I wouldn't suggest getting a dash cam app. Phones don't have the internal battery capacity that dedicated dash cams do, they're pretty useless as dash cams for this reason. Even if they're charging the whole time, the lens on a phone isn't the right kind you'd get on a dash cam, so all the video would be very dark or too light and you'd be unlikely to see number plates. Lots of money is spent on dash cam development specifically because phones just can't hack doing the same job. But you can pick up reasonably cheap dash cams online if you're looking for something that doesn't cost a lot. We reviewed the £20 Lanka Blue here, for example: kit.honestjohn.co.uk/reviews/lanka-blue-dash-cam/