BatteriesBatteries use a chemical reaction to do work on charge and produce a voltage between their output terminals. The basic element is called an electrochemical cell and makes use of an oxidation/reduction reaction. An electrochemical cell which produces an external current is called a voltaic cell. Voltages generated by such cells have historically been referred to as emf (electromotive force).
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Index DC Circuits How Batteries Work | ||||
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Carbon-zinc Batteries
The chemical reactions in this cell may be approximated by
Some of the complexity of this reaction comes from the fact that the reduction of the ammonium ion produces two gaseous products 2NH4+(aq) + 2e- -> 2NH3(g) + H2(g) which must be absorbed to prevent the buildup of gas pressure. That is accomplished with two further reactions in the paste electrolyte. Zinc chloride reacts with ammonia to form solid zinc ammonium chloride and manganese dioxide reacts with hydrogen to form solid dimanganese trioxide plus water (Hewitt). ZnCl2(aq) + 2NH3(g) -> Zn(NH3)2Cl2(s) 2MnO2(s) + H2(g) -> Mn2O3(s) + H2O(l) The voltage of this cell is initially about 1.5 volts, but decreases as energy is taken from the cell. It also has a short shelf life and deteriorates rapidly in cold weather. Oxidation of the zinc wall eventually causes the contents to leak out, so such batteries should not be left in electric equipment for long periods. While these batteries have a long history of usefulness, they are declining in application since some of their problems are overcome in alkaline batteries.
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Index DC Circuits References Floyd Electric Circuit Fundamentals, App. B Ebbing Ch 19 Hewitt Phys. Sci. Ch 20 | |||||||||
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Alkaline Dry CellsAlkaline cells overcome some of the problems with carbon-zinc batteries by using potassium hydroxide in place of ammonium chloride in the electrolyte. Potassium hydroxide is a base or alkaline material, hence "alkaline" batteries. The half-reactions are
These cells have much longer shelf life and perform better under drain and in cold weather. They avoid the use of the zinc-corroding ammonium ions and do not produce any gaseous products.
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Index DC Circuits References Floyd Electric Circuit Fundamentals, App. B Ebbing Ch 19 Hewitt Phys. Sci. Ch 20 | ||||||||
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Go Back |