Inside the battery

If the battery is fully charged, the charges are distributed such that the lower cell is charged negatively and the upper one positively. If they are connected by a conductor, the charges are able to move freely between the cells. The negative charges are attracted by the positively charged cell, so that an electric current is generated.

If the charges between the cells are balanced out, the electric current will come to a standstill. Thus the battery is "empty", if there is exactly the same amount of positive and negative charges in each of the two cells. Consequently, the battery is charged neutrally and so the negative charges are not attracted anymore.