An applied voltage higher than the emf of one cell can be obtained by connecting cells in series. The total voltage available across the battery of cells is equal to the sum of the individual values for each cell. Parallel cells have the same voltage as one cell but have more current capacity. The combination of cells is called a battery.
Series Cell Connection:
Figure (a) shows series adding connections for tree dry cells. Here the three 1.5 V cells in series provide a total battery voltage of 4.5 V. Notice that the two end terminals A and B are left open to serve as the plus and minus terminals of the battery. These terminals are used to connect the battery to the load circuit. As shown in figure C.
In the lead acid battery heavy metal straps connect the cells in series. The current capacity of a battery with cells in series is the same as for one cell because the same current flows through all the series cells.
Parallel Cell connections:
For more current capacity , the battery has cells in parallel, as shown in figure (d). all the positive terminals are strapped together, as are all the negative terminals. Any point on the positive side can be the plus terminal of the battery, and any point on the negative side can be the negative terminal.
The parallel connection is equivalent to increasing the size of the electrodes and electrolyte, which increases the current capacity. The voltage output of the battery, however, is the same as for one cell.
Identical cells in parallel supply equal parts of the load current. For example, with three identical parallel cells producing a load current of 300 mA, each cell has a drain of 100 mA. Bad cells should not be connected in parallel with good cells, however, since the cells in good condition will supply more current, which may overload the good cells. In addition, a cell with lower output voltage will act as a load resistance, draining excessive current from the cells that have higher output voltage.
Series-Parallel Cell Connections:
In order to provide higher output and more current capacity, cells can be connected in series-parallel combinations. Figure (g) shows four No. 6 cells connected in series-parallel to form a battery that has a 3 V output with a current capacity of ½ A. two of the 1.5 V cell in series provide 3 V total output voltage. This series string has a current capacity of ¼ A, however, assuming this current rating for one cell.
To double the current capacity, another string is connected parallel. The two strings in parallel have the same 3 V output as one string, but with a current capacity of ½ A instead of the ¼ A for one string.