Cygwin, gcc-g++ $ make battery

battery.cpp

#include <iostream>
#include <windows.h>

int main(int argc, char *argv[])
{
  SYSTEM_POWER_STATUS powerStatus;
  int percent = 0;
  
  if (GetSystemPowerStatus(&powerStatus) == TRUE) {
    percent = (int)powerStatus.BatteryLifePercent;
  }
  
  std::cout << percent << std::endl;
  
  return 0;
}