What is the difference between series and parallel connection of batteries?

There are primarily two ways to connect two or more batteries:

series connection and parallel connection.

Series connection:
Series connection involves connecting two batteries together to increase the voltage of the battery system but keeps the amp-hour rating the same.
Series connection is established by connecting the positive terminal of one battery with the negative terminal of another until you get the required voltage.
Connecting two batteries with 12 V in series would give you a bank of 24 V system, however the total capacity would remain the same as 100Ah as it is for individual batteries.
In series connection, each battery should have the same voltage and capacity rating. If it is not so, it may result in damaging the battery. The batteries should also be charged individually to avoid dissimilarities.

Parallel connection:
Parallel connection involves connecting two or more batteries together to mainly increase the amp-hour capacity of the battery pac. The voltage in parallel connection remains the same. In order to conncet batteries in parallel the positive terminals of the batteries are connected together via a cable and the negative terminals are connected together until the desired cpapcity is reached. Paralle connections are useful for providing power to the equipment for a longer duration. It is not meant for increasing the voltage output. Batteries connected in parallel also require longer time for charging.