SOLAR IMPULSE - LAB WORK - BATTERIES (ENG)

1/15

BATTERIES Storing energy

This worksheet provides an explanation on how chemical com-pounds are used to produce electricity (voltaic cells) and store it (batteries).

It includes several practical applications. Students can build simple voltaic cells and batteries that illustrate how electrical energy is produced and stored. At the end of the chapter there is a simple application that involves the use of a corrosive com-pound, which is why it is suggested as a possible extension.

Note that the worksheet “SOLAR CELLS” explains how elec-tricity can be produced from light energy. Together with the worksheet on “MOTORS,” these two worksheets provide a good overview of the energy cycle used by Solar Impulse: produc-tion, consumption, and storage.

Project: EPFL | dgeo | Solar Impulse

Writing: Michel Carrara

Graphic design: Anne-Sylvie Borter, Repro – EPFL Print Center

Project follow-up: Yolande Berga

Concepts coveredScience:

• Electricity

• Voltaic piles

• Batteries

Activity durationTheory: 2 periods

Experiment: 4 experiments, 2 periods each

Some of the activities involve a significant amount of preparation by the teacher.

BATTERIES - GUIDE 3/15

HISTORICAL SIDENOTE

The history of storage batteries begins with the history of Voltaic piles.

At the end of the 18th century, the Italian doctor and physicist Luigi Galvani noticed that the muscles of frog legs contracted when they came into contact with metals. Galvani also discovered that the con-traction is stronger when the instrument used is composed of two different types of metals. He referred to this newly discovered phenomenon as animal electricity.

Ten years later, Alessandro Volta made the first stacked voltaic pile, basing himself on Galvani’s work. Volta, an Italian physicist, discovered that it was possible to generate electricity using two metal plates that are connected by a conducting liquid. He stacked up zinc and copper disks separated by round cardboard disks that had been soaked in a saline solution to conduct the current, forming a pile. His invention became known as the Voltaic pile.

Five years later, in 1802, the German physicist and physiologist Johann W. Ritter discovered the prin-ciple behind rechargeable batteries. He made a stack of copper disks that were each separated by cardboard disks soaked in a saline solution. Although his device did not produce a current on its own, he noticed that it could be charged by connecting it to a Voltaic pile. Moreover, the process could be repeated several times. He called his invention a secondary cell. The storage battery, or accumulator, was born.

Also in 1802, Doctor William Cruickshank designed the first electric battery that could be mass-pro-duced. He arranged square sheets of copper that were soldered at their extremities, and placed sheets of zinc of the same size between them. These sheets were then put into a long rectangular wooden box that was sealed shut using cement, to make it watertight. Slots inside the box helped keep the metal plates in place. The box was filled with a saline aqueous solution (lie) or a dilute acid. For quite some time, batteries contained primary elements, which meant that they were not rechargeable.

In 1859, Gaston Planté developed the first lead accumulator, which is still found in car batteries today. His device consists of two sheets of lead that are rolled up into spirals and kept apart with insulating strip. Connecting a battery to his device oxidized the positive electrode, while the iron oxide on the negative electrode was reduced, polarizing the electrodes. Connecting the two terminals, Planté ob-served an electric current that had not been there when the electrodes were in their original state. To increase the surface of the electrodes, Planté rolled up two strips of lead that were separated by two rubber spacers to keep them from touching each other.

In 1901, Thomas Edison invented the iron-nickel battery, which went on to be used to illuminate mines and drive carts.

But it would take almost a century for lithium-ion batteries, first commercialized by Sony Energitech in 1991, to appear. Today, lithium-ion batteries dominate the portable device market (for example in mobile phones).

This worksheet is designed to familiarize your students with batteries. It also introduces some notions of the history of science.

4/15 BATTERIES - GUIDE

BATTERIES

You cannot talk about batteries without talking about electricity. Here are some basic concepts that are needed to understand the topic of this worksheet.

The electrons will quickly reach point A, where they will neutralize the positive charges. At the same time, the flow of electrons away from B will exhaust the supply of electrons. The short-lived flow comes to a halt. To maintain a current (the collective displacement of a group of electrons in a conductor), the electrons have to be removed from A and returned to B. This is what an electri-cal generator does. A generator is a device that acts as an “electron pump.”

In the case of batteries, a redox reaction continuously provides electrons to maintain the flow of the current, and then makes them disappear.

The origin of electric currentConnect two ends of an electric wire to two equal but opposite electric charges (+ and –) and an electric current will flow through the wire: free electrons in the wire will be attracted by the pos-itive charges (+) at point A and repelled by the negative charges (–) at point B.

Salt bridge (filled with NaCl) Copper stripIron strip

CathodeAnode

Cu2+ solutionFe2+ solution

Direction of the electrons

Direction of the current

Fe Fe2+ + 2e–

An oxidation reaction supplies electrons

Cu2+ + 2e– Cu

A reduction reaction removes the electrons

Free electrons in the metal and the direction in which they move

+A B

+ + ––

–

Générator

Conductor

+ –

A current in a conductor,created by a generator


Electrical circuitsAn electrical circuit is made up of an uninterrupted series of conductors that are connected to a gener-ator. If this sequence is interrupted, for example by a switch, the flow of the current through the circuit is interrupted as well. This explains why it is necessary to include a salt bridge connecting the containers of the wet cell: the electrical charges are able to flow only when the circuit is closed.

The direction of electrical currentsThe terminals of an electrical generator are marked with the symbols + and –. By convention, the direc-tion of the current is defined as follows: the current leaves the generator at the + terminal and reenters through the – terminal. This convention was established by André-Marie Ampère (1775 - 1836) at a time when the true nature of electrical currents was not yet understood. In reality, electrons flow through a circuit in the opposite direction: the electrons leave the generator through the – terminal and flow back into it through the + terminal.

The intensity of electric currentsAll collective motions, such as cars driving along a road, or a river flowing along its track, can be char-acterized by their flow rate. The intensity I of an electrical current in a conductor is the rate at which electrical charges flow through it. That means that the intensity of a current is determined by the num-ber of electrical charges that flow through a cross section of the conductor per second. The unit used to quantify the intensity of an electrical current is the ampere [A], in recognition of André-Marie Ampère.

The voltage between two points in a circuit For electrical charges (electrons) to flow through a conductor, they have to be given a large enough amount of energy, which is referred to as the voltage U. Voltage can be compared to what happens in a waterfall. The amount of energy gained for each meter a drop of water falls is the same, whether it falls from 15 m to 14 m or from 1 m to the ground. A charge has an energy because, when it is repelled by the negative and attracted by the positive terminal of a battery, it “naturally” flows from the – to the + terminal. As a charge flows in this direction, its energy decreases. By definition, the voltage between two points in a circuit is the amount of electrical energy consumed to displace a charge from one point to the other. The unit used to quantify voltage is the volt [V], in recognition of Alessandro Volta.

The resistanceThe current that a battery can push through an electric circuit is similar to the water than can be pushed through a tube by a pump: the higher the pressure of the pump, the greater the flow rate through the tube becomes. In the same way, the higher the voltage of the battery, the greater the intensity of the current that is generated in the circuit. The resistance R of a conductor is defined as the ratio between the voltage U that is applied to a conductor and the intensity I of the current that it generates.

Ohm’s Law: U = R ∙ I

R the electrical resistance [Ω] U the voltage [V] I the intensity of the electrical current [A]


Do it yourself: Make a simple dry cell using potatoes (“Agro-Battery”)

Cut the bottom of a potato so that it stays upright on a flat surface.

Make a slit using a knife and insert the copper strip.

Stick in a nail at the chosen distance.

Distance [cm]

Voltage [V]

0.5 0.553

1 0.553

1.5 0.553

2 0.556

2.5 0.550

3 0.551

3.5 0.532

4 0.508

4.5 0.460

Voltage [V]

Distance between the two electrodes [cm]0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

0.6

0.5

0.4

0.3

0.2

0

0.1

Beyond 3 cm, the voltage decreases, which is why a spacing of 2 cm was chosen.

3.5 cm between the electrodes 2 cm between the electrodes

Extension: analysis of the potato cell. (This could serve as the basis for a practical lab session for stu-dents interested in science or physics.)

Influence of the distance between the nail and the copper strip


Number of potatoes

Voltage [V]

1 0.534

2 1.048

3 1.300

4 1.838

5 2.363

6 2.907

7 3.410

Time [min]

Voltage [V]

0 0.540

1 0.539

2 0.537

3 0.536

4 0.535

5 0.535

10 0.532

15 0.529

20 0.525

25 0.519

30 0.516

35 0.520

40 0.517

45 0.514

50 0.513

55 0.513

60 0.512

Voltage [V]

Number of potatoes0 1 32 4 5 6 7

3.5

3

2.5

2

1.5

1

0

0.5

Variation of the voltage over time (1 potato, 2 cm between the electrodes)

The Influence of the number of potatoes (2 cm distance between the electrodes)

Voltage [V]

Time [min]0 5 1510 20 25 30 35 40 45 50 55 60

0.6

0.5

0.4

0.3

0.2

0

0.1

For at least one hour, the voltage remains more or less stable (in 60 minutes the voltage drops by 5%). You can easily work with this battery during a two hour practical lab session.

Connecting the potatoes in a series leads to the expected result: the voltages of each individual battery add up. With 5 to 7 potatoes, we are able to switch on a diode.


TECHNOLOGY: BUILD A BATTERY

A SIMPLE FUEL CELL: A WET ALUMINUM – AIR CELL

Below are the reactions that occur in this type of fuel cell. For the aluminum (anode): Al + 3 OH– Al(OH)3 + 3e–

For the air (cathode): O2 + 2 H2O + 4e– 4 OH–

The overall reaction is: 4 Al + 3 O2 + 6 H2O 4 Al(OH)3

The steel wool does not react. Its purpose is to act as a cathode for the current to flow through.

Here, the voltage is around 300 mV.

The influence of the choice of fruit or vegetable

orange: 0.532 V lemon: 0.601 V shallot: 0.434 V

The same exercise can be repeated with other types of fruits and vegetables.


A SIMPLE FUEL CELL: A DRY ALUMINUM – AIR CELL

Below are the reactions that occur in this type of fuel cell. For the aluminum (anode): Al + 3 OH– Al(OH)3 + 3e–

For the air (cathode): O2 + 2 H2O + 4e– 4 OH–

The overall reaction is: 4 Al + 3 O2 + 6 H2O 4 Al(OH)3

The carbon in the pencil leads does not react. Its purpose is to act as a cathode for the current to flow through.

Here the voltage is about 1,2 V.

This dry cell can power a musical postcard or a small low-power electromotor, such as the one presented in the “MOTORS” worksheet.


A SIMPLE RECHARGEABLE BATTERY: A SECONDARY RITTER CELL

Instructions

1) It might be a good idea to cut out the shapes before carrying out the experiment with the students, as this first step takes a lot of time.

3) The contact between the individual elements can be improved by placing a weight on top of the stack. If the weight is not heavy enough, you may even have to press down on the stack throughout the experiment. The NaCl solution may spill out, carrying copper oxide with it; the workbench will have to be cleaned following the experiment.

If you connect a multimeter to the terminals of the setup at this time, no voltage will be measured. For more information on the details of this type of rechargeable battery, please refer to the measure-

ments reported below.

5) The following chemical reactions can be observed in the Ritter cell.

When it is being recharged, copper oxide Cu2O is formed (using water and oxygen from the air).At the anode: 2 H2O + O2 + 4e– 4 OH–

At the cathode: 2 Cu + H2O Cu2O + 2H+ + 2e–

The overall reaction is: 4 Cu + 4 H2O + O2 2 Cu2O + 4H+ + 4 OH–

Or simplifying: 4 Cu + O2 2 Cu2O

When it is being discharged.At the anode: 2 Cu2O + O2 4 CuO + 2e–

At the cathode: 2 Cu2O + 4e– 4 Cu + O2

The overall reaction is: 6 Cu2O + O2 4 Cu + 8 CuOWhen it is being discharged, the Cu+ (contained in the Cu2O) is transformed into Cu2+ at the anode (contained in the CuO) and into Cu at the cathode; this process is referred to as a disproportionation.

Using 20 copper disks, and after recharging the cell for 3 minutes, the voltage is approximately 120 mV.

At the end of the experiment, we can see that electrical charges have oxidized the copper disks. After three charge / discharge cycles:

Final note: The rechargeable battery presented here dries out quickly, in less than one hour. Once it is dry, it no longer works. All of the materials can be reused to make a new Ritter cell (blotting paper, copper; the copper has to be cleaned using steel wool.)


Analysis of the secondary Ritter cell(Can be used as the basis of a practical laboratory session for students interested in science and physics)

Influence of the number of copper disks on the voltage (charged at 4.5 V for 5 minutes)

Discharge time [min]

Voltage [mV] as a function of the number of copper disks

5 10 15 20

0 100 280 410 310

0.5 52 219 352 239

1 48 177 332 203

1.5 31 134 314 170

2 33 112 301 153

2.5 28 104 288 136

3 21 92 279 133

3.5 17 75 271 127

4 16 70 263 122

4.5 13 72 256 116

5 12 68 248 112


Tension [mV] as a function of the charging time

1 min 2 min 3 min 4 min 5 min

0 210 302 402 467 510

0.5 128 195 268 318 352

1 98 154 200 239 287

1.5 70 120 144 171 229

2 45 64 93 112 164

Influence of the charging time (15 copper disks, charged at 4.5 V)

We can see that the differences in voltage become smaller as we approach 5 minutes of charging using a 4.5 V battery (at t = 0 min in the table on the left), which is why we chose to charge our cell for 5 minutes.

With the copper sheet used here, we can see that we reach an optimum at 15 disks, which is why the remaining measurements will be made using a secondary Ritter cell with 15 layers. The appearance of an optimum can be explained quite simply. By using more elements we increase the maximum available charge, but at the same time, we increase the resistance, as well as problems caused by the contact between the individual elements of the secondary Ritter cell. Therefore, there is an optimum number of elements that depends on various factors, such as the thickness of the copper sheet, the thickness of the blotting paper, the quality of the contact between the individual elements, etc.

Voltage [mV]

400

350

300

250

200

150

0

50

100


5 disks 10 disks 15 disks 20 disks

0 1 32 4 5

Voltage [mV]

500

400

300

200

0

100


Charging time:

1 2 43 5

2 min 3 min 4 min 5 min1 min



Voltage [mV] as a function of the number of cycles

1 2 3

0 702 720 732

0.5 519 491 503

1 417 400 412

1.5 329 327 339

2 270 294 306

2.5 232 262 274

3 207 240 252

3.5 188 221 233

4 170 205 217

4.5 156 192 204

5 144 181 193

5.5 133 172 184

6 124 163 175

6.5 116 156 168

7 108 149 161

7.5 101 143 155

8 96 137 149

8.5 91 132 144

9 86 125 137

9.5 82 122 134

10 79 118 130

Influence of the number of charge/discharge cycles (15 disks, charged at 4.5 V for 5 minutes)

The cell can be recharged several times. We can see that the charge remains essentially the same even after 3 charge / discharge cycles.

Voltage [mV]

700

500

600

400

300

200

0

100


Number of cycles:

0 1 32 9764 85 10

2nd cycle 3rd cycle1st cycle



Voltage [mV] as a function of the number of cycles

1 2 3

0 200 340 360

0.5 160 260 297

1 130 240 212

1.5 130 195 172

2 110 174 140

2.5 120 135 124

3 120 140 110

3.5 110 120 95

4 100 97 88

4.5 110 94 87

5 90 85 83

5.5 95 80 78

6 80 74 71

6.5 80 68 70

7 80 65 69

7.5 60 68 65

8 55 61 63

8.5 40 55 58

9 45 51 62

9.5 35 47 57

10 30 46 55

Influence of the number of charge / discharge cycles (20 disks, charged at 4.5 V for 5 minutes)

Here, the copper disks were “new” during the first charging cycle, which is why the performance was worse than in the following ones. During the first cycle, there was probably still some coating on the copper disks, which would have made the cell less efficient. In fact, during the first charging cycle, the cell heated up more than usual, hinting at a higher consumption of current caused by the coating on the copper disks (and a consequently higher resistance). This heating was not observed in the following two charging cycles.

Voltage [mV]

350

250

300

200

150

100

0

50


0 1 32 9764 85 10

Number of cycles: 2nd cycle 3rd cycle1st cycle


A SIMPLE RECHARGEABLE BATTERY: EDISON’S NICKEL–IRON BATTERY

This experiment is easier to carry out than the Ritter cell, but it requires the use of 0.1 M NaOH, which must be handled with care.

Instructions

1) The piece of nickel used is a former French one franc coin.

2) If you connect a multimeter to the terminals at this moment, a weak voltage of a few mV may be measurable. This voltage decreases over time, dropping from 0.111 V to 0.078 V in three min-utes. It is caused by the different metals coming into contact.

3) When charging the battery, small gas bubbles can be seen on the nail and on the nickel. Applying a voltage of 4.5 V electrolyzes the water, which is decomposed into oxygen (O2) and hydrogen (H2). The oxygen appears on the nickel and the hydro-gen on the iron.

4) The battery gives about 1.5 V. For more details on this battery, see the results of

the analysis given below.

The following chemical reactions can be observed on this battery.

When it is being charged (dipping the iron and the nickel into the NaOH solution creates a layer of iron hydroxide Fe(OH)2 and nickel hydroxide Ni(OH)2 on their surfaces.)At the anode (the iron electrode): Fe(OH)2 + 2e– Fe + 2 OH–

At the cathode (the nickel electrode): Ni(OH)2 + OH– NiO2H + H2O + e–

The overall reaction is: Fe(OH)2 + 2 Ni(OH)2 Fe + 2 NiO2H + 2 H2O

When it is being consumed.At the anode: Fe + 2 OH– Fe(OH)2 + 2e–

At the cathode: NiO2H + H2O + e– Ni(OH)2 + OH–

The overall reaction is: Fe + 2 NiO2H + 2 H2O Fe(OH)2 + 2 Ni(OH)2

Once the cell has been discharged, the electrodes are again covered by iron and nickel hydroxide.


Analysis of the Edison Battery(can be used as the basis of a practical laboratory session for students interested in science and physics)

Influence of charging time (charged at 4.5 V)

Discharge Time [min]

Voltage [V] as a function of the charging time

10 s 20 s 30 s 60 s 120 s

0 1.558 1.558 1.548 1.563 1.521

0.5 1.465 1.467 1.471 1.485 1.446

1 1.395 1.400 1.406 1.427 1.394

1.5 1.328 1.338 1.348 1.371 1.346

2 1.274 1.284 1.301 1.322 1.309

2.5 1.225 1.237 1.257 1.278 1.268

3 1.186 1.195 1.217 1.238 1.240

3.5 1.115 1.164 1.183 1.201 1.209

4 1.113 1.127 1.156 1.172 1.170

4.5 1.079 1.091 1.124 1.148 1.150

5 1.051 1.061 1.090 1.114 1.128

This battery can be recharged very quickly, in just a few seconds, but it is also consumed very quickly. In 5 minutes, it loses 30% of its charge.

Influence of the number of charge / discharge cycles (charged at 4.5 V for 1 minute)

Discharge Time [min]

Tension [V] as a function of the number of cycles

1 2 3

0 1.521 1.532 1.555

0.5 1.446 1.560 1.462

1 1.394 1.401 1.416

1.5 1.346 1.354 1.360

2 1.309 1.313 1.325

2.5 1.268 1.273 1.270

3 1.240 1.239 1.223

3.5 1.209 1.205 1.184

4 1.170 1.178 1.151

4.5 1.150 1.155 1.110

5 1.128 1.134 1.077

5.5 1.109 1.106 1.050

6 1.090 1.080 1.025

6.5 1.067 1.057 1.004

7 1.046 1.037 0.986

7.5 1.026 1.018 0.971

8 1.008 1.001 0.958

8.5 0.991 0.984 0.948

9 0.976 0.969 0.932

9.5 0.961 0.957 0.925

10 0.948 0.945 0.920

Voltage [V]

1.6

1.4

1.0

1.2


Charging time [sec]: 12020 6010 30

1 2 43 5

Voltage [V]

1.6

1.4

1.2

1.0

0

0.8

0.2

0.4

0.6


0 1 32 9764 85 10

Number of cycles: 2nd cycle 3rd cycle1st cycle

Technology

SOLAR IMPULSE - LAB WORK - BATTERIES (ENG)