You also need a third in case one of the first two goes pop, and because batteries don't like being left on the shelf fully charged (or discharged), it needs to

You also need a third in case one of the first two goes pop, and because batteries don't like being left on the shelf fully charged (or discharged), it needs to be actively cycled through. That's N+1 redundancy; N+2 means four in total assuming you can charge as fast as you can discharge.