A battery is an electrochemical cell that is used to store chemical energy for conversion of latter to electric energy. Of the many different types of batteries available, this chapter describes only their principal characteristics and installation.
The two basic types of cells are primary and secondary. A primary cell is chemically irreversible. Such cells cannot be recharged and must be discarded when no longer usable; their active material is consumed during discharge. The secondary cell known as a storage cell is chemically reversible. Such cells may be recharged by passing current through the battery in the reverse direction.
Electrochemical cells are rated in voltage and ampere – hour capacity. The latter represents the number of ampere – hours that can be delivered under specified conditions of temperature, rate of discharge, and final voltage.
2. DRY CELL
The dry cell, shown is the most common type of primary cell. It has carbon rod for the anode, or positive terminal and a zinc container for the cathode or negative terminal. The zinc container has an inner lining of absorbent material and the space between the electrodes is filled with a mixture of crushed coke, manganese dioxide and graphite saturated with a solution of salam- moniac and zinc chloride. Dry cell has an open circuit voltage of 1.6 volts and a rating of approximately 30 amp – hr. The life of a dry cell is limited. It gradually discharges by internal chemical action, called local action – even though no load is connected to its terminals. For this reason some manufacturers stamp a service date on the outer covering of each cell. Dry cells should be stored in a cool, dry place.
3. LEAD- ACID BATTERY.
The lead-acid battery has positive plates of lead peroxide and negative plates of sponge lead. The plates are kept apart by separators made of micro porous rubber, fiber glass, perforated plastic or hard rubber or resin- impregnated cellulose.
The electrolyte is a solution of sulphuric acid and water. During discharge, the positive and negative plates are chemically changed to lead sulphate. The charging process restores the plates to their original chemical makeup of sponge lead and lead peroxide. The open-circuit voltage of a fully charged cell is about 2.05 volts and varies slightly with the temperature and specific gravity of the electrolyte.
Lead – acid batteries should never be left in a discharged condition for any great length of time or the lead sulphate may harden and become non porous, preventing the battery from accepting a full charge. Because the specific gravity of the electrolyte changes with the state of charge or discharge, the battery condition may be determined with a hydrometer. Enough electrolytes should be drawn into the glass barrel to make the hydrometer float. Hydrometer readings should always be taken before the addition of distilled water. If the level of the electrolyte is low, water should be added and the battery should be charged before a hydrometer reading is taken.
The temperature of the electrolyte has considerable effect on its hydrometric values. High temperatures cause the volume of the electrolyte to increase, resulting in lower values of specific gravity whereas low temperatures cause the volume of the electrolyte to decrease resulting in higher values of specific gravity.
Hence, if the specific gravity readings are to be of any value in determining the condition of a battery, corrections to some reference temperature must be made. Table 1. Lists the additions or subtractions must be made to the observed hydrometer readings in order to obtain the specific gravity of the electrolyte for a standard reference temperature of 80 o F. Only alcohol thermometers should be used; mercury thermometers are dangerous, because accidental breakage will cause severe sparking and explosions.
4. DC BATTERY INSTALLATIONS.
The battery bank in a power station is a very important and essential piece of apparatus. Battery installations in general and some of the factors affecting battery life will be discussed in this lesson.
The battery bank in a power station is an essential component of the internal electric power system. There will be more than one battery bank in a station, each bank supplying its own load demands through NO – Fuzz breakers. The duct bases are operated independently of each other under normal operation. However tie switches are provided to tie- in the buses under certain operating conditions.
Typical loads supplied by the batteries are as follows:
- Emergency Lighting
Certain area of the plant requires illumination in the event of complete A.C failure so that operating personnel may carry out their duties.
Nearly all the apparatus in a power station is remotely controlled. Much of the remote control circuiting is fed from one of the D.C control buses.
3. Trip Bus
There are many protective schemes throughout a power station designed to protect personnel and apparatus in the event of a fault condition developing in any D.C circuit networks operating through various relays and contacts to quickly sense and isolate a faulty condition. In most cases, the equipment involved is tripped.
Typical control bus and trip bus loads are as follows:
(1) Typical generator control and protection.
(2) Auxiliary control and protection.
(3) Distribution electricity system control and protection.
(4) Generation electricity system control and protection.
(5) Voltage regulation.
(6) Annunciation system.
(7) Fire Alarm system.
(8) Co2 fire protection system.
5. Step of Battery maintenance
- Battery area – insure the battery area is nit and clean and dry places.
- Check clamp – insure the battery clamp and cable and wire contact is proper.
- Avoid grease – battery terminal cleaning and dry, periodically apply petroleum jelly or Vaseline are provided to battery terminal and cable clamp.
- Distilled water – maintain the distilled water in battery level
- Terminal tightly – battery vent plugs closed tightly.
- Check vent –insure the battery vent hose damaged by the exhaust system
- Regularly check – checking regularly voltage setting
- Service regularly – battery serviced regularly from nearest dealer and maintenance shop.