BATTERIES
	Much of the GMDSS equipment on board is powered by batteries. It is not only in the operator's own interest to make sure that they are maintained in good condition but it is his duty to look after them. For a compulsory GMDSS vessel the operator must make regular tests of the various batteries and must log the results of the tests. Batteries store electrical energy in the form of chemical energy. When electricity is drawn from a battery, a chemical reaction occurs inside the battery. The battery will continue to produce electricity until all the chemicals have been used. On any vessel there will be two types of batteries, primary and secondary. Primary batteries are non-rechargeable, such as are found in a SART or EPIRB. They will have an expiration date and at that date, or if used, they must be replaced with new batteries. Secondary batteries are rechargeable. After use, or after prolonged storage, these batteries can be recharged to restore their capacity. Charging reverses the chemical process inside the battery so the battery can once again supply electricity. Secondary batteries are found in handheld VHFs, stand-by emergency batteries and, particularly on smaller vessels or yachts, secondary batteries will power the main radio systems. Most batteries are made from a series of individual cells. Every battery, and indeed every cell making up a battery, has a positive (marked +) and a negative (marked -) terminal and each cell has a nominal voltage. The nominal voltage of a cell or battery is its average operating voltage. The voltage can be much higher when the battery is being charged and considerably lower than the nominal voltage when the battery is almost discharged. How many volts the nominal voltage is depends on the chemical composition of the cell. For example, a normal car battery is made using lead/acid cells. A lead/acid cell will always have a nominal voltage of 2 volts. When it is fully charged, the voltage could be as much as 2.4 volts and when it is almost discharged it could drop to 1.8 volts, or even less. In a car battery, to reduce the current drawn by the starter-motor to reasonable levels we need a higher voltage than 2 volts. Six of these cells are connected in series. That is, they are joined together in a string, the positive of one cell connected to the negative of the next and so on. This results in the battery as a whole having a nominal voltage of 12 volts. We could just as easily join two 12-volt batteries in series to give us, in effect, one 24-volt battery if that is the voltage that is required. How much power a battery can store is called the capacity of the battery. The capacity of a battery is measured by how much current, measured in amperes, it can supply for how many hours. The units are Ampere hours (Ah). By joining cells or batteries together in parallel, joining positive to positive and negative to negative, the capacity of the battery as a whole is the sum of the capacity of the individual batteries. Primary (non-rechargeable) batteries can be used in series to achieve a particular voltage. For example, two or more batteries are often used in series, i.e. positive to negative in a flashlight to allow a bulb of higher voltage to be used. This will give a brighter light without having to draw a heavier current. However, primary batteries should never be used in parallel because there is a possibility of one battery trying to recharge another. Secondary cells can be used in series, in parallel, or in a combination of both to achieve the voltage and capacity that is required. The only limitation being that each cell is of a similar voltage, capacity and chemical composition. All batteries will lose part of their capacity to produce electricity while they are waiting to be used. They suffer a chemical process other than the main electricity-producing reaction. This is a self-destructive reaction which slowly uses the chemicals which are needed for the electricity-producing reaction. This results in a steady loss of capacity or loss of charge. How much energy is lost per year varies with the type of cell that is used in the battery.
	Primary, Non-rechargeable Batteries
	There are five common types of primary, non-rechargeable batteries that we may find on board. Carbon/zinc batteries Carbon/zinc are the basic type of flashlight batteries. They have a nominal voltage of 1.5 volts per cell. Their advantage is that they are cheap but they tend to lose about 15% of their power per year just by sitting there. 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. On board, they are only likely to be found in flashlights. Alkaline manganese batteries These are the premium 'longer life' batteries such as 'Duracell'. They too have a nominal voltage of 1.5 volts 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. Both carbon/zinc and alkaline batteries can recover a little after a rest. 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. These batteries might also be found in flashlights and may form the basis of battery packs for EPIRBs or SARTS. Mercury batteries Mercury cells have a nominal voltage of about 1.4 volts per cell. Once again they are more expensive, but they have as much as 6 to 8 times the capacity of carbon/zinc batteries and they lose only some 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 and calculators. The nominal voltage is around 1.5 volts 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. Apart from calculators and watches they can be found as back-up batteries for memory circuits in some equipment. Lithium/ batteries These are the most modern, high power batteries. Their voltage can vary from 1.5 to over 3 volts, depending on the construction. 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 equipment because of their long service life. They may also be found in some of the more expensive EPIRBs and SARTs. The operator should be aware of which equipment contains primary batteries and they should be checked regularly for leakage and must be replaced in accordance with the expiration dates given by the manufacturers.
	Secondary, Rechargeable Batteries
	Rechargeable cells also come in a variety of types. In all of them, electrical energy is stored by a reversible chemical reaction. There are four kinds that we might encounter. 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 volts. Most batteries are made with 3 or 6 cells giving a battery voltage of 6 or 12 volts. These batteries are then grouped together to make a bank of the required voltage and capacity. Most vessels use 12 or 24 volts for their battery bank but in older vessels 110 volts is not uncommon. 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 which is very corrosive, so great care must be taken when handling it. Lead/acid batteries are popular because they are cheap and can supply high current when needed. One disadvantage is that they give off hydrogen when being charged and this is a very explosive gas. Any battery compartment must be well ventilated and care must be taken not to cause a spark near a charging battery and of course nobody working in or near the battery compartment should smoke. As part of the chemical reaction which forms the hydrogen gas, the battery uses water which must be replaced. There are some lead/acid batteries which claim to be maintenance free, but in practice most of these eventually require that water be added. The electrolyte level should be checked at least monthly; more often if a lot of water has to be added. 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. It is important to use only distilled water when topping up the electrolyte otherwise impurities will be added which will drastically shorten the life of the battery. One big advantage of the lead/acid battery is that the relative density, or specific gravity, of the electrolyte changes according to the charge there is in the battery - the more charge that there is in the battery, the denser the electrolyte becomes. The specific gravity of the electrolyte in each cell can be easily measured using a hydrometer. This gives an accurate indication of the charge in that cell. 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 1.26, and a fully discharged cell will give a reading around 1.16, depending on the temperature of the electrolyte. The specific gravity reading will indicate the state of charge for each cell, so not only does it give an indication of the state of charge for the battery as a whole, but it also can give a warning of impending problems if one cell differs markedly from the others. If one cell is showing a low specific gravity, then it is an indication that 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. As mentioned, great care must be taken when handling the sulphuric acid electrolyte. A sensible precaution would be to wear rubber gloves and safety goggles. GMDSS regulations stipulate that the voltage of any secondary batteries should be read and recorded each day and in the case of lead/acid batteries, the specific gravity of the electrolyte should be measured and recorded each month. 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 our GMDSS equipment where we are usually interested in drawing a relatively small current for a long time. The only other negative is that we cannot monitor the state of the battery with a hydrometer. The only indication we have is the voltage and this stays relatively constant until the battery is almost flat, so we may have little 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 Nickel cadmium batteries, or NiCads as they are usually called, 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 of. 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. If only part of their capacity is used on a regular basis then they are said to develop a memory* and their capacity will decline to more or less the amount that is regularly used. Any of the nickel batteries will benefit from a periodic discharge to about 1 volt 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. After the discharge, they should be fully recharged according to the manufacturer's recommendations. Nickel cadmium or nickel metal hydride batteries are often found in equipment such as handheld VHFs and portable computers. *Memory Effect: If Ni-Cad batteries are not used correctly they can suffer from the 'Memory Effect'. The problem occurs if batteries are recharged before they are flat, and results in much shorter operating times as well as a shorter useful working life for 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. They are often the battery of choice in top-of-the-range laptop computers. When replacing any battery in a piece of equipment, be it primary or secondary, great care must be taken to connect it in the correct polarity. The positive (+) terminal of the battery goes to the positive (+) terminal of the equipment, which may well be coloured red. The negative (-) terminal of the battery goes to the negative (- ) terminal of the equipment and this is normally coloured black. Connecting the battery the wrong way round is likely to cause serious damage. Likewise, when charging a secondary battery the correct polarity must be observed. When connecting a portable charger, the red or positive lead from the charger must go to the positive (+) terminal of the battery. Connecting the charger the wrong way round will at best damage the battery but could even cause an explosion.
	Battery Chargers
	There are three main types of battery charger. Constant voltage charger The voltage of the charging source is set at the voltage of the fully charged battery - about 25% higher than the nominal voltage for a lead/acid battery, e.g. 15 volts for a 12 volt battery. The charging current will be high at the beginning of the charge, especially if the battery is almost flat to begin with, and then it will taper off as the battery nears capacity and its voltage rises to that of the source. The danger with this type of charger is that the initial current may be so high that the battery will get too hot, and the internal plates of the electrode could buckle and short-out. Trickle charger The charging current is limited to something approaching the average daily loss from the battery. The charging current might be equivalent to 1 or 2 % of the capacity of the battery. Obviously this sort of charger would be no good for recharging a battery that gets heavily discharged on a regular basis especially if the source, such as a generator, is not permanently available. A trickle charger is good for maintaining stand-by batteries in a constant state of readiness. Constant current charger The voltage of the source in this type of charger is perhaps 8 to 10 times higher than the voltage of the battery being charged. The current is limited by a resistor, sometimes a variable resistor, so a suitable current can be selected. The battery will be charged at a constant current with virtually no reduction even as the battery approaches full charge. The danger with this type of charger is that there can be excessive gassing or 'boiling' of the batteries in the final stages of charging. The big advantage is that the battery can be fully charged much quicker than with either of the other chargers. This type of charger is good when the battery in question is discharged regularly and the recharging source, such as a generator, is only available for limited periods. The batteries should be monitored during charging to avoid excessive gassing and fluid loss, or over heating.
	Battery Maintenance
	All batteries should be kept clean and dry. Any accumulation of dirt, moisture or spilled acid can allow a leakage of current between the terminals, which not only reduces the effective capacity of the battery but can also encourage corrosion. The terninals themselves should be kept clean. Lead/acid batteries often suffer a build up of white deposits on the terminals. This deposit can easily be washed off with warm water and the terminals, once they have dried, can be protected with a thin coat of petroleum jelly. As we mentioned earlier, the GMDSS regulations for a compulsory vessel require that the batteries be tested on a regular basis - the voltage of all batteries should be measured daily and hydrometer readings taken monthly for lead/acid batteries and the results logged. For voluntary vessels it is a good idea to follow the same routine. By comparing readings of voltage and the hydrometer readings over several months an early warning of impending problems with the batteries can often be seen.
	Ends