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How is a battery recharged ?
First, let's hark back to what the electricity is.
Electricity is negatively charged particles, the electrons,
that move along in the electrical circuit.
Therefore to produce electricity,
electrons have to be produced.
Within a battery,
these electrons are produced by a chemical reaction.
There are actually a lot of chemical reactions
that release electrons.
For instance,
all we have to do is to plunge some metals
such as zinc, nickel, iron, cadmium or lead
into acid so that these metals yield electrons.
It is very easy to obtain such a chemical reaction.
Let's do a small experiment,
take a paper clip which is made of steel covered with zinc
and knock it in a lemon.
The zinc oxidises in contact with the citric acid
releasing electrons.
Now, how these electrons can be retrieved to light a bulb?
Let's go back to the chemical reaction.
Metal atoms have released electrons
and therefore become electrically charged themselves:
atoms have transformed into ions.
So, there are negatively charged electrons
within the battery or the lemon
as well as positively charged electrons on the other hand.
Electrons are no longer attached to an atom,
they are somewhat “homeless”,
Electrons are going to look for another home:
they will try to find another atom.
So in the battery,
another chemical reaction is occurring,
which this time is going to somewhat absorb electrons.
In a lemon,
this reaction transforms hydrogen ions of the citric acid
into hydrogen gas.
In fact, both chemical reactions,
the one producing electrons and the one absorbing them,
are indivisible.
Electrons can be released
only if they are immediately reabsorbed.
Therefore all the art of making a battery
consists in managing that electrons,
between both reactions,
take the electrical circuit that we want to supply,
rather than going straight away to the positive ion,
that will be their new home.
In the lemon,
the piece of zinc just has to be connected to the bulb
through a copper wire,
and then the other electrode of the bulb to the lemon
thanks to another copper wire:
the bulb lights,
because the electrons go through the circuit
before recombining with the hydrogen ions.
Unfortunately,
the bulb will not remain lit endlessly,
because the chemical reactions
deplete the chemical products
that are within the lemon.
When there will be no zinc or hydrogen ions anymore,
chemical reactions will stop,
and there will be no more current in the circuit.
This is also what happens in a traditional alkaline battery,
where the exchange of electrons
does not occur between zinc and hydrogen,
but between zinc and manganese oxide.
Once the battery has run out of power,
how can we recharge it?
Well chemical reactions involved in the battery
are reversible.
It means that if electricity is provided,
chemical reactions are reversed:
in the lemon,
hydrogen ionises and zinc ions transform back into metal.
Well, it is not that simple because in the lemon,
hydrogen, which is a gas,
has spread in the atmosphere.
So for the lemon battery, that's the end of it!
When the battery has run out of power,
the lemon has to be changed.
So to obtain a rechargeable battery,
an association of chemical reactions that can be reversed
has to be find.
It is the case for instance in NiMH rechargeable batteries,
where metal hydride (MH of NiMH)
transforms into the corresponding metal
by releasing electrons,
while nickel oxyhydroxide (Ni of NiMH)
transforms into nickel hydroxide
by capturing electrons.
The reaction is reversible:
while providing electrons (that is electricity),
initial chemical products form up again,
and the battery can supply electricity again.
Production: Unisciel/ University of Lille 1 (SEMM)
Conception/Production: Maxime Beaugeois, Damien Deltombe and Daniel Hennequin
Editing/Special effects: Benoît Leleu
Music: Sébastien Ride, « Thunder Chacha » (SR Music)
Presentation: Maxime and Nina Beaugeois
Graphic design/Credits animation: Michaël Mensier.