Cells and batteries

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ONE OF THE MOST COMMON AND MOST VERSATILE SOURCES OF DC IS THE CELL.

The term cell means self-contained compartment, and it can refer to any of various different things in (and out of) science. In electricity and electronics, a cell is a unit source of dc energy. There are dozens of different types of electrical cells. When two or more cells are connected in series, the result is known as a battery.

Electrochemical energy

In electricity, one important form of potential energy exists in the atoms and molecules of some chemicals under special conditions. Early in the history of electrical science, laboratory physicists found that when metals came into contact with certain chemical solutions, voltages appeared between the pieces of metal. These were the first electrochemical cells. A piece of lead and a piece of lead dioxide immersed in an acid solution will show a persistent voltage. This can be detected by connecting a galvanometer between the
pieces of metal. A resistor of about 1,000 ohms should always be used in series with the galvanometer in experiments of this kind; connecting the galvanometer directly will cause too much current to flow, possibly damaging the galvanometer and causing the acid to “boil.”

The chemicals and the metal have an inherent ability to produce a constant exchange of charge carriers. If the galvanometer and resistor are left hooked up between the two pieces of metal for a long time, the current will gradually decrease, and the electrodes will become coated. The acid will change, also. The chemical energy, a form of potential energy in the acid, will run out. All of the potential energy in the acid will have been turned into kinetic electrical energy as current in the wire and galvanometer. In turn, this current will have heated the resistor (another form of kinetic energy), and escaped into the air and into space.


Primary and secondary cells

Some electrical cells, once their potential (chemical) energy has all been changed to electricity and used up, must be thrown away. They are no good anymore. These are called primary cells.


Other kinds of cells, like the lead-and-acid unit depicted above, can get their chemical energy back again. Such a cell is a secondary cell. Primary cells include the ones you usually put in a flashlight, in a transistor radio, and in various other consumer devices. They use dry electrolyte pastes along with metal electrodes. They go by names such as dry cell, zinc-carbon cell, alkaline cell, and others. Go into a department store and find the panel of batteries, and you’ll see various sizes and types of primary cells, such as AAA batteries, D batteries, camera batteries, and watch batteries. You should know by now that these things are cells, not true batteries. This is a good example of a misnomer that has gotten so widespread that store clerks might look at you funny if you ask for a couple of cells. You’ll also see real batteries, such as the little 9-V transistor batteries and the large 6-V lantern batteries. Secondary cells can also be found increasingly in consumer stores. Nickel-cadmium (Ni-Cd or NICAD) cells are probably the most common. They’re available in some of the same sizes as nonrechargeable dry cells. The most common sizes are AA, C, and D. These cost several times as much as ordinary dry cells, and a charging unit also costs a few dollars. But if you take care of them, these rechargeable cells can be used hundreds of times and will pay for themselves several times over if you use a lot of “batteries” in your everyday life.


The battery in your car is made from secondary cells connected in series. These cells recharge from the alternator or from an outside charging unit. This battery has cells like the one in Fig. It is extremely dangerous to short-circuit the terminals of such a battery, because the acid (sulfuric acid) can “boil” out and burn your skin and eyes. An important note is worth making here: Never short-circuit any cell or battery, because it might burst or explode.