The direction of positive and negative polarities of emf of a battery is decided at the time of construction.
Explanation:
•      Direction of current flow depends upon direction of emf.
•	In battery chemical process occurs which moves positive ions from negative to positive terminal.
•	The conventional current flow from positive terminal of the battery to negative terminal through closed circuit.
•	However the direction of voltage drop across the component depends on the direction of current.

Context