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Batteries are the heart of the electrical system: they start the engine and power all the boat electrical devices (lights, electric instruments, radio, fridge, etc.).

These two fields of application are markedly different: engine starting needs very high currents (up to 700-900 Ampere) for a short period while electrical devices require very low currents for several hours.

Accordingly engine starting batteries must be different from those used to power electrical devices and the difference lies mainly in their technology.

Batteries were first industrially manufactured in the second half of the last century and, since then, little significant progress has been made in order to improve their performances. Gel batteries and – in the last decade – sealed batteries and fixed-electrolyte batteries (AGM) have been developed and finally  – but still at an experimental stage – lithium batteries.

The full charge voltage of a unit cell is 2.13 V at 25°C. 12V. Batteries consist of 6 cells and so the total voltage is 12.78V for each battery. This value shows the recharge voltage of a battery in the so-called “good conditions”.

Without dwelling too much into technical details let us examine the specifications required by batteries to be installed onboard, first of all considering what is on offer on the market as for construction technology and then the types of batteries suitable either to start the engine or to power electrical devices.


Depending on construction technology the following types of batteries are available:

  • conventional batteries (i.e. with top-up caps to add distilled water)
  • sealed batteries (also called maintenance-free)
  • gel batteries (the electrolyte is fixed in a gel suspension)
  • AGM/V [absorbed glass mat batteries] or immobilized electrolyte batteries (the electrolyte is fixed in synthetic sponges).


AGM battery (on the left) and gel battery (on the right)


Traditional batteries with liquid electrolyte are not suitable for marine application due to the potential risk of acid leakage in case of high inclines and battery case damaging caused by serious impact. Moreover periodic maintenance is required for distilled water top-up.

Sealed batteries are conventional batteries with extra water up to about 10%. These batteries are hermetically sealed except from a small hole for possible gas flow. Electrolyte leakage is thus prevented in case of battery tipping over, however not if the case breaks. These batteries as well as conditional ones, must necessarily be installed on a flat surface. They are a good option thanks to their price/quality ratio.

Gel or immobilized electrolyte batteries are based on the same principle but the electrolyte is not in its liquid state but rather in a gelling state that makes it less dangerous in case of leakage. Gel or immobilized electrolyte batteries can work in any position (even upside down) and do not release/evolve poisonous gases during battery recharge. Moreover, as the water inside the battery does nor evaporate, they are maintenance free. They are the best compromise between cost, long lifespan and reliability

As for cyclic use batteries the most significant parameter to be taken into consideration is the number of uses (charge – discharge cycles) compared to the depth of discharge.


The diagram below describes better the importance of this parameter:

Drawing 3. Charge and discharge cycles



The label shows information on the battery type and capacity


The discharge percentage shows the capacity extracted from a battery – or battery pack – (in percent) before starting to recharge it. The three graph curves show the battery service life. To give a practical example, considering a 40% discharge (namely 120Ah out of the total 300Ah are no more available), the number of predictable cycles varies according to the type of battery: 300 in case of acid battery, about 500 in case of gel battery and about 1200 in case of immobilized electrolyte battery (AGM/V).

The above mentioned graph curves show a major advantage of cyclic use batteries as they can be discharged to zero voltage and then recharged and used again without any problems.

An important point in favour of cyclic use batteries is they can be recharged in a very short time as they accept recharge currents equal to or exceeding 50% of their nominal capacity. This is possible thanks to power alternators.

It is worth noting that the latest versions of cyclic use batteries can easily start the engine in case of need although their inrush current is on average 25-30% lower than that of a comparable start battery.

In order to preserve battery longevity it is essential that batteries are recharged using specific stage  battery chargers (bulk – absorption – float) having variable voltage as specified by the battery’s manufacturer.



Sizing your boat batteries to match your electrical requirement is one of the most common dilemmas, whose solution needs careful thought.

If you often dock in ports the standard sizing suggested by shipyards can meet your electrical requirements. Of coarse you will not have to waste energy as soon as you leave the marina or the batteries will soon run out of charge and even starting the engine will be a problem.

Standard sizing suggested by shipyards is often based on the boat minimum consumption requirements and on the battery bank lowest cost. Therefore it is good to check in advance if there is enough room to install extra batteries, possibly in the same area where standard onse have been located.

If what you need is a larger storage capacity to spend some nights in a bay or have long cruises, then your battery bank amperage risks being insufficient to supply the fridge, the autopilot system and all the other appliances.

The table below is a good point of reference for those who don’t like staying long in the marina. It is an example of battery sizing for a cruise in the Mediterranean sea provided that a charging equipment properly maintained and efficiently managed is available on your boat.

Table data are intended for 12V voltage and increase with increase of the boat length as, accordingly, more electrical appliances to be supplied require higher consumption.



[meters] [Ah]
8 100-200
10 300
12 400
14 600
16 800-1000
18 1400


Batteries long lifespan is guaranteed by their correct and constant recharging also by means of a solar panel e.g. during winter downtime.

One last piece of advice: never discharge your boat batteries below 20% of their capacity. If this happens, to charge them fully will take a long time unless a dedicated battery charger or alternator is provided. Furthermore excessive battery discharge may result in sulphation and consequent permanent damage, especially if your battery bank is undersized.

And so we have seen that the choice of the type and number of required batteries depend on several factors, i.e. if  batteries are used to start the engine or power the electrical devices, the boat dimensions, the frequency of navigation and so on. In my opinion gel or immobilized electrolyte batteries are the best compromise between cost and lifespan but it is you who must carefully consider your boat requirements and then choose the best way to power your boat electrical system.

Davide Zerbinati
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