The various parts of lead acid battery are,
1. Positive Plate or Anode : It is lead peroxide (PbO2) plate of chocolate, dark brown colour.
2. Negative plate or Cathode : it is made up of pure lead (Pb) which is grey in colour.
3. Electrolyte : For the necessary chemical action aqueous solution of sulphuric acid (HSO) is used as an electrolyte.
4. Separators : The positive and negative plates are arranged in groups and are placed alternately. The separators are used to prevent them from coming in contact with each other short circuiting the cell.
5. Container : the entire assembly of plates along with the solution is placed the plastic or ceramic container.
6. Bottom blocks : To prevent the short circuiting of cell due to the active material fallen from the plates, the space known as bottom blocks is provided at the bottom of the container.
7. Plate connector : The number of negative and positive plates are assembled alternately. To connect the positive plates together separate connectors are used which are called plate connectors. The upward connections of the plate connectors are nothing but the terminal of the cell.
8. Vent-plug : these are made up of rubber and screwed to the cover of the cell . its functions is to allow the escape of gases and prevent escape of electrolyte.
The Fig.1 shows the construction of lead acid battery.
The various plates are welded to the plate connectors. The plates are immersed in H2SO4 solution. Each plate is a grid or frame work. Except some special assemblies, wide space between the plates is provided. In an alternate assembly of plates, the negative plate is one more in number than positive. So all the positive plates can work on both the sides.
1.1 Functions of Separators
The separators used have the following functions in the construction of lead acid battery :
1. Acting as mechanical spacer preventing the plates to come in contact with each other.
2. Prevent the growth of lead trees which may be formed on the negative plates and due to heavy accumulation may reach to positive plate to short circuit the cell.
3. Help in preventing the plates from shedding of the active material. The separators must be mechanically strong and must be porous to allow diffusion of the electrolyte through them.
1.2 Chemical Action in Lead Acid Battery
The chemical action in the lead acid battery can be divided into three processes :
1. First charging 2. Discharging 3. Recharging
1. First charging : When the current is passed for the first time through electrolyte, the H2O in the electrolyte is electrolysed as,
The hydrogen ions as positively charged get attracted towards one of the electrodes which acts as cathode (negative). The hydrogen does not combine with lead and hence cathode retains its original state and colour.
The oxygen ion as negatively charged gets attracted towards the other lead plate which acts as anode (positive). But this oxygen chemically combines with the lead (Pb) to form lead peroxide (PbO2). Due to the formation of lead peroxide the anode becomes dark brown in colour.
Thus anode is dark brown due to the layer of lead peroxide deposited on it while the cathode is spongy lead electrode.
So there exists a potential difference between the positive anode and the negative cathode which can be used to drive the external circuit. The electrical energy obtained from chemical reaction is drawn out of the battery to the external circuit, which is called discharging.
2. Discharging : When the external supply is disconnected and a resistance is connected across the anode and cathode then current flows through the resistance, drawing an electrical energy from the battery. This is discharging. The direction of current is opposite to the direction of current at the time of first charging. The discharging is shown in the Fig.2.
During the discharging, the directions of the ions are reversed. The H+ ions now move towards anode and the SO4- - ions move towards cathode.
This is because H2 SO4 decomposes as,
At the anode, the hydrogen ions become free atoms and react with lead peroxide alongwith the H2SO4 and ultimately lead sulphate Pb SO4 results as,
At the cathode, each SO4- - ion become free SO4 which reacts with the metallic lead to get lead sulphate.
Thus discharging results into formation of whitish lead sulphate on both the electrodes.
3. Recharging : The cell provides the discharge current for limited time and it is necessary to recharge it after regular time interval. Again an e.m.f. is injected the cell terminals with the help of an external supply.
The charging is shown in the Fig.3.
Due to this recharging current flows and following reactions take place,
Thus the PbO2 gets formed at anode while lead sulphate layer on the cathode is reduced and gets converted to grey metallic lead. So the strength of the cell is regained. It can be seen from the reaction that water is used and H2SO4 is created. Hence the specific gravity of H2SO4 which is the charging indicator of battery, increases.
Note : More the specific gravity of H2SO4 better is the charging.
The specific gravity is 1.25 to 1.28 for fully charged battery while it is about 1.17 to 1.15 for fully discharged battery. The voltage also can be used as a charging indicator. For fully charged battery it is 2.2 to 2.5 volts.
The chemical reaction during charging and discharging can be represented using single equation as,
1.3 Features of Lead Acid Battery
The various features of lead acid battery are,
1. The capacity is about 100 to 300 ampere-hours.
2. The voltage is 2.2 V for fully charged condition.
3. The cost is low.
4. The internal resistance is very low.
5.The current ratings are high.
6.The ampere-hour efficiency is about 90 to 95% with 10 hour rate.
1.4 Conditions of a Fully Charged Battery
For identifying whether the battery is fully charged or not, following conditions must be observed,
1. The specific gravity of H2SO4 must be 1.25 to 1.28.
2. The voltage stops to rise and its value is about 2.2 to 2.5 V.
3. Violent gasing starts as battery is fully charged.
4. The colour of positive plate becomes dark brown while the colour of negative plate becomes slate grey.
1.5 Miantanences and Precautions to be taken for Lead Acid Battery
The following steps must be taken in the maintenance of the lead acid battery,
1. The battery must be recharged immediately when it discharges.
2. The level of the electrolyte must be kept above the top of plates so the plates remain completely immersed.
3. The rate of charge and discharge should not be exceeded as specified by the manufacturers.
4. Maintain the specific gravity of the electrolyte between 1.28 to 1.18.
5. The loss of water due to evaporation and gasing must be made up using only distilled water.
6. The connection plugs should be kept clean and properly tightened.
7. It should not be discharged till its voltages falls below 1.8 V.
8. When not in use, it should be fully charged and stored in a cool and dry place.
9. It should not be kept long in dicherged condition. Otherwise PbSO4 gets converted to hard substance which is difficult to remove by charging. This is called sulphating. Thus sulphating should be avoided.
10. The battery must be given periodic overcharge at half the normal rate to remove white sulphate.
11. The temperature of the battery should not exceed 45 °C otherwise plates deteriorate rapidly.
12. The battery terminals should not be shorted to check whether battery is charged or not.
13. Always keep the surface of the container dry.
14. No sulphuric acid should be added till it is sure that low specific gravity is due to under charge and not due to white sulphate formed on plates.
15. The acid used must be pure without any impurity and colourless.
16. The sparks and flames must be kept away from the battery.
1.6 Testing Procedure for Lead Acid Battery
1. Using hydrometer : The testing basically involves the checking of specific gravity of the sulphuric acid. It can be checked by the use of hydrometer. The hydrometer consists of a glass float with a calibrated stem placed in a syringe. The reading on hydrometer are shown in the Fig.5.
2. Using cell tester : Another method of testing the lead acid battery is called high discharge test or short circuit test. The cell tester is used for this test. It consists of 0 – 3 V voltemeter shunted by a low resistance. The low resistance shunt is connected between two prongs as shown in the Fig.6 the prongs are pressed against the cell terminals. A high discharge current flows through the low resistance and cell voltage is indicated by the voltmeter.
The battery with full charge, without sulphatation occurred in it, shows proper reading on voltmeter. But battery which is sulphated, showing other indications of full charge will show low voltage reading. Thus this is reliable method of testing the lead acid battery.
Thus if the reading of voltmeter is less than 1.8 V, the battery needs charging while if it is more than 2.5 V it is overcharged. According to specific gravity reading the distilled water should be added to bring specific gravity back to its normal value.
1.7 Applicatons
The various applications of lead acid battery are,
1. In emergency lighting systems.
2. In automobiles for starting.
3. Uninterrupted power supply systems.
4. Railway Signalling.
5. Electrical substations and the power stations.
6. For compensating feeder drops in case of heavy loads.
7. For energizing the trip coils of the relays and the switch gears
8. As a source of supply in mines and telephone exchanges.Apart from these applications, the lead acid batteries are used in various other areas also like telephone systems, aeroplanes, marine applications etc.