Batteries are made up of a group of cells where a cell is the smallest individual electrochemical unit. These cells are connected in series, in parallel, or in a combination of both in order to obtain the required voltage and capacity.
A typical battery rating consists of the following terminologies:
1. Nominal Voltage
Battery nominal voltage depends on the nominal voltage of the cell and the connection of the cells. The nominal voltage of the cell depends on the combination of the active chemicals used in the cell. For a lithium-based cell, it’s usually slightly over 3V. For the battery in the above figure, the nominal voltage is 3.7V.
2. Nominal Capacity
The nominal capacity of the battery quantifies the amount of charge it is rated to hold. It is specified in ampere hours (Ah) or milliampere hour (mAh) as current is the rate at which charge flows and multiplying it with time would basically give us the amount of charge. For the battery shown in the figure, the nominal capacity is 5000mAh or 5Ah.
There is also something called the “C rate” which is a relative measure of the battery current. It is usually the constant current charge or discharge rate that the cell can sustain for one hour. For example, a 120Ah battery should be able to deliver 120A which is called “1C” for 1h or 12A which is called “C/10” for approximately 10h. For the battery shown in the figure, the 1C rate is 5A.
3. Energy and Power
A battery stores energy in an electrochemical form. This energy is later used to do work. It is expressed in Watt hours (Wh). The energy capacity of the battery is approximately the nominal voltage (V) multiplied with its nominal capacity (Ah). It is not always specified on the battery as it can be easily obtained from the nominal voltage and the nominal capacity.
For the battery shown in the figure,
which is as rated on the battery.
If you’re given the capacity in watt-hours but not in amp-hours, you can simply flip this equation around.
It is also worth noting here that when we connect similar batteries in a series combination, their capacity will remain the same whereas the voltage will be the arithmetic sum of the voltages of the individual batteries. When we connect them in a parallel combination, their voltage will remain the same whereas their capacity will be the arithmetic sum of capacities of individual batteries. But in either case, the energy capacity will be the total of the energy capacities of the individual batteries.
Thus, if we are given the nominal voltage and either the energy or power capacity, we can calculate many of the most important factors of the battery and decide which battery is most suitable for our application.