They're not recharging batteries but they are extending range. The whole time they're descending, they're moving forward and overcoming air resistance, without needing as much fuel as if they were flying level the same distance and altitudes.

I think that compared to energy expenditure X in level flight at cruising altitude, the aircraft would expend X+Y in the climb and a little less than X-Y in the descent, assuming all horizontal distances are the same.

To the extent that they deliberately increase drag, that wouldn't hold true, but I'm guessing that's mostly on final approach.