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BATTERIES Storing energy

On a sunny day Solar Impulse produces more electrical energy than it needs. But on cloudy days or at night it needs extra power. That is why surplus electricity from sunny days has to be stored, and this is where batteries come in.

In this worksheet you will learn about how electrical energy can be stored and how to build a battery yourself.

Project: EPFL | dgeo | Solar Impulse

Writing: Michel Carrara

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

Project follow-up: Yolande Berga

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WHAT IS A BATTERY?

Once its solar panels have transformed light into electrical energy, Solar Impulse uses this energy to power its motors that are driven by electricity. At the same time, some energy has to be set aside to drive the motors during the night or on less sunny days. That is why, while the sun is out, Solar Im-pulse’s solar panels produce more current than they need. The surplus is stored in its batteries and can be used when the solar panels stop working (see the worksheet SOLAR CELLS).

A dictionary definition of a battery would be a “usually large group of similar people, things, or ideas that work together, are used together.” But what kind of “things” are we talking about here? In this particular case, we are talking about accumulators, or storage batteries. And what is an accumulator? According to Wikipedia, it is a rechargeable battery, a type of electrical battery that can be charged, discharged into a load, and recharged many times. In summary, an accumulator stores electricity so that it can be delivered later on, and a battery is made up of several accumulators.

Electric eels

A characteristic property of these fish is that they have elec-trical organs – electroplaques – in the rear part of their body. These electroplaques can generate electric discharges of 100 to 700 volts that are powerful enough to fatally electro-cute humans.

opencage (CC-BY-SA) Stan Shebs (CC-BY-SA)

Solar Impulse is equipped with four batteries that are locat-ed in four engine nacelles under its wings. Each battery is made up of multiple lithium polymer cells that are connected to each other to achieve the required voltage. What makes Solar Impulse’s batteries stand out is the excellent ratio be-tween their weight, their efficiency, and their life-span. The HB-SIB plane’s batteries are particularly revolutionary. Their secret lies in the com-plex chemical formula that minimizes oxidation, reducing wear. The new batteries guarantee power for up to 2000 flight hours, compared to 500 in the case of the batteries used by HB-SIA. The batteries are located next to the engines to minimize losses.

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ELECTROCHEMICAL CELLS

To understand what a battery is we first have to understand what electrochemical cells are. A battery is a device that transforms the energy stored in its components into electricity. The energy in a battery is stored as chemical compounds that react with each other when the battery generates an electric cur-rent. It is this reaction and the resulting exchange of electrons between the components that produce the electric current. The reactions are referred to as redox reactions. Because these chemical reactions produce electricity, the devices are also referred to as electrochemical cells.

WET CELLS

A wet cell is composed as follows:

• Two electrodes made of conducting materials (usually metal or carbon).

• One or more solutions containing different types of salts (which can also take the form of gels in dry batteries).

• A salt bridge.

The most common types of electrochemical cells are:

• wet cells

• dry cells

• fuel cells

These three types of electrochem-ical cells all work according to the same principle. In each type of cell electrons are exchanged between two conductors, or reactants (usu-

The salt bridge is made of a hollow U-shaped tube that is filled with a conductive and concentrated gel (or of a piece of blotting paper soaked in a concentrated saline solution). The ions that are present in the salt bridge, such as the Na+ and Cl– ions from the chlorine and the sodium in the NaCl) do not participate in the reaction that produces the electric current. They are said to be chemically inert. All they do is create a passage for the current inside the battery, as electric circuits always have to be closed.

ally metals). In the process the reactants transform into products. The electrons are not exchanged directly, but via an external circuit, such as an electronic device, thereby creating an electric current.

Reactants Electriccurrent

External circuit(a device that needs electricity to work)

Electriccurrent

An electric current is a flow of electrons

Products

electrons electrons

Saline bridge Metalelectrode

Metalelectrode

Salinesolution

Salinesolution

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Never throw your batteries in the garbage!

Close to 3800 tons of batteries are sold each year in Switzerland – a figure that has remained close to constant over the past years. Most of these batteries are non-rechargeable. Whether or not they are rechargeable, batteries are the most polluting objects that find their way into our trash bins. They contain a high con-centration of heavy metals and other substances that are dangerous to both our health and the environment. Their impact can easily be reduced. All we have to do is collect them separately so that they can be treated by dedicated recycling facilities.

John Seb Barber (CC-BY-SA)

Source : vd.ch/themes/environnement/developpement-durable/dd-au-travail/fiches-dd-info/piles-et-batteries

Ever since the turn of the century, the Canton of Vaud has collected about 65% of the 300 tons of batteries consumed on its territory. Compared to the 20% that were col-lected in 1990 the progress is encouraging, but it is not enough. The national goal has been set at 90%!

Making non-rechargeable batteries consumes more than 50 times the energy that they provide (for alkali batteries). By contrast, the ratio is between 3 and 5 for rechargeable batteries, such as the ones used in mobile phones.

Here are some good habits to adopt when using batteries:

• Choose devices that you can plug in rather than devices that run on batteries.

• Use rechargeable batteries.

• Dispose of your used batteries in designated bins that you can find in most super-markets and waste collection sites.

• When you throw away electronic devices (alarm clocks, toys, gadgets, etc.) don’t forget that they too often contain batteries. Be sure to remove them first.

Let’s take a look at a battery made of copper (Cu) and iron (Fe). The salt bridge is made of a piece of cloth soaked in a concen-trated saline solution (NaCl).

If we connect iron and a solution that contains Cu2+ (formed when a copper salt, e.g. copper sulfate CuSO4, dissolves), these reactants exchange electrons and the iron oxide turns black:

Fe + Cu2+ Fe2+ + CuReactants Products

BATTERIES 5/12

When iron is dipped into the copper sulfate solution, it loses electrons (e–) and is transformed into Fe2+. The transformation of the iron alone can be written as Fe Fe2+ + 2e–, and we say that the iron is oxi-dized and dissolves in the solution. As for the dissolved copper Cu2+, it takes up two electrons and turns solid. This reaction can be written as Cu2+ + 2e– Cu, and we say that the copper is reduced. This is why the overall reaction is called a redox reaction: one compound is reduced, while the other is oxidized.

To make a battery using these two components, you have to separate the copper and the iron to recov-er the electrons that are exchanged between the Cu2+ and the Fe so that they form an electric circuit. The battery, or electrochemical cell, is made by connecting two half cells with a salt bridge. A beaker with the copper sulfate solution (CuSO4) with a copper strip as an electrode can be used as one of the two half-cells. The second one can be a beaker with an iron sulfate solution (FeSO4) with an iron strip or a nail as its electrode.

Salt bridge (filled with NaCl) Copper stripIron strip

CathodeAnode

Cu2+ solution Fe2+ solution

Voltmeter

In this part of the battery, the iron is oxidized:

Fe Fe2+ + 2e–

Metal iron is “consumed” and ends up dissolved in the solution as Fe2+ ions.

The iron strip dissolves.

In this part of the battery, the copper is reduced:

Cu2+ + 2e– Cu

Cu2+ ions take up electrons that end up being deposited on the copper strip.

The Cu strip becomes ”fatter.”

But nothing happens when you dip a strip of copper in a solution that contains Fe2+.

No reaction between Fe2+ and Cu The Cu2+ and the Fe react: the nail turns black.After 12 hours, you can clearly see that the copper has coated the nail.

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OTHER TYPES OF BATTERIES

Dry cells and fuel cells work in the same way as wet cells, with electrons being exchanged between two conducting materials. The main difference is that the reactions do not take place in a liquid, but rather in a thick paste or in a solid.

Wet and dry cells are the most common types of cells (classic cells). Fuel cells, which are more recent, are only now beginning to be used in electric cars.

There is one big difference between classic cells and fuel cells.

By contrast, in fuel cells, the raw materials are in-troduced as they are consumed. In principle, this type of cell can produce electrical energy as long as it is fed with reactants.

In classic cells, the reactants are added all at once, in a finite amount, when the batteries are made. When the reactants are spent, the battery is replaced by a new one. This is the case with the “agro-cells” that we will see later on.

FabricationFactory

Electricity

Classic cells

Reactants

ProductsRecycling

FabricationFactory

Electricity

Fuel cells

Reactants

ProductsRecycling

How do you store the energy produced by solar cells as efficiently as pos-sible? What solutions can help guarantee the availability of electricity in dif-ficult flight conditions, over and over again, and remain reliable for as many cycles as possible?

STEFAN GEHRMANN, AERONAUTICAL ENGINEER AND FOUNDER OF AIR ENERGY

PORTRAIT

After obtaining a degree in aeronautical engineering in Germany, Stefan Gehrmann founded his own battery manufacturing company, Air Energy. As founder and CEO, his job is to design and build batter-ies for prototypes in a broad range of areas, for planes, cars, or submarines. Because he was one of the few people in Europe who made lithium batteries, Stefan was contacted by Solar Impulse in 2002 before the first plane was built. Each battery in the plane is made of lithium polymer cells. These cells, built by a Korean company, are assembled and packaged by Air Energy before they are shipped to Solar Impulse. Air Energy also designed and built the control systems for the batteries on the HB-SIA and the HB-SIB planes. These systems, which monitor and control how the batteries are consumed and recharged, handle all communication between the plane and its batteries. What Stefan loves about his work is that he gets to participate in all kinds of projects, often at the prototype stage. It takes a a lot of flexibility and creativity to face new challenges and find solutions for projects with such different objectives and needs.

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RECHARGEABLE BATTERIES

Rechargeable batteries work just like ordinary batteries when they are used to produce electrical ener-gy. As we saw earlier, this energy is the result of chemical reactions.

Oxidation reaction at the negative terminal: Fe Fe2+ + 2e– Reduction reaction at the positive terminal: 2 H2O + 2e– 2 OH– + H2

Note that instead of using potatoes, you can use fruits (lemons, oranges, etc.), pots filled with moist soil, or vegetables (e.g. shallots). The inside of the plants acts as a salt bridge, closing the electric cir-cuit. That is why so you can use such a large variety of fruits and vegetables.

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

When the reactants are used up, the electric current can be reversed using an external pow-er source. Reversing the current recharges the batteries, as the electric energy regenerates the reactants from the products.

Contrary to ordinary batteries, rechargeable bat-teries can electro-chemically transform energy in both directions.

Fabrication Recycling

ElectricityElectricity

Accumulator

dischargingcharging Reactants

Products

Factory

Nail Nail Nail NailCopper Copper Copper Copper

LED, motor or multimeter

Materials

• 7 to 8 potatoes

• nails

• copper (a strip or 5 euro cent coins)

• electric wires

• crocodile clips

• a multimeter

• an LED or a low powered electric motor

Stick a nail and a strip of copper into each potato, about two centimeters apart. The copper is the positive terminal of the battery and the iron (nail) the negative one. If you hook up the diode to a single potato battery, it will not light up.

To get the diode to light up, you have to line up several potatoes, between five and seven, in series, or in a line. Be sure to connect the potatoes correctly, otherwise the agro-battery will not switch on the diode: the negative terminal of one potato (the nail) has to be connected to the positive terminal (the copper strip) of the next one in the series.

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Planté’s rechargeable battery is made up of two lead electrodes that are submerged into an acidic me-dium containing sulfuric acid H2SO4. One of the electrodes is covered in lead oxide (PbO2). The sulfuric acid provides the H+ ions that are used in the chemical reaction at the positive terminal, as outlined below.

At the negative terminal, this reaction takes place during discharge: Pb Pb2+ + 2e–

At the positive terminal, this reaction takes place during discharge: PbO2 + 4H+ + 2e– Pb2+ + 2H2OThe natural reaction between these pairs is: PbO2 + 4H+ + Pb 2Pb2+ + 2H2O To recharge the battery, an electric current is sent through the battery in the opposite direction, regen-erating the components (PbO2 et Pb). The voltage imposed at the terminals must be higher than the voltage that is generated by the reactions that create the current.

Gaston Planté (1834 - 1889) was a French physicist and inventor, who is best known for inventing rechargeable batteries (lead batter-ies).

The first rechargeable bat-tery to be commercialized was invented by Gaston Planté in 1859. Batteries of this type are still com-monly used today, for ex-ample in cars.

PbPb covered with PbO2

CathodeAnode

SO42–

H+

Direction of the electrons

Direction of the current

Oxydation

Pb2+ + 2H2O PbO2 + 4H+ + 2e–

Réduction

Pb2+ + 2e– Pb

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TECHNOLOGY: BUILD A BATTERY

A SIMPLE FUEL CELL: A WET ALUMINUM – AIR CELL

Materials

• 60 g of NaCl (table salt)

• 300 ml of water

• 1 large sheet of household aluminum

• 1 large sheet of kitchen paper

• Steel wool

• 1 container that holds at least 500 ml

• 1 multimeter

• 2 insulated copper wires

• 2 paperclips of crocodile clamps

Instructions1) Pour the salt into the container and dissolve it in 300 ml of water. 2) Crumple the aluminum foil into a ball and poke holes into it with a fork to release as much air out of

the ball as possible. 3) Use a crocodile clamp to attach a wire to the aluminum ball.4) Place the aluminum in the container and pack it tightly against the bottom.5) Completely cover the aluminum with the kitchen paper to insulate it from the next layer (steel wool). 6) Use the other crocodile clamp to attach a wire to the steel wool.7) Submerge the steel wool in the saline solution.8) The fuel cell is ready to go. The fuel it uses is the oxygen in the air. You can measure the voltage of

the fuel cell using a multimeter.

A SIMPLE FUEL CELL: A SOLID ALUMINUM – AIR CELL

Materials

• 1 sheet of household aluminum foil (15 × 15 cm)

• 1 piece of kitchen paper 9 × 12 cm

• 5 to 6 pencil leads (2 mm in diameter)

• 1 metal clamp

• 1 gas burner

• 1 saturated NaCl solution *

• 1 beaker of 50 ml or a glass

• 1 multimeter

• 2 insulated copper wires

• 2 paperclips or crocodile clamps

* NaCl (sodium chloride) is table salt. To prepare a saturated solution of table salt, pour small amounts of salt into hot, ideally distilled water, and stir until it is completely dissolved. Continue adding more salt until it no longer dissolves in the water (saturation). Once the solution has cooled down to room temperature, it is ready. Some salt may have precipitated as the solution cooled down. This is because it is slightly less soluble at low temperatures than at high tempera-tures. At 25°C, a saturated saline solution contains about 350g/l of NaCl.

Steel wool

NaClsolution

Kitchen paper

Aluminium

Volt

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Instructions 1) Wrap the extremity of the metal clamp with the aluminum foil so that you grasp all of the pencil leads

at the same time.

2) Hold the pencil leads using the clamp and stick them into the flame of the gas burner. Heat them up so that they become red hot for one minute and let them cool down.

3) Keep the pencil leads in a tight bundle and wrap them up using the kitchen paper, leaving 1-2 cm of the unburned side of the pencil leads exposed to the air.

Once you have wrapped the leads with half of the length of the kitchen paper, fold up the excess paper and wrap the remaining paper around the leads.

4) Next, wrap the same bundle of pencil leads with the aluminum foil. Leave about 1 cm of kitchen paper uncovered. Once you have wrapped the bundle with about half of the aluminum foil, fold it up and wrap the remaining foil around them.

5) Submerge the wrapped bundle of leads into the saturated saline solution until the kitchen paper is completely soaked. Remove the bundle and let it drip dry.

6) Hook up an electric wire to the end of one of the pencil leads and another to the aluminum foil. Your fuel cell is ready. The fuel used is the oxygen contained in the air. Measure the voltage of your fuel cell using the multimeter.

Volt

BATTERIES 11/12

4) Connect a 4.5 V battery for 5 min-utes to charge your battery.

5) Now, connect a digital multime-ter to the terminals and you can read the voltage. Your battery is charged.

Instructions

1) Cut out 10 to 15 shapes (circles, rectangles, etc.) from the sheet of copper and the same number of shapes from the blotting paper. Let a flap stick out of two of the copper shapes. Later, we will use them to attach the crocodile clamps. Copper is often covered with a protective coating, so be sure to rub it off using steel wool (and not with a detergent).

2) Soak the blotting paper with the saturated saline solution.

A SIMPLE RECHARGEABLE BATTERY: A SECONDARY RITTER CELL

Materials

• 1 saturated NaCl solution *

• Sheets of copper

• Sheets of blotting paper

• Styrofoam (at least 5 cm thick)

• 4 wooden skewer sticks)

• 1 stone or a weight

• 1 beaker of 50 ml or a glass

• 1 pair of scissors

• 1 flat 4.5 V battery

• 1 multimeter

• 2 insulated copper wires

• 2 paperclips or crocodile clamps

A stack of alternating sheets of copper and blotting paper on a block of Styrofoam

4 wooden skewer-sticks help keep the stack in place

* NaCl (sodium chloride) is table salt. To prepare a saturated solution of table salt, pour small amounts of salt into hot, ideally distilled water, and stir until it is completely dissolved. Continue adding more salt until it no longer dissolves in the water (saturation). Once the solution has cooled down to room temperature, it is ready. Some salt may have precipitated as the solution cooled down. This is because it is slightly less soluble at low temperatures than at high tempera-tures. At 25°C, a saturated saline solution contains about 350g/l of NaCl.

3) Make a support structure to hold the cut out shapes in place using the Styro-foam and the skewer sticks. Stack up the shapes starting with one of the copper shapes that have a protruding flap, and then alternate between blotting paper and copper. End with the second copper shape with a flap. Your rechargeable bat-tery is ready.

To optimize the contact between the elements, add a weight on top of the stack.

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

Materials

• 1 solution of 0,1 mol/l NaOH *

• 3 nails

• 1 pair of diagonal pliers

• 1 piece of nickel

• 1 screw-terminal

Instructions

1) Push one of the two nails through one side of the screw-termi-nal. On the other side, attach the crocodile clamp, (which you will probably have to bend into shape to make it fit) and the top half of the second nail (cut with the diagonal pliers) as shown in the picture.

Use the crocodile clamp to hold the nickel.

2) Submerge the electrodes you just assembled into the NaOH solution. Your battery is ready.

3) Charge your battery for 30 sec-onds to a minute by connecting the positive terminal of the 4.5 V battery to the nail that is in con-tact with the piece of nickel.

4) Now, hook up the digital multi-

meter to the terminals and you will see a voltage. Your battery is charged.

* NaOH (sodium hydroxide) is caustic soda. To make a 0.1 mol/l solution, dissolve 4g of NaOH into 1 liter of water. BE CAREFUL: solid sodium hydroxide is very corrosive. Students must not handle it when it is solid. They can, however, handle the 0.1 mol/l solution.

• 1 beaker of 50 ml or a glass

• 1 flat 4,5 V battery

• 1 multimeter

• 2 insulated copper wires

• 2 paperclips or crocodile clamps

nail

screw-terminal

shortenednail

NaOHsolution

nail

crocodileclamp

nickel