Batteries

Batteries 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).

Carbon-zinc batteriesLead-acid batteries
Alkaline cells
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Carbon-zinc Batteries

The venerable carbon-zinc cell or Lechlanche' cell was invented in 1866 and was the most common small battery throughout most of the 20th century until largely supplanted by alkaline cells. The oxidation at the zinc electrode (the anode) is straightforward and similar to that in other cells like the Daniell cell. The other reactions involve the MnO2 which is contained near the carbon center rod and the NH4Cl and ZnCl2 which make up the bulk of the paste between the cathode and anode.

The chemical reactions in this cell may be approximated by

Zn(s) -> Zn2+(aq) + 2e-

Anode

2NH4+(aq) + 2MnO2(s) + 2e- -> Mn2O3(s) + H2O(l) + 2NH3(aq)

Cathode

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.

Batteries
Index

DC Circuits

References
Floyd
Electric Circuit Fundamentals, App. B

Ebbing
Ch 19

Hewitt
Phys. Sci. Ch 20
 
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Alkaline Dry Cells

Alkaline 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

Zn(s) + 2OH-(aq) -> Zn(OH)2(s) + 2e-

Anode

2MnO2(s) + H2O(l) + 2e- -> Mn2O3(s) + 2OH-(aq)

Cathode

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.

Example of internal resistance
Batteries
Index

DC Circuits

References
Floyd
Electric Circuit Fundamentals, App. B

Ebbing
Ch 19

Hewitt
Phys. Sci. Ch 20
 
HyperPhysics***** Electricity and Magnetism R Nave
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