Rating of a battery is usually expressed in

• An Amp Hour (Ah) is the amount of current a certain battery can supply for a certain period of time.
• If we have the amp hour rating of the battery, we can compute how long the battery will last while supplying a certain current. 
• The ampere-hour rating of a battery is given by multiplying the current (amperes) by the discharge time (hours).

Series connection:

• Increases output voltage.
• Capacity remains the same (measured in ampere-hours).

Parallel Connection:

In order to increase the ampere-hour rating of a battery, cells are connected in parallel.

This is explained with the help of the following diagram:

• The positive terminals of all batteries are connected together, or to a common conductor, and all negative terminals are connected in the same manner.
• The final voltage remains unchanged while the capacity of the assembly is the sum of the capacities of the individual batteries of this connection.