Working of a Battery:
A battery or an electrochemical cell is a device which converts chemical energy to electrical
energy. It consists of a number of voltaic cells; each voltaic cell consists of two half cells connected
in series by a conductive electrolyte containing anions and cations. One half-cell includes electrolyte and the electrode to which anions (negatively charged ions) migrate, i.e., the anode or negative electrode; the other half-cell includes electrolyte and the electrode to which cations (positively
charged ions) migrate, i.e., the cathode or positive electrode. In the redox reaction that powers the
battery, cations are reduced (electrons are added) at the cathode, while anions are oxidized
(electrons are removed) at the anode. The electrodes do not touch each other but are electrically connected by the electrolyte. Some cells use two half-cells with different electrolytes. A separator between half cells allows ions to flow, but prevents mixing of the electrolytes.
Each half cell has an electromotive force (or emf), determined by its ability to drive electric current from the interior to the exterior of the cell. The net emf of the cell is the difference between the
emfs of its half-cells, as first recognized by Volta. Therefore, if the electrodes have emfs V1and V2 , then the net emf isV1-V2 ; in other words, the net emf is the difference between the reduction potentials of the half-reactions.
Figure 2.1: A
Demonstrative Electrochemical cell consisting of two half cells and a salt
The electrical driving force or across the terminals of a cell is known as the terminal voltage
(difference) and is measured in volts. The terminal voltage of a cell that is neither charging nor discharging is called the open-circuit voltage and equals the emf of the cell. Because of internal resistance, the terminal voltage of a cell that is discharging is smaller in magnitude than the open- circuit voltage and the terminal voltage of a cell that is charging exceeds the open-circuit voltage.An
ideal cell has negligible internal resistance, so it would maintain a constant terminal voltage of
until exhausted, then dropping to zero. If such a cell maintained 1.5 volts and stored a charge of one coulomb then on complete discharge it would perform 1.5 joule of work.In actual cells, the internal resistance increases under discharge, and the open circuit voltage also decreases under discharge. If the voltage and resistance are plotted against time, the resulting graphs typically are a curve; the
shape of the curve varies according to the chemistry and internal arrangement employed.
As stated above, the voltage developed across a cell's terminals depends on the energy release of
the chemical reactions of its electrodes and electrolyte. Alkaline and carbon-zinc cells have different chemistries but approximately the same emf of 1.5 volts; likewise NiCd and NiMH cells have
different chemistries, but approximately the same emf of 1.2 volts. On the other hand the high electrochemical potential changes in the reactions of lithium compounds give lithium cells emfs of 3 volts or more.