Batteries are devices that convert stored chemical energy into useful electrical energy. A battery may be thought of as a clever variant of a standard exothermic chemical reactor that yields chemical products with lower energy content than the chemical reactants. In such a chemical reactor, the overall chemical reaction proceeds spontaneously (possibly requiring a catalyst and/or elevated temperature) when the reactants are brought into physical contact. In a battery, the overall chemical reaction is divided into two physically and electrically separated processes: one is an oxidation process at the battery negative electrode wherein the valence of at least one species becomes more positive, and the other is a reduction process at the battery positive electrode wherein the valence of at least one species becomes more negative.
A battery consists of three important parts, among others:
- The electrodes
- The electrolytes
- The sulphate
An electrode in an electrochemical cell is referred to as either an anode or a cathode (words coined by Michael Faraday). The anode is now defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode as the electrode at which electrons enter the cell and reduction occurs. Each electrode may become either the anode or the cathode depending on the direction of current through the cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell. Electrodes are used to provide current through nonmetal objects to alter them in numerous ways and to measure conductivity for numerous purposes.
An electrolyte is a substance that ionizes when dissolved in suitable ionizing solvents such as water. This includes most soluble salts, acids, and bases. Some gases, such as hydrogen chloride, under conditions of high temperature or low pressure can also function as electrolytes. In batteries, two materials with different electron affinities are used as electrodes; electrons flow from one electrode to the other outside of the battery, while inside the battery the circuit is closed by the electrolyte’s ions. Here, the electrode reactions convert chemical energy to electrical energy. The electrolyte must have high ionic conductivity to allow the development of a high-power battery, be stable at both the high potential of the battery cathode and the low potential of the battery anode, be compatible with a physical separator (usually a porous polymeric material) that prevents physical or electronic contact between the anode and cathode, adequately wet the anode, cathode, and separator, be environmentally benign and have low vapor pressure.
Sulphation occurs when a lead acid battery is deprived of a full charge. This is common with starter batteries in cars driven in the city with load-hungry accessories. A motor in idle or at low speed cannot charge the battery sufficiently. During use, small sulphate crystals form, but these are normal and are not harmful. During prolonged charge deprivation, however, the amorphous lead sulfate converts to a stable crystalline that forms a deposit on the negative plates. This leads to the development of large crystals, which reduce the battery’s active material that is responsible for high capacity and low resistance. Sulphation also lowers charge acceptance. Sulphation charging will take longer because of elevated internal resistance.