I have a DdPai at the back and a Goluk T3 at the front. Both great little cameras. Very compact and unobtrusive and I barely see the front one from the driver's seat because it is just a little cylinder that sits on the passenger side, hidden behind the rear view mirror. No screens or any external controls. All controls and the screen are on the app on the smartphone.
They have small battery powered bluetooth emergency buttons that when pressed, save six second before and six seconds afterwards to a protected file. So no fumbling around on the camera itself for the emergency button. All files remain on the card until they are overwritten apart from protected files which remain until the card is formatted. All those files or portions of files can be transferred to the smartphone or just viewed without downloading, without removing the microSD card from the camera. A tablet works just as well. Either device is only needed when needed if you see what I mean. I can go for months with no need to view or download.
Wiring these in is easy. You can wire the back one into the rear power socket, if your car has one, and on Volvo the power shuts off after 10 minutes or so from shutting the ignition off. The only drawback is that, in my case, I have a big fridge semi-permanently strapped down in the back and on the odd occasion when it is used, such as when shopping for frozen food, the camera loses its power point. It would be easy enough to splice another power point in if only I could be arsed.
Shouldn't take more than about two hours to wire both in with no trace of wiring visible at the front but with a wire crossing the seal of the hatchback at the back.
I have wired a linked set of front and back cameras into the Qashqai, which records both cameras to the same card in the front one. While again very compact, the emergency button is fiddly to reach from the driver's seat, if you can remember which one it is even, and the card has to be removed and inserted into a card reader with an adapter to see the files. Took about an hour and a half to wire that in. Started to route the cable front to back along the floor but found that was a no-go because the cable would be too short. The roof route was much much easier and just required the cable to be pushed under the headlining and door seals. I used a special tool called a teaspoon handle to aid this job.
That camera is this model.
https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1
I've had it a year and only checked it a couple of times to make sure it works. I formatted the card a few weeks ago as routine maintenance, as I did with the either the Goluk or DdPai, I forget which. The other one was left because I did not find a format control on the app. They do all work fine.
None of these have a battery. They do have capacitors which last far longer apparently.