Everything I have read about the Raspberry Pi 3, model B says that it will not run reliably on anything below 4.6vdc. I have a Pi 3 and I am running it at around 4V DC with a single 18650 lithium battery. It boots up just fine on the Volumio 2 OS and it sounds wonderful. Can anyone explain why this is so?
I am not running any peripherals other than the Allo Boss HAT DAC. I know that the lithium battery has tons of headroom as far as amperage goes. I read this section on StackExchange which might suggest that only the HDMI port requires 5V DC:
Power In from the μUSB connector goes through a polyfuse and an ideal diode to provide 5V which is the 5V rail on the expansion header and provides all power to the Pi including the following:
Power in: a dual step-down converter (PAM2306) which provide 3V3 and 1V8 via a MOSFET to 5V_CORE and a Step-Down Converter (RT8088A) which generates the VDD_CORE (nominal 1.2V). This was performed by the SOC in the original Pi. voltage monitor chip (APX803) Power Switch (RT9741) which produces H5V for HDMI
The Richtek RT8088A and RT9741 listed above both have an input voltage range of 2.7vdc to 5.5vdc so that's good.