to 80% of its potential charge.

Likewise, with Lithium, it is generally accepted that the most economical and practical depth of discharge (DOD) for an AGM battery is 50%. For Lithium-iron-phosphate (LiFePO4 or LFP) which is the safest of the mainstream Li-ion battery types, it is 80% DOD. So, although the Ah rating may appear low, users may draw upon more of the Ah available in the battery.

The main disadvantage of AGM batteries over flooded lead-acid batteries is the additional cost. AGM automobile batteries are typically about twice the price of flooded-cell batteries in a given BCI size group; gel batteries as much as five times the price. Li-ion battery types will be vastly more expensive again.

Amp-Hour Rating

Battery ratings can help you determine the capacity you're buying but the problem is that the rating system is not completely standardised. The Amp hour rating, written as Ah, will tell you how much amperage is available when discharged evenly over a 20-hour period. For example, a battery with 100 AH will deliver 5 amps per hour for 20 hours. Twenty hours has been the standard time length for rating batteries, although shorter or longer time variables may be used depending on the application.

Unfortunately, it can only be used as a guide as there is a complicated formula to accurately estimate this and it is impossible to find consistency among manufacturers. Likewise, it’s not necessary that a 100 Ah will fully deliver 100 amps, so always choose the battery that has more amp hour rating than the actual amount you required.

Group or Size

Marine batteries are sold by physical case size represented by the group number. They are determined in groups; for example, U1, 22, 24, 27, 31, 30 H, 4D, 6D, 8D, etc with smaller groups indicating a generally larger battery, whereas larger group indicating a generally smaller battery. The groups are managed by the Battery Council International which regulates the distribution of lead-acid batteries and so determines their group number categories. If you are replacing a battery in an established battery box or compartment, it is important to make certain the battery fits in the box.


Marine batteries can last from between 1 - 6 years but it varies considerably depending upon how they are used, how they are cared for and the storage strategy adopted. The longest life of a standard lead acid battery, when working, is about the 48-month mark and it is achieved by keeping the depth of discharge of less than 20%. Marine deep cycle batteries may be discharged deeper in accordance to their individual specification but keeping them within the first of 20% discharge is always optimal.

Any cycle that has a discharge of over 80%, i.e. deeper than 20% or first ⅕, of the rated capacity is classed as a deep discharge. Each of these deep discharge cycles reduces the battery's lifespan and the deeper the discharge cycle the deeper the ageing of the battery and the fewer the number of cycles the battery can supply. However, keeping inside the top 20% of the bank capacity is difficult when disconnected from the grid in the seagoing environment. So there is a trade-off between deeper discharge and long-term battery durability.

Add to this cost, capacity, size, weight and maintenance and it is most likely the correct battery choice for your use case will be a function of all of these elements in a varied combination. To properly apprehend this please see eOceanic's care and maintenance of batteries.


Battery installation depends on a number of factors:

  • • The battery voltage. Usually batteries are available in 6V, 12V or 24V potentials but some battery types are made as single cells (usually 2V).

  • • The desired system voltage, e.g. 12V or 24V.

12V v 24V is a decision that needs to be made early. The main advantage of 24V systems is that half the current needs to be carried to deliver the same power as in a 12V system. For example, to power a 1200W anchor windlass at 12V requires 100A but it only requires 50A of current at 24V. The downside is that most marine electrical components are only easily available in 12V versions, with 24V versions sometimes being available at an extra cost.

It's also possible to have a hybrid system - 12V for the regular components and 24V for some components.

Series and Parallel Connections

Series Connected Batteries
Photo: batterystuff.com
Series connections means that you are connecting the + terminal of one battery to the - terminal of the next battery. This is done to deliver a higher voltage, e.g. to deliver 24V from 2 x 12V batteries or to deliver 12V from 6 x 2V batteries.

Parallel Connected Batteries
Photo: batterystuff.com
Parallel connected batteries means that you are connecting the + terminals of one or more batteries together, and also connecting the - terminals of those same batteries together. This provides the same voltage, but at a higher capacity. For example, 2 x 12V 100Ah batteries connected together in parallel would still deliver 12V but would have a combined capacity of 200Ah.

It's possible to cable a set of batteries in both a series and a parallel configuration. In order to visualise this:
  • • First consider each battery as a separate unit, with a specific voltage that is less than the voltage you want to deliver. e.g. you may have multiple 6V batteries but wish to deliver 12V.

  • • Connect a set of batteries together in series, to form a single battery bank delivering the correct voltage. In this example, connect 2 x 6V batteries together in series to create 1 x 12V battery bank.

  • • Continue to connect all of your batteries in series banks until you have a number of banks delivering the correct battery voltage. e.g. if you have 6 x 6V batteries, then wire them together as 3 pairs of 2 batteries in series, where each pair is a battery bank delivering 12V.

  • • Now connect each battery bank together in parallel. So you would connect each battery bank's + terminal together and each battery bank's - terminal together. This can either be done by wiring the batteries together terminal to terminal, or (less frequently) by wiring each battery bank to a pair of large capacity bus bars.

  • • Do NOT make "diagonal" or "partial" interconnects between the battery banks. In the example above, each battery bank (pair of 6V batteries) can be considered a single 12V battery bank. Do not make any connections between the pairs of +/- terminals which are connected together -- only make connections between the outer + terminals of each battery bank and also connections between the outer - terminals of each battery bank.

In theory, you can connect as many batteries together as you want. But when you start to construct a tangled mess of batteries and cables, it can be very confusing, and confusion can be dangerous. Keep in mind the requirements for your application, and stick to them. Also, use batteries of the same capabilities. Avoid mixing and matching battery sizes wherever possible

Always remember to be safe, and keep track of your connections. If it helps, make a diagram of your battery banks before attempting to construct them.

Battery Connections

Battery cables provide the link between the batteries, equipment and charging system. Faulty connections can lead to poor performance and terminal damage, meltdown or fire. To ensure proper connections, battery cables should be sized to handle the expected load, torque values should be used for tightening connectors and terminal protectors should be fitted.

The Wire Gauge Size table provides an indication of the maximum current carrying capacity (amps) based on the cable/wire gauge size for cable sizing assuming lengths of more than 1.8 metres. In series/parallel battery banks, it is preferable for all series cables to be the same length and all parallel cables to be the same length.

It is important that all cable connections are tightened to the proper specification to make sure there is good contact with the terminals. Over-tightening the connection to the terminal can result in terminal breakage and loose connections which can result in meltdown or fire.
Please note

Use a wrench with a rubber handle when making battery connections.

Lead Off and Feed In

With banks of lead-acid batteries connected in parallel, it's frequently considered a good idea to take the main feed off from different batteries in the bank. For instance, in the diagram presenting two batteries wired in parallel, notice that the + (red) lead comes from one battery and the - (black) lead comes from the other battery. Although it would be possible to run both the - and + leads from one of the two batteries in the bank, it's better not to do this because in high current situations it may be the case that the frontmost battery in the bank is the only one used to supply the power and the rear battery in the bank does not get used very much or at all (although in theory, this should never happen, resistance in the interconnecting leads or terminals, or perhaps a small amount of terminal corrosion, can sometimes cause this problem). Wiring the lead off from opposite batteries in the bank circumvents this issue.

An example of a correctly wired battery
Photo: David Elson
Feed in, for example charging connections from a shore power battery charger, should be run into the diagonally opposite terminals. So, in the example pictured, the + and - terminals that do not have the feed off coming from them should be the terminals that are used to connect the battery charger.

This example of a correctly wired battery shows a bank of 4 x 6V batteries wired with 2 banks of 2 batteries each in series and with each bank of 2 batteries wired in parallel. The black lines show the - cabling, the red lines show the +12V cabling, and the blue lines show the interconnects within each battery bank (these are at 6V).

Usefull further articles

See eOceanic's electrical power generation on a sailing yacht for charging. Likewise, there is an old saying that 'batteries don’t die, they are killed'. Regardless of the technology adopted, good management practices provide the best insurance against early failure, so eOceanic's care and maintenance of batteries will provide a useful read. Below are an additional set of useful external research links to depart upon further reserch:

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation with author restrictions as may be seen on CruisersWiki.

With thanks to:
Delatbabel at the World Cruising Wiki

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What are battery amp hours

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