Boat Batteries

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Boat Batteries

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 a 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.

Battery Types

Trojan T105 deep cycle battery

Wet Cell (flooded) Lead Acid Batteries

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. 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.

Lead–acid battery sales account for 40–45% of the value from batteries sold worldwide.

Deep Cycle vs Starting Batteries

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" imply 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

AGM 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.

AGM batteries can be used for either deep cycle or starting purposes.

The main disadvantage of AGM batteries over flooded lead acid batteries is the additional cost.

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. 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 Battery -

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 batteries offer much better energy density than lead acid batteries, as well as significant weight advantages, 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:


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 -

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 -

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.


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.

e.g. in the diagram shown of 2 batteries wired in parallel, notice than 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 front most 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.

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.

An example of a correctly wired battery bank is attached here. This 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).


See Power Generation


See Battery Care


List links to discussion threads on partnering forums. (see link for requirements)


  • Miner Brotherton, Ed Sherman, The 12V Bible for Boats, International Marine, ISBN 0713667036


Also See

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