I found that my battery powered power supply was outputting 4.89V after checking it with a multimeter. Is this not enough to power a pi zero? When I plug the pi into the wall everything is fine. However, when I plug it into the battery powered power supply, it just keeps going to the rainbow screen, the startup screen, and then back to the rainbow screen. Is there anything I should check on the power supply besides the voltage to see why it won't effectively power the pi?