I did build one years ago, as I was using a portable radio which would eat D cells on a regular basis. So I built one ( not too complex, was an Elektor article in the 1980's), and at least it was able to increase battery life from a week to around 3 months, though only for the cheap non alkaline cells. Then they would show signs of leakage, and I would toss them, and go out and buy another 4 D cells. modified an old NiCd charger to do it, using the case and a little modification of the inner side ( and removal of most of the rest of the components on the board along with the switch for selecting battery type) board to do it.