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Science, Maths & Technology

# Challenge: Recharge a Battery

Updated Monday, 28th January 2008

Learn about the science behind charging batteries.

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Batteries store electrical energy. Some, like conventional torch batteries, are used then thrown away. Others are rechargeable so how easy is it to 'top up.' In this experiment our amateur scientists have only a generator and the challenge was to recharge an old car battery.

How does a battery work?

Every atom consists of a positively charged nucleus called a proton surrounded by a number of smaller, negatively charged particles called electrons. If atoms lose or gain electrons, they are left with a positive or negative charge and they are then called ions.

In an electrically conducting solid such as wire, some of the negatively charged electrons are able to move around fairly freely around the fixed array of positively charged protons. Although these 'free' electrons can move, there is a force which keeps the electrons - and so the charge - evenly spread along the length of the wire.

If all the electrons are concentrated at one end of the wire they would want to rush back to cancel out the charge imbalance. In doing so they would create an electric current for a very short time.

A conductor in its natural state - negative electrons (blue) and positive ions (red) evenly distributed along its length.

The same conductor in a state of imbalance where all the negative particles (blue) have been forced to one end.

If we want to maintain the flow of electrons we would need some sort of electrical pump. This is essentially what a battery does. It 'pulls' electrons from one end of a wire and pushes them to the other end. The energy to drive this flow is provided by chemical reactions within the battery.

Electrons flowing through the filament of the bulb cause it to become warm and emit light.

How do we go about charging a car battery?

A car battery is also known as a 'Lead Acid' battery. It is constructed of positive plates, negative plates and an electrolyte. The electrolyte connects the negative and positive plates chemically.

A battery only contains a fixed amount of reactants. When a lead acid battery becomes discharged both plates undergo chemical changes to form lead sulphate and eventually the battery becomes flat.

To recharge a flat battery, the electrical current is forced backwards. This causes chemical reactions which restore the plates to their original chemical composition.

Because we don't want to contaminate the battery with salts and minerals we must use distilled water to refill it.

What voltage are car batteries?

Car batteries are made up of six 'cells' each giving a voltage of 2.2 volts (2.2v) when fully charged and about 1.8v when flat. When a car battery is fully charged it has a voltage of 13.2v. So to fully charge a car battery we somehow have to produce a voltage of at least 13.2v (we'll aim at 14v) and force this back into the battery.

How do we charge a battery without mains electricity?

We can make electricity by constructing a basic generator. This would allow us to change kinetic energy into electrical energy, which in turn could be stored as chemical energy in the battery.

The simplest generator consists of a coil of wire and a bar magnet. When you push the magnet through the middle of the coil, an electric current is produced in the wire as electrons are forced along it. This method of generating electricity is called induction.

The amount of electricity can be increased by coiling the wire around an armature to concentrate the magnetic field. The armature can then be turned in such a way that many lengths of wire keep passing through the magnetic field. The voltage can be raised by increasing the number of 'windings' of the coil, increasing the strength of the magnetic field or increasing the speed at which the generator turns.

We now need to 'harvest' the electricity and take it to the battery using wires. The coil can't just be connected directly to the battery because as the armature turns, the wires would quickly become twisted. In addition, to charge a battery we need direct current rather than alternating current.

One answer would be to use a rectifier. Rectifiers need special materials, so on the island we used a commutator to produce a direct current.

How can we take electricity from the generator?

In order to take electricity from the generator we need to use a type of sliding connection.

By connecting the sliding connection brushes to each end of the coil in turn we capture a direct current as the armature turns.

How do we get this direct current electricity back into the battery?

We need to generate about 14v to charge the battery. If we produce much less we will never charge it and, more importantly, if we generate a lower voltage than the battery already has then electricity will begin to flow from the battery back into the generator. This will cause the battery to go flat and could cause a dangerous short circuit.

To avoid this, we incorporated a second circuit from the generator. In old fashioned car circuits this would be called a shunt circuit. When the current flowing through the shunt circuit reaches a certain level it creates a magnetic field that is strong enough to move a piece of iron, in our case a nail. This can act as a switch connecting the generator to the battery when the voltage is high enough. This is a basic solenoid.

Because we have a battery to store electrical energy, we only need to use the generator when it's convenient to do so. As we couldn't always depend on the wind or waves we took the easy option on the island and used muscle power.

References and links

How Batteries Work - on the How Stuff Works site

Advanced Physics by Tom Duncan, John Murray

Understanding Electricity (Science for Fun), Copper Beech Books

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