Solar Impulse: around the world without fuel to promote clean technologies.

1. 1/12
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

2. 2/12 BATTERIES
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.

3. BATTERIES 3/12
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
Electric
current
External circuit
(a device that needs electricity to work)
Electric
current
An electric current is a flow of electrons
Products
electrons electrons
Saline bridge Metal
electrode
Metal
electrode
Saline
solution
Saline
solution

4. 4/12 BATTERIES
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+
+ Cu
Reactants Products

5. 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+
solutionFe2+
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.

6. 6/12 BATTERIES
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.
Fabrication
Factory
Electricity
Classic cells
Reactants
ProductsRecycling
Fabrication
Factory
Electricity
Fuel cells
Reactants
Products
Recycling
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.

7. BATTERIES 7/12
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.

8. 8/12 BATTERIES
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+
+ 2H2O
The 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
SO4
2–
H+
Direction of the electrons
Direction of the current
Oxydation
Pb2+
+ 2H2O PbO2 + 4H+
+ 2e–
Réduction
Pb2+
+ 2e–
Pb

9. BATTERIES 9/12
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
Instructions
1) 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
NaCl
solution
Kitchen paper
Aluminium
Volt

10. 10/12 BATTERIES
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

11. 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.

12. 12/12 BATTERIES
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
shortened
nail
NaOH
solution
nail
crocodile
clamp
nickel