Cells and Batteries

Learner Workbook

Version 1

SAMPLE Training and Education Support Industry Skills Unit

Meadowbank

Product Code: 5641

Cells and Batteries

Table of contents Introduction ...... 5

Section 1: Primary cells ...... 7

Review questions ...... 15

Section 2: Secondary cells ...... 19

Review questions ...... 27

Section 3: Cells - configurations ...... 31

Skill practice 3.1 ...... 37

Skill practice 3.2 ...... 39

Review questions ...... 42

Section 4: Secondary batteries ...... 45

Review questions ...... 52

Section 5: Battery Charging ...... 55

Review questions ...... 62

Section 6: Storage battery banks ...... 65

Skill practice 6 .1 ...... 74

Review questions ...... 77

Answers to Review Questions...... 79 SAMPLE

Cells and Batteries

Section 1: Primary cells

Objectives At the end of this section you should be able to:

• Define a primary cell • List the basic components of a typical primary cell • State applications of common types of primary cells • State the terminal voltages of common types of primary cells • Describe correct storage, handling and disposal procedures for primary cells.

Chemical sources of emf The conversion of chemical energy into electrical is done by chemical cells.

A cell is made when two different metals are immersed in a liquid conductor called an electrolyte.

Cells are classified into two general types; 1. Primary 2. Secondary When two or more cells are connected in series or parallel, (or a combination of both) they form a battery.

The chemical change that takes place within the cell causes a potential difference between the cell electrodes.

When an external circuit is connected this potential causes a current to flow.

SAMPLE

Figure 1.1 Conventional current flow and electron flow.

Cells and Batteries

Primary cells Historically primary cells were the first source of dc electricity capable of producing a steady current flow.

Primary cells convert chemical energy into electricity by an internal chemical process which depends on the materials used in the cell construction.

Primary cells undergo an irreversible chemical change, that is, once all the appropriate chemicals have been converted the cell is no longer able to produce electricity.

At this point the cell is then replaced by a new one.

Primary cells undergo an irreversible chemical change, that is, once all the appropriate chemicals have been converted the cell is no longer able to produce electricity.

At this point the cell is then replaced by a new one.

Figure 1.2 The Simple Voltaic Cell

The chemical action between the electrolyte and the dissimilar electrodes causes a potential difference of approximately 1V between the electrodes.

Conventional current will flow through an external load from the positive electrode (copper) to theSAMPLE negative electrode (zinc). Polarisation The simple voltaic cell is not very practical since the positive electrode becomes covered with hydrogen gas reducing the chemical change and decreasing the cells output. This build-up of hydrogen is known as "polarisation" and all primary cells are affected.

Various materials are used in the construction of the cell to decrease the effect of polarisation by combining the hydrogen gas bubbles with another substance.

These materials are known as "depolarising agents".

Page 8 of 88 © TAFE NSW (TES, Industry Skills Unit Meadowbank) 2012

Cells and Batteries

Cell potential The choice of electrode material determines the potential developed.

Some materials react more readily in their ability to accept electrons.

The electromotive force series tabulates the electrode potentials when referenced to hydrogen. Table 1.1 The Electromotive Force Series.

Metal Electrode potential in volts

aluminium - 1.67 zinc - 0.76 chromium - 0.71 iron - 0.44 cadmium - 0.40 lead - 0.13 hydrogen 0 copper + 0.34 silver + 0.80 gold + 1.42

The higher potential electrode becomes the positive electrode.

The potential of the other electrode is subtracted from the positive electrode potential to determine the cell emf.

Example: Copper - zinc 0.34 - (-SAMPLE 0.76) 0.34 + 0.76

= 1.1 V

Iron - zinc

0.44 - (- 0.76)

0.44 + 0.76

= 0.32 V

Cells and Batteries

Local action Small local cells are formed by impurities in the negative electrodes. These local cells produce heat and consume chemicals even when no external load is connected.

Figure 1.3 Local Action

Local action is decreased by using electrodes of the highest purity, or by coating the electrode with a material to neutralise the effect

Example: Zinc electrodes are coated with mercury, (mercuric amalgamation).

Shelf life Due to the effects of local action cells will discharge internally while not in use. Shelf life is approximately one year for carbon zinc primary cells.

Cell types The Leclanche cell This cell was the first successful attempt to overcome the polarising effects of the basic voltaic cell.

The original version was a "wet cell" as shown in Figure 1.4. SAMPLE

Figure 1.4 The Leclanche Wet Cell

Page 10 of 88 © TAFE NSW (TES, Industry Skills Unit Meadowbank) 2012

Cells and Batteries

The electrode materials are carbon and zinc immersed in a solution of ammonium chloride which is commonly called "sal ammoniac". The depolarising agent is manganese dioxide which combines with hydrogen to produce water.

The Leclanche "dry cell" is what today’s carbon zinc cells are modelled on.

Instead of a liquid electrolyte the dry cell uses a conductive paste which is unspillable. The terminal emf is approximately 1.5V .

Figure 1.5 The Leclanche Dry Cell

SAMPLE

Figure 1.6 Carbon Zinc Cell

Cells and Batteries

Figure 1.7 Alkaline Cell

Figure 1.8 Mercury Cell

Modern primary cells Carbon Zinc Cells Characteristics Emf 1.5V Low cost Low current SAMPLE Many sizes

Applications: Portable equipment used intermittently with medium power requirements

Alkaline Cells (zinc - manganese dioxide)

Characteristics Emf 1.5V Medium cost Longer life

Cells and Batteries

Higher current Applications: Portable equipment requiring heavy current drain

Mercury Cells (zinc mercuric oxide)

Characteristics Emf 1.3 V (nearly constant over entire life, dropping sharply when exhausted) High cost Compact Long shelf life Applications: Equipment with limited available space

Silver Oxide (zinc silver oxide) Characteristics Emf 1.5V (constant over entire life) High cost Compact (miniature) Excellent shelf life More service life than a mercury Applications: Extremely miniaturised electronic equipment

SAMPLE

Figure 1.9 Silver oxide Cell equipment.

Cells and Batteries

Lithium (Lithium thionyl chloride) Characteristics

Emf 3.0 V to 3.6 V High cost

Very long life

Compact Applications:

Portable equipment with low power drain and high supply voltage.

Cell use and disposal Never mix cells of different types in batteries.

Replace all cells in the battery when the battery fails.

Remove cells from equipment that is not being regularly used.

Never attempt to recharge primary cells as the cells may explode.

Never dispose primary cells in a fire or store them in areas of extreme high temperatures as the cells may explode.

Consider the environment when disposing cells, especially those which contain mercury.

SAMPLE