Have been doing similar devices to friends for many years.. For eg. device with three AA batteries, one battery is replaced with small 4.5V switcher power supply (with small ground wire to first battery). Keeps constant 4.5V output until batteries are very dead. Was specially useful with old flashlights, you could increase voltage to bulb limits and turn old Maglite to light cannon
:D Well okay, maybe not that impressive, but better still..