Page 55 - Handbook of Battery Materials

1.4 Criteria for the Judgment of Batteries 21

1.4
Criteria for the Judgment of Batteries
The need to operate electrically powered tools or devices independently of stationary
power sources has led to the development of a variety of different battery systems,
the preference for any particular system depending on the field of application. In
the case of a occasional use, for example, for electric torches in the household
or for long-term applications with low current consumption such as watches or
pacemaker, primary cells (zinc–carbon, alkaline manganese, or lithium–iodide
cells) are chosen. For many other applications such as notebooks, MP3-players,
cellular phones, or starter batteries in cars only rechargeable battery systems, for
example, lithium-ion batteries or lead–acid batteries, can be considered from the
point of view of cost and the environment.
The wide variety of applications has led to an immense number of configurations
and sizes, for example, small round cells for hearing aids or large prismatic cells like
lead–acid batteries for use in trucks. Here the great variety of demands has the con-
sequence that nowadays no battery system is able to cope with all of them. The choice
of the ‘right’ battery system for a single application is therefore often a compromise.
The external set-up of different battery systems is generally simple and in
principle differs only a little from one system to another. A mechanically stable cell
case carries the positive and negative electrodes, which are kept apart by means of
a membrane and are connected to electrically conducting terminals. Conduction
of the ions between the electrodes takes place in a fluid or gel-like electrolyte [13].
To assess the different battery systems, their most important features need to be
compared.

1.4.1
Terminal Voltage

During charging and discharging of the cell the terminal voltage U between
the poles is measured. Also, it should be possible to calculate the theoretical
thermodynamic terminal voltage from the thermodynamic data of the cell reaction.
This value often differs slightly from the voltage measured between the poles of
the cell because of an inhibited equilibrium state or side reactions.

1.4.2
Current–Voltage Diagram

An important experimentally available feature is the current–voltage characteristic.
This gives the terminal voltage provided by the electrochemical cell as a function of
the discharge current (see Figure 1.9). The product of current and the accompanying
terminal voltage is the electric power delivered by the battery system at any time.

P = I · U terminal voltage (1.32)