Dash cameras 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. You don't say why you're reluctant to get a camera hard-wired, but that's what we'd suggest. Although, if you're dead set against it, you could get a dual-port charger which will split your 12v socket into being able to power two USB-powered devices. They're fairly cheap to buy (about £7-8) and we've reviewed an Anker one here: kit.honestjohn.co.uk/reviews/review-anker-24w-powe.../

Then you can trail the cables into the lining of your car's interior so you can't see the wires. That's what we do. There are plenty of instructional videos on YouTube. Alternatively, you could power a dash camera with an external battery bank. This isn't ideal because it would constantly need to be charged, but if you do go for this option - make sure you get a power bank which will support your dash cams charging capacity.