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Emergency power or temporary emergency power can be provided by automatic connection of a battery at loss of main power. A simple arrangement of Ni-Cd batteries are used this type of secondary cell loses charges gradually over a period of time. Rate of loss is kept to a minimum by maintaining the cells in a clean dry state, but it is necessary to make up the losses of charge.
Proper maintained battery storage will instantly supply electrical power when required. This feature makes a battery the key element on-board the ships in the provision of emergency power supplies for essential services like radio equipment, telephone exchange, fire detection, general alarm circuits normally the main supply for each services are fed from rectifiers.
Emergency battery supplies for starting the emergency generator and for emergency lighting are used in a standby role to provide power when the main supply fails. A ship’s batteries are usually rated at a nominal voltage of 24 V D.C. In some cases a battery system of 110V or 220V may be used where a large number of emergency lights are required or where a battery is the only source of emergency power. Remember, when supplying emergency lighting loads, the storage battery’s initial voltage must not exceed the standard system voltage by more than 5%.
The nominal cell voltage is approximately 2V for lead acid and 1.2V for alkaline types. Hence, 12 lead acid cells or 20 alkaline cells must be connected in series to produce a nominal 24V. More cells may be connected in parallel to increase the battery capacity, rated in Ampere-hour (Ah). The capacity of a ship’s battery is usually rated in terms of its discharge at the 10 hour rate. Thus a 350 Ah battery would be expected to provide 35A for 10 hours. However, the battery will generally have a lower capacity at a shorter discharge rate.
The manufacture’s discharge curves must be checked for such details. After 10 hours discharge a lead acid cell’s voltage falls to about 1.8 V. the equivalent figure for alkaline battery is 1.14V.
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Nickel-Cadmium Flooded Batteries Used On-board Ships
This type of battery possesses good qualities such as a high-rate of discharge, low temperature capability, flat voltage and excellent cycle life than that of the lead-acid batteries. The active materials of positive and negative plates in each cell of a charged nickel-cadmium battery and nickel hydrate and cadmium the chemicals are retained in the supporting structure of perforated metal plates and the design is such as to give maximum contact between active compounds and electrolyte.
The strong alkaline electrolyte is a solution of potassium hydroxide in distilled water (with an addition of lithium). The ions produced in formation of potassium hydroxide solution (k+ and OH_) act as current carriers and take part in an iron transfer.
1. Discharge Action
During discharge the complicated but uncertain action at the positive plates (hydrated oxide of nickel) causes hydroxyl irons (OH) to be introduced into the electrolyte. As the action progresses the nickel hydrate is changed to nickel hydroxide. Simultaneously hydroxyl ions from the electrolyte forms cadmium hydroxide with cadmium of negative plates. Effectively, the hydroxyl ions OH moves from one set of plates to the other leaving the electrolyte unchanged. There are no significant changes in specific gravity through the discharge cycle and the state of charge cannot be found by using a hydrometer.
The gases evolved during charging are oxygen (at the positive plates) and hydrogen (at negative plates). Rate of production of gases increases in periods of overcharge. When hydrogen in air reaches a proportion of about 4% and up to 74% it constitutes an explosive mixture. Good ventilation of battery compartments is therefore necessary to remove the gas. Equipment likely to cause sparking or arcing must not be located or introduced into battery spaces. Vent caps are non-return valves, so that gas is released by contact by electrolyte with the atmosphere is prevented.
The electrolyte readily absorbs carbon dioxide from the atmosphere and deterioration results because of the formation of potassium carbonate. For this reason cell vent caps must be kept closed.
3. Topping up
Gassing is a consequence of the breakdown of water in the electrolyte. This together with certain amount of evaporation means that topping up with distilled water will be necessary from time to time. High consumption of distilled water would suggest overcharging.
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Potassium hydroxide solution is strongly alkaline and the physical and chemical properties of potassium hydroxide closely resemble those of caustic soda (sodium hydroxide). It is corrosive, so care is essential when topping up batteries or handling the electrolyte. In the event of skin or eye contact, the remedy is wash with plenty of clean water (for 15 min) to dilute and remove the solution quickly. Speed is vital to prevent burn damage and water, which is the best flushing agent, must be readily available. Neutralizing compounds, usually weak acids, cannot be located easily, although they should be available in the battery compartment.
The electrolyte slowly attacks glass and various other materials. Containers are therefore of welded sheet steel which is then nickel plated or molded in high-impact polystyrene steel casings are preferred when batteries are subjected to shock and vibration. Hardwood crates are used to keep the cells separate from each other and from the support beneath. Separation is necessary because the positive plate assembly is connected to the steel casing.
6. Battery charging
A direct current supply for charging is obtained from A.C mains, through the transformer and rectifier in the battery charger. The positive of the charge supply is connected to the positive of the cell, and negative to negative terminal. Flow of current from the charging sources reverses the discharge action.
- Charging from D.C Main
The circuit for changing from D.C mains includes a resistance connected in series, to reduce the current flow from the higher main voltage. Feedback from the battery on charge is prevented, at mains failure by a relay (which de-energizes) and spring arranged to automatically disconnect the battery. The contacts are spring operated; gravity opening is not acceptable for marine installations.
- Charging from A.C Main
Main A.C voltage is reduced by transformer to a suitable value and then rectified to give direct current for charging. The supply current may be taken from the 230 volt section and changed to say 30 volts for charging 24 batteries. Smoothing is not essential for battery charging but would be incorporated for power supplies to low pressure D.C systems with standby batteries.
7. Capacity of Battery
Battery capacity refers to the total amount of energy stored within the battery. It is measured in ampere-hour (Ah). The capacity is always given at a specified rate of discharge (10-hour rate in U.K, 8-hour rate in the U.S.A). However to be on the safer side, 20 hour rate (calculated at 20 C) are also acceptable. Capacity of the battery mainly depends on the five major factors: rate of discharge, temperature, density of electrolyte, quantity of active material and history of the battery.