From 2GreenEnergy Intern Fabio Porcu – An Introduction to Batteries
In my last post, I covered the various types of energy storage. Now it’s time to analyze in detail the possibilities that the batteries offer in this field. We will discuss the various types of batteries, and note the characteristics of different battery types.
The working principle of a battery is a reversible chemical process called reduction-oxidation (or “redox”), a process in which one component is oxidized (loses electrons) and the other is reduced (gains electrons). These are processes whose components are not consumed or lost, but merely changed in their oxidation state and which may return to its original state in appropriate circumstances. These circumstances are closing an external circuit during the discharge process, and applying a voltage from an external source during charging.
Nineteenth century researchers devoted a great deal of effort in observing and clarifying this phenomenon, which was called polarization. This process is common in other relations between chemicals and electricity, e.g., in a process called electrolysis.
A battery is a device in which the polarization reaches its achievable limits, and generally consists of two electrodes, the same or different material, immersed in an electrolyte.
I will write about the different kind of materials in the next article but now we will focus in the general parameters that characterize batteries.
The first parameter is the capacity of the battery, usually expressed through an integral expression, i.e., the integral from zero to T (the time at the complete discharge of the cell is achieved) of the current flowing within the cell itself; this is the definition of capacity. The capacity is a quantity that is normally used to define the stress state of the cell. The value of current is referred to its nominal value C (i.e. the nominal capacity), and must not be confused with the energy stored in the system, because the value of the energy is given by the integral in the same period of time, between the produce of the voltage and the current flowing from the cell.
Other important parameters to classify and catalog the batteries are:
• Energy density: the quantity of energy that is delivered by the battery as a function of its volumetric dimensions and therefore the value of the energy density is expressed in Watts/liter.
• Specific energy: the battery energy compared to its weight. The value of energy density and specific energy are thus two different concepts.
The value of the specific energy is very important in automotive systems, and in all other systems that are characterized by mobility; the weight affects the ability to make effective use of the energy that is stored. Energy density becomes important where we have problems of space, for example, where we have the desire to create extremely small structures, such as the mobile phone.
• Specific power: power per unit of weight. This is another parameter that is used where power (energy discharged in a unit of time) is more important than the amount of total energy. The applications for which this is important are those to power quality where instead we need to have powers in brief moments. This happens for example when we have to supply the wind energy plans. You know that the wind centrals need to have a voltage to be referred. When the net, for every problem cannot give the voltage usually you can use batteries. But to do this you need a fast and good quality voltage signal.)
• Efficiency: Another fundamental parameter in the selection of the batteries is efficiency. In batteries, that assume different connotations because we have a value of amorphous efficiency (let’s call it) which is the value of the load and discharge capacity. The energetic efficiency instead is the ratio between the energy used in the discharge stage and the energy that is used in the charging phase.
