Batteries are meant to provide energy to a machine, the more power the battery consist the more efficiently the machine run and when the battery gets empty machine also stops. These batteries could be the metaphor for many things lets say one can resemble the power in the battery to the level of motivation in the employee, the more they are motivated the better result will be produced. Or one can also use them to portray resources available, they may have ample resources or they may have scarcity of resources, both can be illustrated using different batteries with different colors.