Connecting two 12-volt batteries in series adds their individual voltages together to create a 24-volt battery pack. The series arrangement does not increase the capacity of the batteries.
Connecting the positive battery post to the negative post of a second battery and then connecting the leads of the load to the remaining two battery posts results in a series connection. This is the arrangement used in most flashlights, where the batteries stack on top of one another. Connecting a series battery pack to a device designed to operate on a single battery often burns out motors and sensitive electronic parts.
Connecting batteries in parallel increases the capacity of the batteries without increasing the voltage. The parallel connection indicates the positive post and negative post of one battery connect to their corresponding post on the second one. The load then connects to the posts of one battery. A parallel battery pack allows the load to operate twice as long. Doubling both voltage and capacity of a battery pack requires connecting two two-cell series battery packs together in parallel.
Regardless of how batteries connect, it is essential to match their type and capacity. Differences in battery chemistry result in differences in battery voltage. These differences in capacity cause one battery to drain the second below the normal level. As a result, the over-drained battery is often unable to recover from the deep discharge, shortening its life.