I recently ordered, received, and built a MintyBoost, but i am having a problem with it charging my USB devices. I tried connecting 2 D batteries in series as the input for the device, but it seems that the output voltage keeps dropping down below the 4.5 volt level which makes the device toggle back and forth between saying it's charging and not charging. I also tried connecting 4, 3.7 volt batteries all in parallel as the power supply. The open circuit voltage for the batteries is 4.2 volts, but i am still having the exact same problem as before with the output of the MintyBoost dropping too low. Has this been an issue before?