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I would recommend just getting one of the commercial battery packs for dash cams.
I've got the blackvue 900s dashcam, and an older battery (couple years) called CellLink B.
It does what you're asking for, powers the cam while driving using car's power, charges the battery while driving, and switches to the external battery while parked, not interfering with the car. The external battery charges full from empty in about 20 minutes. The battery provides about 20 hours of front and rear dash cam power.
There are some new battery models, that probably have even betters stats.
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