As I understand it... the luxI, and luxIII versions of the G&P drop ins work from a 3.7V source fairly well with a reduced output... And the new Cree-dropin modules also work from a 3.7V source- but on all of them: not as much of the run is regulated output.. like it would be from a 6 or 9V source. These drop-ins have buck regulators, no boost.. So you will be forced to use a li-ion rechargeable cell to drive one. (primaries would be very dim). If I'm wrong on this (possible) hopefully someone will step in and correct me.