Update:
One month later and the Ace Dashcam OBD2 power cable for the Blackvue DR-900X/750X/590X series cams is working perfectly. No troubles at all.
Also, in my non-scientific testing of the Blackvue DR-900X built-in battery protection feature, it works as advertised with no complications due to the OBD2 power vs. being fusebox hardwired.
I have a 700CCA brand-new battery (car is new), no dedicated aftermarket battery for the cams at all, vehicle battery only. BV firmware battery protection settings at 12 volts/12 hours. Haven't driven the car for a week (home on COVID isolation, so perfect time to try this out...). Started it every morning long enough to wake up the camera but not to appreciably charge the battery. Cam then connected to my home network and stayed awake in parking mode, full 4K UHD, front and back cams recording, wi-fi on, for the next 12 hours. It took 4 days of this before the cam notified me that it would be shutting down to protect the vehicle's battery, which it did.
So, a month in, the BV DR900X-2CH and the Ace OBD2 power cable are 5 stars, totally recommend.
My only gripe with BV at this point is their cloud service. Although cheaper than it used to be, still more than I wanna pay. Twelve bucks a month isn't really that bad, but I despise having to sign up for anything that hits my credit card on a recurring monthly basis. Now, if they offered a slight discount for signing up for a year at a time, I'd be much more likely to do that. Remote viewing, notifications, cloud upload, etc., are really crippled on the free plan. Just not enough data allowance available to make it useful, IMO.
I hesitate to throw this out there, but...
If you'd rather not hardwire to the fuse box, there is a OBD2 option available. I'm testing it now with my DR900X.
There's a place in Los Angeles called "Ace Dashcam" that makes a OBD2 power cable specifically for the DR750X and DR900X. He makes them himself in his shop.
I was a little wary, obviously, so I looked for reviews and other sellers, but found none--other than his EBay store where he also sells them. So I called him and we chatted. Got a good vibe, so I figured I'd give it a shot. Worst that could happen is it fails and I go back and hardwire the cam.
I've been running one for a couple of weeks now without any problem at all. As far as the cam is concerned, it thinks it's hardwired to the fusebox. Parking mode and all other normal functions work perfectly, and all battery protection functions in the phone app work perfectly. The cam thinks it's hardwired.
You can find his website and see it under Electronics > Parking Mode Add-Ons. Like I said, it doesn't come from somewhere else, the guy makes these on site. And looks like pretty good work as well.
YMMV, but I'll report back after another month or so with an update. I have high hopes.