There are two parameters that measure battery performance: voltage and capacity. In very simple terms, the voltage is the force propelling each of the electrons coming out of a battery and the capacity is the number of electrons that can be obtained from a battery. How these parameters relate to batteries is explained below.
Voltage
The voltage of a battery cell is determined by the materials used in it. The reduction and oxidation reactions mentioned in the "How a Battery Works" section, each produce a fixed potential. The sum of the reduction and oxidation potentials is the voltage of the cell. For example, the discharge reaction at the positive electrode for a lead-acid cell is PbO2 + SO4-2 + 4H+ + 2e- ® PbSO4 + 2H2O which has a potential of 1.685 volts. The reaction at the negative electrode is Pb + SO4-2 ® PbSO4 + 2e- which has a potential of .356 volts. This means that the overall voltage of a lead-acid cell is 2.04 volts. This value is known as the standard electrode potential. Other factors, such as the acid concentration can also effect the voltage of a lead-acid cell. The typical open circuit voltage of commercial lead-acid cells is around 2.15 volts.Thus the voltage of any battery cell is established depending on the cell chemistry. Nickel-cadmium cells are about 1.2 volts, lead-acid cells are about 2.0 volts, and lithium cells may be as high as nearly 4 volts. Cells can be connected together so that their voltages accumulate. This means lead-acid batteries with nominal voltages of 2v, 4v, 6v, etc. are possible.
Capacity
While the voltage of a cell is fixed by its chemistry, cell capacity is variable depending on the quantity of active materials it contains. Individual cells may range in capacity from fractions of an ampere-hour to many thousands of ampere-hours.
The capacity of a cell is essentially the number of electrons that can be obtained from it. Since current is the number of electrons per unit time, cell capacity is the current supplied by a cell over time and is normally measured in ampere-hours.
Battery vs. Cell
Voltage and Capacity
Batteries normally consist of multiple cells that are electrically connected. The way that the electrical connections are made determines the voltage and capacity of the battery. If the positive terminal of one cell is connected to the negative terminal of the next and so on through the battery the result, as illustrated in Figure 2, is called a series-connected battery. The voltage of this type of battery is the sum of the individual cell voltages. For example, a 12-volt automobile battery consists of 6 2-volt lead-acid cells connected in series. Although the voltages add, the cell capacity is fixed at the value for the individual cell.
The other way to connect cells within a battery is to connect the negative terminal from one cell to the negative of the next cell and to connect the positive terminal to the positive terminal. When this is done throughout the battery, the result is the parallel-connected battery shown in Figure 3. Here the capacities of the individual cells add to make the battery capacity but the battery voltage remains as the voltage of the individual cell.
Series-connected batteries are far more common than parallel-connected. Usually it is easier to get added capacity by just using a larger cell rather than a parallel-connected battery.
All of the battery connections may be made internally so that it is difficult to determine the number of cells by external examination. However, knowing the voltage of the basic cell, it is easy to determine the number of cells by dividing the cell voltage into the battery voltage.
Cells used for batteries should always be identical. Mixing cells of different chemistry or different size may be hazardous and should be avoided.