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What is a battery?
An electrochemical battery - or, more precisely, a "cell" - is a device in which the reaction
between two substances can be made to occur in such a way that some of the chemical energy
is converted to useful electricity. When the cell can only be used once, it is called a "primary"
cell. When the chemical reaction can be reversed repeatedly by applying electrical
energy to the cell, it is called a "secondary" cell and can be used in an accumulator
or "storage" battery.
Certain cells are capable of only a few charge-discharge cycles and are, therefore, technically
"secondary" cells. Such is the case with certain silver oxide-zinc batteries. These
batteries are not capable of the repeated cycling required of a satellite battery system,
and are, therefore, considered to be "rechargeable primary" rather than storage batteries.
To define a battery in another way, it is an arrangement whereby an "electrochemical"
reaction can be made to take place so that the "electrical" part of the reaction proceeds
via the metallic path of the external circuit, while the "chemical" part of the
reaction occurs via ionic conduction through electrolyte.
The type of chemical reaction that can be used in an electrochemical cell is known as an
"oxidation-reduction" reaction - a reaction in which one chemical species gives electrons
to another. By separating the two species and controlling the flow of ions between them,
battery engineers make devices in which essentially all of these electrons can be
made to flow through an external circuit, thereby converting most of the chemical energy to
electrical energy during the discharge of the cell.
Some of the components common to all cells are:
- The "cathode" or "positive" electrode, which consists of a mass of "electron-receptive"
chemical held in intimate contact with a metallic "plate" through which the electrons arrive
from the external circuit.
- The "anode" or "negative" electrode, which consists of another chemical which readily
gives up electrons - an "electron donor" - similarly held in close contact with a metallic
member through which electrons can be conducted to the external circuit.
- The "electrolyte," usually a liquid solution that permits the transfer of mass necessary
to the overall reaction. This movement takes place by "migration" of "ions" - positively or
negatively charged molecular fragments - from anode to cathode and from cathode to anode.
![[Schematic of basic cell elements]](../images/batt2.gif)
A schematic diagram of these basic cell elements is shown above. The cell is shown connected
to a load - representing the discharge reaction. Charging is accomplished by connecting an electrical source in place of the load, thereby reversing the entire process.
Excerpts from "Space Batteries" by Howard T. Francis. National Aeronautics and Space Administration, 1964.
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