A lead acid battery is a rechargeable battery. So it is a secondary battery. We name this battery such because it uses lead metal for its electrode. Also, it uses dilute sulfuric acid as its electrolyte. Among these two electrodes, one is positive electrode or anode. Another one is the negative electrode or cathode. The anode consists of lead peroxide (PbO2). And the cathode consists of pure lead (Pb).
Electrodes of Lead Acid Battery
Pure lead or sponge lead is porous enough. Hence the contact surface between electrolyte and electrodes is large enough. But the main problem of lead is its mechanical weakness. So we often use few different alloys for the purpose instead of using pure lead. For example, the lead-antimony, lead-calcium, and lead-antimony-calcium are such alloys.
Advantages and Disadvantages of Using Antimony in Electrode Plates
Although, using alloys instead of pure lead, invites some problems in the performance of the lead acid battery. For example, lead – antimony alloy cheap in cost but prone to sulfation. So if we leave the lead acid battery at a low state of charging for a long period the sulfation may significantly arise on the plate. Also, lead-antimony alloy increases the gassing of the battery during charging. As a result, a higher loss of water occurs. So frequent topping up of water becomes essential. Also, a lead-antimony plate battery has a high discharge level.
Advantages and Disadvantages of Using Calcium in Electrode Plates
The lead-calcium alloy is a little bit costlier than lead-antimony alloy. But it reduces the gassing of the lead acid battery. But a lead calcium battery is the less deep discharge battery than a lead-antimony battery.
We can achieve the advantages of both the alloys by adding both antimony and calcium in the lead. But lead-antimony-calcium alloy is costlier than the previous two alloys.
Physical Configuration of Electrodes
Besides the material for electrode plates, the physical configuration of the electrodes also plays an impact role on the rate of the charging and discharging and also on the lifetime. Thin electrode plates allow quick charging and discharging.
But a thin plate is mechanically weak and more prone to the shedding of material from the plates. So we have to maintain a moderate thickness of the plates. So that it can serve a moderate lower charge and discharge times, with longer lifetime.
Electrolyte of Lead Acid Battery
We use the dilute sulfuric acid (H2SO4 + H2O) as the electrolyte in a lead acid battery. Each molecule of sulfuric acid in its dilute form splits into two positive monovalent hydrogen ions and one negative bivalent sulfate ion. The specific gravity of the dilute sulfuric acid ranges from 1.2 to 1.23.
Charging of Lead Acid Battery
During charging we connect the positive plate (anode) with the positive end of the DC source. At the same time, we connect the negative plate (cathode) with the negative end of the dc source.
Initially, the negative and positive both plates contain a layer of lead sulfate (PbSO4). The layer of this lead sulfate has formed during discharging of the lead acid battery. We yet not have discussed the discharging of the lead acid battery. But we shall discuss it later in this article.
Reaction in Positive Plate
Due to the current passing through the electrolyte, during charging, some water molecules split into H+ and O— ions. Due to electrostatic forces, the negative oxygen ions (O—) come to the lead sulfate covered anode. Here the oxygen ions (O—) replace the sulfate and form lead PbO2. Then the sulfate ions get dissolved in the electrolyte with the hydrogen ions of the water. Hence the concentration of sulfuric acid increases in the electrolyte.
Reaction in Negative Plate
Due to electrostatic forces, the positive hydrogen ions (H+) come to the lead sulfate covered cathode. Here the hydrogen ions (H+) react with the lead sulfate and form pure lead with sulfuric acid. In other words, the sulfate and hydrogen both ions dissolve in the electrolyte solution. Then the sulfate ions get dissolved in the electrolyte with the hydrogen ions of the water. Hence again the concentration of sulfuric acid increases in the electrolyte.
Discharging of Lead Acid Battery
Reaction in Positive Plate
During discharging, PbO2 reacts with dilute sulfuric acid and forms lead sulfate (PbSO4). As Pb is tetravalent and sulfate is divalent, the extra two electrons the reaction gets from the electrode itself.
So, this reaction of lead acid battery absorbs dissolved sulfate from electrolyte and deposits lead sulfate on the positive plate. Also, it contributes water to the electrolyte. Therefore the concentration of sulfuric acid decreases in the electrolyte.
Reaction in Negative Plate
Here in the negative plate the pure Pb atoms give up its two valence electrons to the electrode itself and react with sulfuric acid to form lead sulfate.
Due to absorption of sulfate ions again in the negative plate, the concentration of sulfuric acid further decreases. As a result, the overall specific gravity of the lead acid battery cell becomes low.
Gassing in Lead Acid Battery
As we have already told, when a current flows through the electrolyte, the water molecules disassociate in hydrogen and oxygen ions. But the lead sulfate absorbs these ions during the formation of PbO2 in positive and Pb in negative plates respectively.
Please refer to the above charging reactions. But once charging completes there will be no more lead sulfate to react. After that, if we continue to charge the lead acid battery, these hydrogen and oxygen ions form hydrogen and oxygen gases. Here at the positive plate the oxygen ions give up their extra electrons and become atoms. Then these atoms attach in pair and form gaseous oxygen. Consequently, there will be a bubbling of oxygen gas coming out along the positive plate. At the same time, the hydrogen ions receive electrons from the negative plate and become hydrogen atoms. Similarly, they attach in pair and form hydrogen gas. So, at the negative plate, there will be a bubbling of hydrogen gas. We call this phenomenon as gassing of lead acid battery.
- Faraday’s Law of Electrolysis First Law and Second Law
- Types of Electric Conductors Electrolytes and Nonelectrolytes
- Ionization of Electrolytes or Dissociation of Electrolytes
- Electrolysis and Electrodes Reactions
- Battery and Battery Cell
- Lead Acid Battery Working Principle of Lead Acid Battery
- Construction of Lead Acid Battery
- Maintenance of Lead Acid Battery
- VRLA Battery or Valve Regulated Lead Acid Battery