Using batteries to store weeks worth of energy would be foolish. Batteries are too expensive per unit of energy storage capacity. They're suited for diurnal storage.
Long term storage doesn't need to be efficient, because the number of charge/discharge cycles is small and the "cost of inefficiency" is proportional to the number of such cycles.
Long term storage doesn't need to be efficient, because the number of charge/discharge cycles is small and the "cost of inefficiency" is proportional to the number of such cycles.