Cells and batteries
What is a cell?
Earlier we mentioned that Italian physicist Alessandro Volta developed the voltaic pile, which led to the creation of the battery. A battery is just a source of energy that you can use as the power supply in a circuit. A cell is a single unit of power capable of supplying energy. A battery is a group of cells that collectively supply energy. Most people interchange these two terms.
In operating a device that requires a battery, the energy supplied by the battery is then converted into other types of energy and some energy may be directed elsewhere in the conversion. This demonstrates the law of the conservation of energy that states that energy is neither created nor destroyed, but is converted from one form to another. Think of a battery in a torch used to produce light. Not all the energy will convert to light - some of the 'lost' energy creates a little heat.
The most basic type of battery consists of two metal rods called electrodes placed in an electrolyte, a type of conducting fluid. The electrodes must be made from different metals. Some combinations work better than others, though we know that zinc and copper work well together. The electrolyte is an acid, base or salt solution - you can even use the citric acid in a lemon.
A wet cell works by using the energy from the chemical reaction that takes place when you place the electrodes in the electrolyte. As the components react, displaced electrons flow from one electrode to the other, producing a charge.
To make a battery, choose a juicy lemon, then take a galvanised (nickel-coated) nail and a length of copper wire and insert both, a few centimetres apart, into the fruit. Using alligator clips, attach the ends of the nail and wire to a small load, such as a small light bulb (or a voltmeter, if you want to measure the voltage). Complete the circuit to activate your load.
Alessandro's voltaic pile consisted of a stack of similar components: alternate copper and zinc discs separated by cloth soaked in an acidic, basic or saltwater solution. The zinc became the negative terminal at the top of the pile and the copper became the positive terminal at the bottom. Refer Image 1
It is inconvenient to carry around the components for a wet cell, so modern batteries are slightly different. A common household battery uses carbon on the positive electrode and zinc on the negative and the electrolyte is a chemical paste rather than a liquid. All the components are contained within a casing to prevent the battery from discharging accidentally.
When you put the battery in a device, the conducting wires within the device complete the circuit, joining the positive and negative terminals. When you switch on the device, the charge flows from the negative to the positive, powering it. Refer Image 2
There are different types of batteries that use different electrodes and electrolytes, depending on the battery's use. You might have heard of alkaline, lithium or mercury batteries. Each of these has advantages and disadvantages according to its rate of discharge, its longevity (length of life) and its size.
Many devices require more than one battery. Batteries used in combination will increase the overall voltage, but you must be careful about using different types of batteries together, or batteries that have discharged at different rates, because the variance could affect the strength and rate of the discharge.
Rechargeable batteries, although they are dry cells, work in a slightly different manner. When the energy from the chemical reaction has been used up, you can recharge the battery by using another source of electricity. The external electricity makes the chemicals return to their original state so that when the battery is reused, the chemicals react to produce energy again.
Earlier we mentioned that semiconductors are substances that have a variable ability to conduct electricity, depending on factors such as temperature, humidity, pressure and the presence and absence of light.
Solar powered batteries use semiconductors that react to light. The cells become activated when sun strikes the panel and their movement produces a flow of electricity. A battery created in this manner is called a photovoltaic cell. Refer Image 3