On any vessel there will be two types of batteries and their cells, primary and secondary.

Type of cell

Area of use


Primary cell EPIRB, SART, ... No
Secondary cell Main battery (for VHF radio, NAVTEX receiver, ...), handheld VHF, ...

Type of cells

Primary cells

A primary cell is not rechargeable. It is filled with a variety of chemicals whose reaction is not designed to be reversible so this means that when the chemical reaction is exhausted the battery is dead.

Primary cells

Primary cells

Primary cells:

  • can be connected in series to achieve a particular voltage,
  • should never be connected in parallel because there is a possibility of one cell trying to recharge another.

There are five common types of primary cells used in batteries that may be found on board.

Carbon/zinc batteries

They have a nominal voltage of 1.5 V per cell. Their advantage is that they are cheap but they tend to lose about 15% of their capacity per year in storage. They must never be left inside equipment when they are exhausted, as they are liable to leak very corrosive chemicals which can cause expensive damage to the equipment. If the battery is getting low, then by turning the equipment off for a while and letting the battery rest the useful life can be prolonged. Continuous discharge reduces the capacity of the battery.

Alkaline manganese batteries

These are the premium "longer life" batteries such as "Duracell". They have a nominal voltage of 1.5 V per cell. They are considerably more expensive than carbon/zinc batteries, but they have about 3 times the capacity and normally lose only about 7% of their capacity per year in storage. If the battery is getting low, then by turning the equipment off for a while and letting the battery rest the useful life can be prolonged. Continuous discharge reduces the capacity of the battery.

Mercury batteries

Mercury cells have a nominal voltage of about 1.4 V per cell. They are more expensive, but they have as much as 6 to 8 times the capacity of carbon/zinc batteries and they lose only about 6% of their capacity per year in storage. Because of the environmental problems associated with the disposal of mercury batteries, they are used less frequently now.

Silver oxide batteries

These are familiar as the little round silver coloured batteries found in watches, calculators and as back-up batteries for memory circuits in some equipment. The nominal voltage is around 1.5 V per cell. They have a capacity similar to the alkaline manganese batteries, at considerably greater cost, but their big advantage is that they only lose about 4% of their capacity per year in storage.

Lithium manganese dioxide batteries

Li-MnO2 are the most modern, high power batteries. Their nominal voltage is 3 V. Their capacity is approaching that of the mercury batteries, but best of all, they generally lose less than 2% of their capacity per year. They are ideally suited as back-up batteries inside some equipment and in EPIRB and SART equipment because of their long service life.

Secondary cells

These are rechargeable and are referred to as storage batteries. These are used aboard to power onboard electrical equipment such as the VHF radio and are recharged from either the vessel’s engine, generator or through a battery charger connected to mains power. Charging the battery reverses the chemical process inside the battery so the battery can once again supply electricity.

Secondary cells

Secondary cells

Secondary cells can be used in series, in parallel, or in a combination of both to achieve the voltage and the capacity that is required. The only limitation being that each cell is of a similar voltage, capacity and chemical composition.

There are four common types of secondary cells used in batteries that may be found on board.

Lead/acid batteries

This is the most common type of large rechargeable battery. This is the same as the ubiquitous car battery. Each battery is made from a number of individual cells, each having a nominal voltage of 2 V. Most batteries are made from 3 or 6 cells giving a battery voltage of 6 or 12 V. These batteries are then grouped together to make a bank of the required voltage and capacity. Most vessels use 12 or 24 V for their battery bank.

Lead/acid cells consist of a series of lead plates immersed in a liquid called the electrolyte. The electrolyte in these batteries is sulphuric acid.

Lead/acid batteries are popular because they are cheap and can supply high current when needed, for example for starting an engine.

Lead/acid batteries may be found in two versions: unsealed and sealed.

Unsealed lead/acid batteries offer the access to each of their cells through batteries caps that enables accurately determining the state of charge of each cell.

Unselaed battery

Unselaed battery

This can be done by measuring the specific gravity of the electrolyte with a hydrometer, because the more charge that there is in the battery, the denser the electrolyte becomes.



A hydrometer consists of a glass tube containing a float. At one end of the tube there is a rubber bulb which is used to draw a sample of the electrolyte into the tube. The float inside the tube indicates the specific gravity of the electrolyte according to how deeply or otherwise it floats in the liquid. The less dense the liquid, the deeper immersed is the float. The readings for the specific gravity of the electrolyte can be read directly off the stem of the float. The electrolyte of a fully charged lead/acid battery will have a specific gravity of about 1.27, and a fully discharged cell will give a reading about 1.16, depending on the temperature of the electrolyte. Usually,the float is also colour coded to help the user determine the state of charge of the cell.

Hydrometer in use

Hydrometer in use

The specific gravity measurement should be repeated for each cell and if one cell is showing much lower specific gravity than others, then it is an indication that particular cell is no longer taking a full charge and it could suggest that the battery is coming to the end of its useful life.

When the specific gravity falls uder 1,22 the cell is 75% charged and must be recharged.

Lead/acid batteries uses water from the electrolyte when being charged as part of the chemical reaction. Distilled water should be added to the electrolyte. The recommended level of the electrolyte is usually marked inside the battery in some way. If not, then the electrolyte should be kept at such a level that the tops of the lead plates are never exposed but not so full that the electrolyte overflows when the battery is being charged (usually 5 mm).

Sealed lead/acid batteries have closed cells and they shouldn't be open by force because they are filled under pressure that causes that water from the electrolyte is not being used when the battery is beaing charged. For this reason they are known as maintenance free batteries.

Sealed battery

Sealed battery

The only way to determine the state of charge of sealed batteries is to measure its voltage. Fully charged it should read 12,6 V.

When the voltage falls uder 12,4 V the battery is 75% charged and must be recharged.

The measurment could be done by accurate digital voltmeter (analog ones are not accurate enough). Digital voltmeter should be adjusted to 20 to 40 V SC scale and meter leads should be placed to the battery terminals.

Battery measurement with voltmeter

Battery measurement with voltmeter

This way could be measured also unsealed batteries, but this is not recommended because they should be measured with hydrometer; each cell separately.

Gel batteries

These are the modern version of the lead/acid battery. As the name suggests, the electrolyte is in the form of a gel rather than a liquid. This has the great advantage that the electrolyte cannot be spilled. Another advantage is that they do not give off hydrogen when being charged, so the possibility of an explosion is reduced and water does not need to be added. Gel batteries can tolerate being completely discharged which lead/acid batteries cannot, and they can usually accept a charge at a higher rate than a lead/acid battery without suffering any harm. Against all these benefits there are a couple of negatives. The first is the cost. They are at least twice the price of the equivalent lead/acid battery but generally they have a longer service life. Gel batteries do not like supplying large currents such as for starting an engine, but this is not generally a concern for batteries powering GMDSS equipment which needs relatively small current for a long time. The only other negative point is that the state of the battery can be monitored only with measuring its voltage and this stays relatively constant until the battery is almost flat, so it doesn't exist any indication of the true state of charge of the battery. The solution is to adopt a regular pattern of charging to keep the battery well-charged.

Nickel cadmium / Nickel metal hydride batteries

NiCd batteries face the same enviromental problems of disposal, as the mercury batteries. It is the cadmium which poses the problem, and they are being largely replaced by nickel metal hydride batteries. These have similar properties, but are much safer to dispose off. Both of the nickel batteries perform best if they are almost fully discharged and then fully recharged. If they are just partially discharged and then recharged on a regular basis they can lose some of their capacity. Any of the nickel batteries will benefit from a periodic discharge to about 1 V per cell. They should not be allowed to go below this voltage because if the battery is flattened completely, some of the cells may suffer a reversal of polarity which effectively ends the useful life of the battery.

Lithium ion batteries

These are state-of-the-art rechargeable batteries. They offer at least twice the capacity of nickel metal hydride batteries and have little tendency to form a memory. The snag is that they are about three times the price of nickel metal hydride batteries. They are found in applications where a lot of power is needed but where weight or bulk must be kept to a minimum.

The SOLAS convention requirements

A reserve source(s) of energy to supply radio installations must be provided on every SOLAS vessel for the purposes of conducting distress and safety radio communications in the event of failure of the vessel’s main emergency sources of power. The reserve source of energy must be capable of simultaneously operating the VHF radio installations, and either the MF/HF radio installation or the INMARSAT ship’s earth station (as appropriate for ship’s sea area operation).

The capacity of the reserve source of energy should be sufficient to operate the particular installation with the highest power consumption for the appropriate period specified:

  • Ships with emergency generators: 1 Hour
  • Ships without emergency generators: 6 Hours

The batteries must be recharged to required minimums within a 10-hour period. The capacity of batteries must be checked, using an appropriate method, at intervals not to exceed 12 months.

Last modified: Saturday, 25 April 2020, 7:50 PM