In this topic, you study the definition, diagram and working of the lead acid battery and also the chemical reactions during charging and discharging.
The combination of two or more than two cells suitably connected together is known as a battery. In case of lead acid cell, the cell has got the following parts.
Parts of lead acid battery. The different parts are studied independently:
(a) Container. It is used to accumulate all the parts Of the cell or battery viz. plates, separators, electrolyte etc. The container is divided into a number of chambers or compartments equal to the number of cells used for that battery. There is a mud space provided in the bottom where the fallen active material may be collected. The container is made of hard rubber compound, celluloid and glass etc. It is open from the top. After the assembly the cell and top are sealed by a sealing compound.
Plates. There are two types of the plates the positive plate and negative plate. The active material of the positive plate is Pb02 (lead peroxide) and spongy lead for negative plate. According to the construction the plates are divided into the followings:
(i) Plante type. (ii) Faure type.
(i) Plante type plates. These are prepared from the pure lead by repeated charge and discharge. The grid of pure lead is immersed in dilute H2S04 and d.c. is passed through them, after several charge and discharge the plate is covered with a layer of Pb02, i.e. positive plate. The other plate is covered with spongy lead and is known as negative plate. This type of plates have comparatively longer life.
(ii) Faure type plate. In this case the active material is filled in the pockets of lead orbits. The paste of the active material is filled in the pockets of a grid which is prepared by the lead antimony alloy. The paste is a mixture of oxides of lead and dil. H2S04. After filling them properly, the plates are put on charge. The plates are prepared, the active material of positive plate is lead per oxide and of negative plate the srongy lead. Such plates are heavier then the Plante type plates. The colour of positive plate is of chocolate brown colour and the negative plate is of grey colour.
9.12. Parts of lead acid battery.
The positive plates are joined at one terminal which is known as positive terminal and the negative plates which another terminal which is known as negative terminal. The batteries are categorised according to the number of plates i.e. 15 plates, 17 plates and 19 plates, etc.
(c) Separators. The separators are used between the positive and negative plates, to avoid the short circuit. These are made of perforated wood, perforated rubber, perforated celluloid or the glass wood. It has one side plane and other side grooved to enable the electrolyte to come in contact with the positive plates. The number of separators are always double than the number of positive plates or one less than the total number Of plates (positive and negative). These should have adequate mechanical strength.
(d) Plate connector or ter’rninal. These are made of lead. All the positive plates are connected to one positive pole. The plates are welded or soldered with the connector. There are two such connectors per cell, one positive and other as negative.
(e) Cell connector. These are used to connect the cell in series. These are made of lead and antimony. The plates connector is welded with cell connector. The cell connector must be so thick to withstand the current capacity of the load without heating.
(f) Cell cover. A cell cover is used to cover the cell. It is made of moulded hard rubber compound and are sealed with the container to cover the cell. There are two holes for connections and one for filling the electrolyte or distilled water etc.
(g) Vent plug. A vent plug is used to cover the filling hole. It contains so many small holes to escape the gas.
(h) Sealed compound. It is used to seal the cell cover and also to acid tight joints.
(i) Electrolyte. The electrolyte is a solution Of dil. H2S04. It is a carrier of current between the positive and negative plates. The active material of the plate will not be effective unless it is covered with electrolyte. The distilled water is added in the H2S04 to make it dilute. The ratio of acid and water depends upon the specific gravity required or as per makers instructions.
While preparing the electrolyte the acid is added drop by drop into the distilled water and stirred with a rod of glass. In hot condition the specific gravity is between 1280-1300 and while Cold the specific gravity should be 1250-1280. The internal resistance Of the cell depends upon the electrolyte.
Chemical action. While the cell is fully charged the positive plate is of dark chocolate brown and negative of grey colour. In Case Of discharging the H2S04 decomposes into its ions the hydrogen and sulphate.
H2 + H2S04 —A’bS04 + 2H20
and when it is recharged
and
PbS04 + H2S04
PbS04 + SO, + + 2H2SO,.
Thus the complete equation can be given in one reversible reaction
Pb02+2H2S04+ Pb
PbS04 + H20 + PbSO
The charging and discharging is concluded as under:
Discharging
- It supplies the load.
- Both the positive and negative plates becomes whitish in colour (PbSO,).
- The specific gravity decreases.
- The voltage/cell decreases.
Charging
- It works as the load.
- Both PbSO, changes to Pb02 for anode and Pb for cathode.
- The specific gravity increases.
- The voltage/cell increases.
What are the main defects and their remedies Of the lead acid battery?
There may be the following main defects in a lead acid battery.
(a) Sulphation. Formation of the lead sulphate layer on positive and negative plate is known as the sulphation.
Effects. The capacity, life and the efficiency Of the cell is decreased.
Reasons. There are the following reasons:
(i) Keeping the battery in charging and discharging conditions for a long time.
(ii) Higher rate Of charging and discharging.
(iii) Improper level Of electrolyte.
(iv) Over discharging condition.
Remedy. The defect can be overcome by the following methods:
(i) By charging the battery at a low rate i.e. trickle charging for a long time.
(ii) If the layer is thick then take out the plate and remove the sulphate layer by mechanical means. If the plates are in working condition then reassemble the battery and charge again.
(b) Buckling. Deformation of plates is known as buckling.
Effects. Life is reduced.
Reasons. There are the following reasons:
(i) High rate charging and discharging.
(ii) Short circuit inside or outside the cell.
(iii) Over charging and discharging.
Remedy. (1) Take out the plate, make them straight and if the plates are in a condition to be used, reassemble and charge.
(ii) Change the battery plates.
(c) Sedimentation. Falling of active material is known as the sedimentation. The active material may be positive plate (Pb02) or negative plate (Pb).
Effects. Capacity and life is reduced.
Reasons. (1) Overheating due to flow of overload current.
(ii) Vibrations.
Remedy. Dismantle the battery, take out the fallen material from the mud space, clean the container with distilled water, check the plates, if these are straight, reassemble the battery and charge it.
(d) Internal short circuit. The plates, positive and negative, are coming in contact with each other without any external resistance, the defect is known as internal short circuit.
EffecL
(i) Low voltage.
(ii) Imw discharged potential difference.
(iil) Fully charged cell will be discharged very soon even at low current.
(iv) Overheating in an idle condition also.
Remedy. Check and clean the plates, separates the mud space and electrolyte and if these are in working condition then assemble and recharge the battery.
(c) Loss of electrolyte. If the proper level is not maintained then it is said the loss Of electrolyte.
Effect. The voltage and capacity of the cell is reduced.
Reason. Crack in container Or hole in the container or in cell cover etc.
Remedy. Change the container.
What are the care and maintenance of the lead acid cell?
There are the following precautions to be observed for the care and maintenance Of the lead acid battery:
- Strictly observe the makers instructions and rate of charging and discharging.
- Do not keep the battery idle for a long time.
- For storage, charge the battery, remove the electrolyte, dry the battery and keep it in cool dry place after reassembling.
- Keep the tenninals clean and dry and apply grease or Vaseline.
- Check the level of electrolyte, it should be at least 10 mm above the top of the plates.
- Do not fully discharge the battery, send for charging after half discharge. In no case the voltage per cell be less than 1.7 volt/cell.
- Add distilled water to the battery for recharging.
- The charging rate should be prorrr.
- Do not use high specific gravity electrolyte.
- DO not short circuit the battery.
- In mobile vanes the battery must be kept carefully tightened and fixed.
- The temperature must not exceed 500C and below the freezing point of water.
- Keep the plugs tight and vent holes cleaned.
- Never bring naked flame near the battery while charging the battery and room should be well ventilated.
- Never charge the lead acid and NiFe batteries together.