Back up Battery for head unit

I did a search for back up batteries and found some stuff, but I would like to know if I can get one of these little siren backup batteries and hook it up to the constant of my head unit because I am always unhooking my battery and in another car I have (which is a classic and hard to fix the wiring) the battery slowly goes dead and in both cases I always lose all of my preset stations and EQ settings and so on. So the question is, what is the best unit to get for this problem? Will it damage my head units? I have also heard of kits, will these do the correct wiring for me, such as charging the battery and making it so it only powers the radio and not the rest of the car if the primary battery is disconnected. One last thing, I do not want to turn the radio on with no battery, just keep my settings if the battery goes dead or is disconnected. Thanks for your help and suggestions.

Check to see if your cigarette lighter plug has constant power on it or not.  What I've done before is connect a pigtail (fused) from the cigarette lighter outlet to my 12v DeWalt drill battery.  It works like a charm.  Note that if your lighter outlet is on accessory power, you'll have to turn the key on for this to work- so shut all your accessories off when you do this (a/c, radio, defrost, etc.).  That way your drill battery is not under too much strain.  I like the DeWalts, because insulated male disconnects will fit snugly over the battery terminals, so you won't have to hold any wires in place when you do this.My ex once told me I have a perfect face for radio.