Primary Cells

Topics: Battery, Electrode, Zinc Pages: 9 (3158 words) Published: May 19, 2011
Jaycar Electronics Reference Data Sheet: BATTPRIM.PDF (1)

Many portable electrical and electronic devices are designed to be powered from batteries — and in a lot of cases, from primary or non-rechargeable batteries. This is the familiar kind of battery which has a fixed amount of energy stored in it during manufacture, and once that energy has been used up the battery is simply thrown away and replaced. There are many different types of primary battery now available, each with its own strengths and weaknesses. The aim of this data sheet is to give you a basic idea of these different types, how they work and the things to consider when you’re selecting the most suitable type for a given job. Before we look at each type in turn, though, let’s clarify a few points about primary batteries in general. electrode, which gains electrons lost by ions from the electrolyte and becomes negatively charged — i.e., the cell’s n e g a t i v e e l e c t r o d e . Conversely the reduction reaction occurs at the other electrode, which loses electrons to ions in the electrolyte and hence becomes positively charged — becoming the cell’s positive electrode . Now if there’s no external connection between the two electrodes, the charges on them simply build up until the voltages between them and the electrolyte block any further oxidation and reduction, and the reactions stop. However if we connect the two electrodes via an external circuit, this allows electrons to flow between them and the electrochemical reactions keep going. Most batteries developed before about 1860 used cells in which two electrodes of metal or a metal compound were immersed in a liquid electrolyte, all housed in a jar or vat which had to be kept upright. However in most modern batteries the cells either have the electrolyte absorbed in a porous separator material (like Volta’s salt-soaked cardboard) or they use a non-liquid electrolyte such as a paste or gel. This allows them to be packaged in essentially sealed containers, and known as d r y c e l l s and dry batteries. By the way, strictly speaking a cell is a single electrochemical system with its own positive and negative electrodes and electrolyte, while a battery is a number of these individual cells packaged together — and usually connected in series, to produce a higher voltage. Most common cylindrical ‘batteries’ are in fact single cells, but those in rectangular and other package shapes are often multiple-cell batteries. The terminal voltage is usually a good guide.

Primary batteries have been around for about 200 years. Italian scientist Alessandro Volta is credited with developing in 1800 the first practical battery, which consisted of a stack of small electrochemical cells — each consisting of a silver plate and a zinc plate separated by a sheet of cardboard which had been soaked in salt water. Each cell generated a small amount of electrical energy, and by connecting all of the cells of his ‘pile’ in series, Volta could generate a useful voltage for his experiments. It’s in honour of Volta’s achievement that we call the unit of electromotive force the v o l t , of course. We also tend to call this kind of cell a voltaic cell . All primary cells effectively have energy stored in them when they’re made, in the form of their component materials and chemicals. As we draw energy from them, the electrochemical energy is converted into electrical Cell performance energy and some of the chemicals are used up. The performance of primary cells depends on a number How do the cells actually produce electrical energy? In of factors. For example the gross amount of electrodetail that’s a fairly involved process, and a full explanation is beyond us here. However in simple terms it’s by taking advantage of two main effects. One is that when any metal electrode is placed in an electrically conducting liquid (i.e., an e l e c t r o l y t e ), the atoms at its surface tend to exchange electrons...
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