I’ve got a Thinkware F800 front and rear cam which was pain-in-the-arse expensive to buy but relatively easy to fit and set up, although I sought the help of a local in car hi fi shop to do the hardwiring into the fuse box.
It’s been excellent – it’s got loads of settings and when parked (when the ignition’s off) it’s got different options for how to record. It’s also got a battery level detector so that if your battery drops below a certain point it will shut down, so you’re not left with a dead battery. This hasn’t been a problem for me, but I guess if you’ve got an old battery it might be – that said, a new battery is a lot cheaper than the piggyback batteries and would mean your car starts more reliably!
I leave it in a mode that starts recording if it senses a bump or motion. The advantage of this is that it uses less power most of the time, but that it takes a second to “wake up” and start recording so won’t start until after the impact. The other option is to have it in a mode that’s permanently recording to a memory buffer but only writes to the memory card the contents of the buffer once it senes an impact. The advantage of that is it will have in the memory the minute or so before the impact, so you’ll have a better record of what happened, but the permanently recording mode uses more power so will drain your battery faster.
The mode I use seems to work fine for a week or two of parked monitoring. That said, the recording function is set to only start if the camera senses a physical movement. Which means it won’t start recording if someone is having a go at the car parked behind you unless they jostle your car as well. If you want it permanently recording (and a 128GB SD card can store at least a few days’ worth of recordings), you’ll need to set it to do that, but be aware of the drain on the battery.
HTH!