I would say no less than the Electronize 30Amp range with the external relay mod (Technobots part 1500-022) Youd need one per channel, but they should be able to handle the 4 drills (Ive been running four 9.6V drills @ 14.4V off them without any problems). There are also the larger options if your money allows (like the Sidewinder) I certainly wouldnt go below 30A per channel.