A series hookup allows for higher voltages without affecting capacity, and a parallel hookup increases capacity while preserving the voltage. Voltage is the amount of power the battery delivers, while capacity is the length of time the battery maintains its voltage.
Wiring batteries in a series provides more power by doubling the voltage output. For instance, wiring two 12-volt batteries in series provides an output of 24 volts. A series hookup has no effect on the capacity, or life, of the batteries. If two batteries in a series each have a capacity of 20 amp hours, the series hookup also has a 20-amp-hour capacity.
Wiring batteries in parallel has no effect on the voltage but doubles the capacity. This means that two 12-volt batteries, each with a 20-amp-hour capacity, have a 40-amp-hour capacity but still only yield 12 volts when they are wired in series.
Connecting batteries in series is as simple as connecting the negative terminal of one battery to the positive terminal of the next, on down the line until the desired series is complete. For a parallel hookup, like terminals connect to one another instead of to the opposite terminals. It is best to use the same type and capacity of batteries, as mixing batteries leads to decreased battery life and can even cause batteries to explode.