Dash cams aren't really designed to run off battery power, and those that say they do don't really have big enough batteries to be any use. Imagine recording a video on your phone for an entire commute, it would be completely dead by the end of your drive. I'd suggest getting the dash cam hard wired in your car, it's pretty inexpensive. Or, like I do, you can trail the wire up into the lining of the cabin. This video shows you how: www.youtube.com/watch?v=xwD8Ilk-MHU

Alternatively, you could power it with an external battery bank. This isn't ideal because it would constantly need charged, but if you do go for this option - make sure you get a power bank which will support your dash cams charging capacity.