I have a SolarForce Gladiator with one of their 1-4 cell dropins. No extra runtime with extra batteries over using 1 cell. They just wasted the extra voltage in the driver. I know of no 17mm driver that lowers voltage and increases run time by raising current potential. One possible option is to use a Cree XHP35 LED that is build for 12 volts and much lower current for equal output.