Batteries are electrochemical power sources. Chemical reactions take place within the battery, and these generate electrical energy independently of the mains supply.
The smallest unit in a battery is known as a cell. A battery generally consists of a number of such cells, connected together electrically to form a single unit. One lead-acid cell produces a nominal voltage of 2 volts. Thus, for instance, a 12V battery has 6 cells (6 cells x 2V = 12V nominal).
The nominal voltages 6V, 12V, 24V, 36V, 48V and 80V have emerged as the most widely-used ratings.
Two or more batteries can be connected in series. The voltages of the two batteries are added together, and their capacity remains the same:
Example: 2 x 12V batteries, each 100Ah => 24V 100Ah
When batteries are connected in parallel, the voltage remains the same while capacity is increased to the sum of the individual batteries' capacities:
Example: 2 x 12V batteries, each 100 Ah => 12V 200Ah
For both connection configurations it is important that batteries of the same type and capacity are used. The batteries should also be the same age, and they should be fully charged before use.