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PART 1
Universal Characteristics of Batteries
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Chapter 1
Definitions and Methods of Measurement
1.1. Introduction
This chapter provides definitions of the most frequently encountered terms used to express the electrical characteristics of a battery. These terms are used throughout the following chapters and so it is essential to gain a clear understanding of their meaning. Readers who work in the field of battery technology will be familiar with these terms, but it remains important to provide a reminder and, where necessary, clarify their precise meaning.
In addition to these definitions, we present the methods used to measure these characteristics, particularly those used in testing standards1.
1.2. Terminology
Before entering into technical detail, we must define the objects with which we are to work to avoid any ambiguity in what follows. It is essential to understand the characteristics of the types of batteries we are dealing with, along with terms such as “anode” and “cathode”.
1.2.1. Accumulators
Accumulators, of which rechargeable batteries are an example, store (accumulate) energy, converting electrical energy2 into a form of chemical energy, and then rendering up the energy on demand. An accumulator can therefore be considered to be a reversible generator; this differentiates secondary batteries from non-rechargeable primary cells.
While “battery”, in the strictest sense of the term, refers to a collection (or “pile”) of several electrochemical cells, in common parlance the word is also applied to single cells. For reasons of simplicity, this will also be the case here.
1.2.2. Cells, elementary cells and electrolyte
A cell of a battery, or elementary cell, is composed of two electrodes immersed in an electrolyte. These two electrodes constitute the couple: {PbO2/Pb} for lead batteries, {NiOOH/Cd} for nickel-cadmium (NiCd), or {lithiated metal oxide/lithiated carbon} for certain lithium batteries.
1.2.3. Electrodes and half-cells
An electrode is occasionally referred to as a “half-cell”. The two electrodes are known, respectively, as the positive and the negative electrodes. We shall avoid using the terms “anode” and “cathode” here as the electrodes switch roles depending on whether the battery is being charged or discharged.
1.2.4. Oxidation, reduction, anode and cathode
When discussing electrodes, we should go by certain definitions.
Oxidation is the reaction in which an atom or an ion loses one or more electrons. An electron donor is a reducer.
Reduction is a reaction by which an atom or an ion gains one or more electrons. An electron acceptor is an oxidant.
A reduction-oxidation reaction (redox reaction) may be written, generically, as follows:
The anode is the electrode where the oxidation reaction occurs and the cathode is the electrode where the reduction reaction occurs.
An electrode thus takes the role of a cathode or an anode depending on the direction of the current, i.e. whether the battery is charging or discharging. However, the positive (or negative) electrode will remain positive (or, respectively, negative) in both cases. The behavior of the two electrodes is shown in Table 1.1.
Table 1.1. Behavior of electrodes when charging and discharging
| Charging | Discharging |
| Positive electrode | Anode | Cathode |
| Negative electrode | Cathode | Anode |
Electrochemists very often misuse these terms, using “cathode” to designate the positive electrode and “anode” for the negative electrode, as in the early days of electricity generation by electrochemical reactions (at the beginning of the 19th Century), all batteries took the form of primary cells, in which the terms are indeed synonymous. Secondary (rechargeable) batteries came later, toward the middle of the 19th Century.
1.2.5. Active material
The chemical products used in the charging and discharging reactions constitute the active material of a battery. In a lead-acid battery, the active material is made up of lead (Pb), lead dioxide (PbO2), and sulfuric acid (H2SO4).
The container, separators, electrical connections, and chemical products that are identical to the active material but not accessible and so not involved in the charging/discharging reactions, are referred to as inactive materials.
1.2.6. Voltage
A cell produces a certain voltage. This is of the order of:
– 1.2 V for a nickel-cadmium (NiCd) or nickel-metal hydride (NiMH) cell;
– 2 V for a lead cell;
– from 1.8 V to almost 4 V (and even 5 V in research laboratories), depending on the choice of electrodes, for couples using lithium. The nominal voltage is usually from 3.6 V to 3.8 V for cells used in portable or mobile devices of the lithium cobalt nickel aluminum/graphite type. It is closer to 3.2 V for lithium phosphate/graphite cells, but only 1.8 V for LiFePO4/Li4Ti5O12 cells. The voltage at cellular level is noted as Vpc (V per cell).
1.2.7. Battery series, monoblocs, packs and BMS
With the notable exception of cellular phones, which operate using a single 3.7 V Li-ion cell, these voltages are too low for most applications, such as powering electronic devices, notably electric vehicles. Thus, these cells are rarely used singly; a series of cells will be used to obtain the desired voltage — a “battery” of cells, in the strictest sense of the term.
Rechargeable batteries are available:
– either in the form of a single cell;
– or as an indivisible association of several cells (usually three or six) in the same container, known as a monobloc.
The starting, lighting, and ignition (SLI) batte...
