DC SOURCES OF ELECTRICITYbatteries is divided into two groups
- primary cell ( dry cell / leclanche cell )
- secondary cell
now we are going to discuss about secondary cell
SECONDARY CELL.
-known as storage cell.
-any kind of electrolytic cell in which the electrochemical reaction that releases energy is reversible.
-must be charged before used.
-examples of usages :
- laptop
- handphone cell
types of secondary cell
- LEAD ACID CELLS
- NICKEL CADMIUM CELL ( NiCAD )
LEAD ACID CELLS
purpose :
- power supply to aircraft during emergency and starting.
construction:
- +ve plate used lead peroxide fill with antimony
- -ve plate used spongy lead and has extra plate than +ve plate
- electrolye is dilute sulphuric acid with water 30% acid and 70% water
- casing made up of high impact plastic
- vent screw type which allow distilled water to adjust electrolyte level.
- separator prevents +ve and -ve plate from touching but allow sulphuric acid to pass through.
- 6 / 12 cells, each cell rate 2.1 volts.
indication of fully charged condition:
-terminal voltage
- 2.7V/ cell on charged
- 2.1 V/ cell off charged
- 1.8 V/cell discharged
terminal voltage between 30.0 to 32.4 V ( on charge ), if terminal voltage below 28.5 V , it should be withdrawn from service.
voltage test :
fully discharge
- specific gravity of electrolyte - 1.275 and 1.300 at 80. use hydrometer to check
- gassing freely
- sulphation occur and electrolyte become water.
- S.G will be low (1.150) due to internal resistance becomes high.
- both plate becomes white lead sulphate : act as high resistance
- sulphation occurs when battery discharged
capacity
- ability to produce a given amount of current for specific time e.g 1A for 1 Hour
- more than 80% efficiency fit for aircraft use
efficiency (%) = output capacity / rated capacity X 100
factor affecting capacity of a battery :
- num of plate
- plate area
- strength of electrolyte
- temperature
proper servicing is required on this cell :
- keep battery clean , all terminals tight and free of corrosions.
- electrolyte level should 1/4 above the plates , if low add distilled water and not acid.
- when mixing electrolyte always add ACID to WATER.
- coat the battery terminals with petroleum jelly or general purpose grease in order to prevent corrosion or oxidization on the terminals.
two types charging :
- Constant current charging
- most effective
- charge battery by supplying current at a constant rate ( manually )
- when charging more than one battery, it must be connected in series and battery voltage must not exceed the charging voltage.
- several batteries in series , charge at the lowest battery capacity.
- need monitoring during charging
- temperature must not exceed 60 C as for the electrolyte it must be below 43C
- when cells commence gassing, the voltage and relative density should be measured periodically.
-Constant voltage charging
- when battery connected in an aircraft , car or a other vehicles
- generating voltage system slightly higher than battery voltage
- the amount of current flow into a battery is determined by the state charge of battery.
- no need monitoring device
capacity test :
- carried out after initial charge
- carried out 3 monthly
- at any time if capacity in doubt
- acceptable capacity for use in aircraft is 80% and above
insulation test :
- carried out at a periodic of time
- at any time the electrolyte suspected leakage.
- place a fully charged battery on metal plate , test between terminal and plate using 250 V insulation tester ( megger ), reading must not less than 1 Mili Ohms.
storage :
- stored clean, dry, well ventilated area
- always stored in fully charged condition this is to prevent sulphation to occur.
- give re freshening charged every 2 to 4 weeks.
safety and precaution :
- lead acid and Nicad cell must be stored in separate room.
- do not wear rings, or metal strap.
to be continued......................
