I know what I want but not sure how to achieve it! I have car dash cam supplied by 5 volts. I've set up a 5volt power bank to run the dash cam when parked and wired up a relay operated by an Accessory circuit to switch from car battery power (via a 5v usb adapter) to power bank battery when the car is parked. Very rough circuit diagram below. Problem is that the fraction of a second it takes for the relay contacts to switch over causes the camera to switch off and back on, causing data loss and/or having to reformat the SD card. What I need is a 'simple' capacitor circuit to maintain the 5v on the camera for that split second! I'm no expert so would really appreciate some advice on what components to use, and suggested values.
You do not need a relay on the ground side.
You may get away with a single relay and two SCHOTTKY 60V 5A DO201AD diodes acting as bypass while the relay moves as shown below. It will depend if the camera can handle the voltage dropping to about 4.7V briefly. They are cheap though so a quick test is easy enough. NOTE: You may not even need the relay with this circuit.
However, your issue may be a sequencing one. If the battery pack shuts off the outputs before the USB adaptor completely comes up it may still be an issue. If so you will need to add a large capacitor in after the diodes.