uninterupted power relay circuit

Hello all, I am brand-new to the forum. I've decided to build a portable boom box using some left over equipment I have lying around. The system will consist of a Pioneer head unit, a Pioneer 4ch amp pushing MB Quart 6.5" components, and an 8" subwoofer. Power will be supplied by a 12v 33a AC-DC power supply. Now to my question: I want the head unit to retain memory/settings with a small battery backup. I'm thinking I can achieve this with a small rechargeable battery on a relay circuit that is charging when the unit is powered up and switches to provide power for the stereo's memory when it is powered down. I was wondering if someone could could explain how to set this up, particularly how to choose the correct relay(s). I have never had a good grasp of electrical circuits and I get confused quickly.

Usually it's merely done with diodes - the memory's +ve supply thru a diode and the battery +ve thru a diode with diode line ends joined towards the "memory" end. That avoids the need for the memory to hold thru the relay switching time (usually up to 5 mSecs).

Ok, the very small amount of understanding I have for diodes is starting to make since. I guess now I just need to figure out what size(s) I need. So I guess a relay is completely unwarranted?

A relay would be used if the diode voltage drop is unacceptable - eg, if charging a battery. But the DC supply should be (say) 0.6-0.7V above the battery's charging voltage and likewise the battery at least ~0.6V above what the memory requires (ignoring discharging voltage drops). Or ~0.3V is Schottky diodes are used. A cap might also be suitable in lieu of a battery....