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Lead-acid Battery & Alkaline Battery for Ships

The battery is a convenient means of storing electricity. It is used on many ships as an instantly available emergency supply. It may also be used on a regular basis to provide a low-voltage d.c. supply to certain equipment. To provide these services the appropriate size and type of battery must be used and should be regularly serviced. Two main types of battery are used on board ship: the lead—acid and the alkaline type, together with various circuits and control gear.

Lead-acid battery

The lead - acid battery is made up of a series of cells. One cell consists of a lead peroxide positive plate and a lead negative plate both immersed in a dilute sulphuric acid solution. The sulphuric acid is known as the 'electrolyte'. A wire joining these two plates will have a potential or voltage developed across it and a current will flow. This voltage is about 2.2V initially with a steady value of about 2V. A grouping of six separate cells connected in series will give a 12V battery. The word 'accumulator* is sometimes used instead of battery.

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Actual construction uses interleaved plates in the cell in order to produce a compact arrangement with a greater capacity. The complete battery is usually surrounded by a heavy-duty plastic, hard rubber or bitumen case.

In the charged condition the battery contains lead, lead peroxide and sulphuric acid. During discharge, i.e. the providing of electrical power, some of the lead peroxide and the lead will change to lead sulphate and water. The sulphuric acid is weakened by this reaction and its specific gravity falls.

When the battery is charged, i.e. electrical power is put into it, the reactions reverse to return the plates to their former material and the water produced breaks down into hydrogen gas which bubbles out.

Alkaline battery

The basic cell of the alkaline battery consists of a nickel hydroxide positive plate and a cadmium and iron negative plate immersed in a solution of potassium hydroxide. The cell voltage is about 1.4V. A grouping of five cells is usual to give about seven volts.

An interleaved construction is again used and each cell is within a steel casing. This casing is electrically 'live' being in contact with the electrolyte and possibly one set of plates. A battery consists of a group of cells mounted in hardwood crates with space between each. The cells are connected in series to give the battery voltage.

In the charged condition the positive plate is nickel hydroxide and the negative plate cadmium. During discharge oxygen is transferred from one plate to the other without affecting the specific gravity of the potassium hydroxide solution. The negative plate becomes cadmium oxide and the positive plate is less oxidised nickel hydroxide. Charging the battery returns the oxygen to the positive plate.

Choosing lead—acid or alkaline type

Large marine battery applications are provided by the following types:- The use/storage of the two different types in the same system/room is not permitted. The maker’s instructions are to be carefully adhered to.

The choice between the lead—acid or alkaline type of battery for various running machinery onboard will be based upon their respective advantages and disadvantages.

The lead-acid battery uses fewer cells to reach a particular voltage. It is reasonably priced but has a limited life. It does, however, discharge on open circuit and requires regular attention and charging to keep it in a fully charged condition. If left in a discharged condition for any period of time a lead-acid battery may be ruined.

The alkaline battery retains its charge on open circuit and even if discharged it can be left for long periods without any adverse effect. Although more expensive it will last much longer and requires less attention. Also a greater number of cells are required for a particular voltage because of the smaller nominal value per cell. Both types of battery are widely used at sea for the same basic duties.



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