Using MAX641 integrated circuit, can be made a doubler voltage regulator. In this scheme high voltage electronic controller allows a maximum output current up to 1A. Low battery voltage detector input compare LB1 with internal reference of 1.31 V. LBO output goes in low state when the voltage at pin 1 falls below 1.31 V. The threshold voltage for "low battery", is determined by voltage divider R1-R2.
LED D1 illuminates the LBO output when the input voltage falls below 2.62 V.
Input voltage must remain below 5 V. The maximum effectiveness is 80% conversion.