The main drawback to that suggestion is that aircraft really don't need more charge as they descend.
The high-energy flight segments are take-off and climb. If you're charging batteries on descent ... the only gain is that the next take-off/climb phase can use that recovered energy.
But from a mass and capacity standpoint, which seem to be the real challenges for electric flight, you're gaining very little. The ability to get rid of the battery you've spent on take-off/climb would be far more useful.