In principle, a Li-ion battery cell, e.g., the most widely used 18750, has a 3.7v output. So, if you put them in serial, you can get 7.4, 11.1, 14.8, 18.5, 22.2, 25.9, ...volt.
"Clean HDMI Output" simply means the output video does not contain overlay text such as Battery Life, SD card Capacity, Apture, Shutter Speed, ISO, ...... on it.