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 following figureacquire a persistent potential difference. This can be detected by connecting a galvanometer between the pieces of metal. A resistor of about 1000 Ω must be used in series with the galvanometer in experiments of this kind, because 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. All the chemical energy in the acid will have been turned into 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 chemical energy has all been changed to electricity and used up, must be thrown away. These are called primary cells. Other kinds of cells, such as the lead-and-acid type, can get their chemical energy back again by means of recharging. 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, or alkaline cell. Go into a department store and find a rack of batteries, and you’ll see various sizes and types of primary cells, such as AAA batteries, D batteries,Secondary cells can also be found in consumer stores. Nickel-based cells are common. 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 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 above figure. It is dangerous to short-circuit the terminals of such a battery, because the acid (sulfuric acid) can bubble up and erupt out of the battery casing. Serious skin and eye injuries can result. In fact, it’s a bad idea to short-circuit any cell or battery, because it can get extremely hot and cause a fire, or rupture and damage surrounding materials, wiring, and components.
The Weston Standard Cell
Most electrochemical cells produce 1.2 to 1.8 V. Different types vary slightly. A mercury cell has a voltage that is a little less than that of a zinc-carbon or alkaline cell. The voltage of a cell can also be affected by variables in the manufacturing process. Most consumer-type dry cells can be assumed to produce 1.5 V.
There are certain cells whose voltages are predictable and exact. These are called standard cells. A good example is the Weston cell, which produces 1.018 V at room temperature. It has a solution of cadmium sulfate, a positive electrode made from mercury sulfate, and a negative electrode made from mercury and cadmium. The device is set up in a container, as shown in following figure.
Recall that the common electrical units of energy are the watt-hour (Wh) and the kilowatt-hour (kWh). Any electrochemical cell or battery has a certain amount of electrical energy that can be obtained from it, and this can be specified in watt-hours or kilowatt-hours. More often, though, it’s given in ampere-hours (Ah). A battery with a rating of 2 Ah can provide 2 A for 1 h, or 1 A for 2 h, or 100 mA for 20 h.
Simplified drawing of the construction of a Weston standard cell.
There are infinitely many possibilities here, as long as the product of the current in amperes and the use time in hours is equal to 2. The limitations are the shelf life at one extreme, and the maximum deliverable current at the other. Shelf life is the length of time the battery will last if it is never used; this can be years. The maximum deliverable current is the highest amount of current that the battery can provide before its voltage drops because of its own internal resistance. Small cells have storage capacity of a few milliampere-hours (mAh) up to 100 or 200 mAh. Medium-sized cells can supply 500 mAh to 1 Ah. Large automotive or truck batteries can provide upward of 50 Ah. The energy capacity in watt-hours is the ampere-hour capacity multiplied by the battery voltage.
An ideal cell or ideal battery (a theoretically perfect cell or battery) delivers a constant current for a while, and then the current starts to drop following figure. Some types of cells and batteries approach this level of perfection, which is represented by a flat discharge curve. But many cells and batteries are far from perfect; they deliver current that declines gradually, almost right from the start. When the current that a battery can provide has tailed off to about half of its initial value, the cell or battery is said to be weak. At this time, it should be replaced. If it’s allowed to run all the way out, until the current actually goes to zero, the cell or battery is dead. The area under the curve in following figure is a graphical representation the total capacity of the cell or battery in ampere-hours.
A flat discharge curve. This is considered ideal.