Friday, March 06, 2015

Important Characteristics of Lead-Acid Batteries

Primary and Secondary Cell

Batteries are used by industrial plants to provide supply for motor controls, alternator excitation supply and for emergency power. There are two types of batteries namely the primary cells and secondary cells. The primary cells are not typically used in the since that the said cells are not rechargeable and it could not provide large and steady current for longer time. The secondary cells are the most practical type of battery for a simple reason which is contrary to the former.

Common types of secondary cell

There are three commonly used secondary cell in the industry namely,

  • Lead Acid
  • Nickel- iron-alkaline cell
  • Nickel-cadmium-alkaline cell
Among the three the most common type of secondary cell is the lead-acid cell. The reason why it is common and widely used is because of its characteristic compare to others. Therefore it is noteworthy to study the important characteristics of this battery.

Important Characteristics of a Lead-Acid Cell

  • Terminal Voltage - When the battery delivers current, the voltage terminal voltage is less than its EMF due to its internal resistance. Lead acid cell has less lead sulphate that will clogged the pores of the battery once there is continous flow of current.
  •  EMF- The emf of a fully charged L-A cell is relatively higher, its open circuit voltage will reach as high as 2.2 Volts while other has 1.2 Volts only.
  •  Capacity - The capacity of the cell is defined as the quantity of electricity which it can give out during single discharge until its terminal voltage falls to 1.8 V. Battery capacity is measured by Ampere-hours and the capacity of lead-acid cell is not permitted to discharged beyond 1.8 V, thus it has high capacity.

  • Efficiency - There are two ways which we can measure the efficiency of the lead-acid cell that is ampere-hour efficiency and watt-hour efficiency. The L-A cell has 90% Ampere-hour efficiency and 75% Watt-hour efficiency. These data is relatively higher compare to other secondary cells.
Ampere-hour efficiency = (Amp-hr provided on discharge / Amp-hr of charge) x 100 

Watt-hour efficiency = (Energy given on discharge / Energy input of charge ) x 100
 Applications of Lead-Acid Cells

  • In most industrial plants, lead acid batteries are used to energized the control equipments. 
  • These batteries are widely used in automobiles. The fact that its internal resistance is very low then it will provide a large output current required for car starting.
  • Hospital facilities that needs steady DC supply.
 Maintenance of Lead-Acid Cells
  1.  Keep the top of the battery clean and dry
  2. Keep the electrolyte at the proper level
  3. Always monitor the hydrometer reading.
  4. Do not charge the battery at high rate.
  5. Do not leave the battery in a discharge condition for a long time.

Sample Battery Problems
  •  A lead-acid cell maintains a constant current of 1.5 A for 30 hours before its terminal voltage  falls to 1.8 volts. What is the capacity of the cell?
           Capacity = Id x Td = 1.5 A x 30 h = 45 Ampere-hours
  •  A lead acid cell is charged at the rate of 18 A for 10 hours at an average voltage of 2.26 Volts. It is discharge in the same time at the rate of 17.2 A; the average voltage during discharge being 1.98 V. Calculate the Ampere-hour eff of the cell?
         Ampere-hour eff = Id x Td / Ic x Tc = (17.2 * 10) / (18 * 10) x 100 = 95.5%

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