Electrolysis is the process of passing electrical current through an ionic solution to produce a chemical reaction. For this process, an electrical current is applied across a pair of electrodes immersed in an electrolyte (chemical solution). Each electrode attracts the decomposed ions of opposite charges and the cells are used to produce electricity through the chemical reaction between the electrodes.
Electrolysis only takes place while the cell is being recharged. The reverse process of electrolysis takes place while the cell is delivering a load. In the process of electrolysis, the chemical substances that are present in the electrodes react together in the presence of the electrolyte. The resulting chemical reactions in both the electrodes result in the flow of ions that produces electricity.
It is always better to learn the process of electrolysis before we start to look at the working of cells. A battery can also be defined as a group of cells connected together either in series or parallel.
Faraday's Law of Electrolysis
Faraday deduced two fundamental laws through his experiments. These laws govern the phenomenon of electrolysis.
The Laws are:
First Law: The mass of ions liberated in an electrode is directly proportional to the quantity of electricity i.e. the charge which passes through the electrolyte.
Second Law: The masses of ions of different substances liberated by the same quantity of electricity is proportional to their chemical equivalent weights. Alternating current (A.C.) does not cause any chemical change when passed through an electrolyte and does not help in electrolysis.
Workings of a Lead Acid Cell
The working of a lead-acid cell is based on a simple chemical reaction. When two dissimilar metals are immersed in a particular electrolyte a chemical reaction takes place which produces a voltage. Based on this reaction, the lead-acid battery can be discharged or charged. Discharging and charging cycles can be split into four stages.
The first is a fully charged stage where the positive plates are fully covered with lead oxide (PbO2) and negative plates are fully covered with sponge lead (Pb). Water and sulphuric acid constitute the electrolyte.
The second stage is the discharging stage, when the current flows from the positive to the negative plates and the sulphuric acid in the electrolyte splits into hydrogen (H2) and sulphate (SO4). The free sulphate combines with the lead of both the lead oxide and the sponge lead and forms lead sulphate (PbSO4). The free hydrogen combines with oxygen and forms water and dilutes the electrolyte.
The third stage is fully discharged stage in which both the plates are fully sulphates and the electrolyte is mostly diluted with water.
The fourth stage is the charging stage in which the chemical reaction that took place during discharging is reversed. The sulphate on the plates combines with the hydrogen and forms sulphuric acid (H2SO4). In the negative plate, hydrogen bubbles are formed while in the positive plate, free oxygen (O2) combines with lead (Pb) and forms lead oxide (PbO2).
The battery case for alkaline batteries is generally made out of steel. In some batteries, the battery case also acts as one of the active materials. For example, a zinc can in a Leclanche cell. The lead-acid chemical reaction and charging-discharging cycle are explained in the media given below.
Factors That Affect Battery Efficiency
Battery efficiency is greatly reduced at lower temperatures. During winter, a battery's efficiency is reduced to 40% to that of its efficiency during summer. If a battery has to be replaced, it has to be replaced with a battery of the same or higher CCA.
Conclusion
Efficiency is an important issue in component selection due to the relatively high cost of power and capacity. You have learnt that overall battery efficiency is specified by two categories: Ampere hour efficiency and Watt-hour efficiency.
You have also learnt about the ratings of battery capacity which depends on different factors and there are different ratings to measure it.
