DC SOURCES OF ELECTRICITY

batteries 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terminal voltage between 30.0 to 32.4 V ( on charge ), if terminal voltage below 28.5 V , it should be withdrawn from service.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 :

• specific gravity of electrolyte - 1.275 and 1.300 at 80. use hydrometer to check
• gassing freely

fully discharge

• 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......................