electronic challange for all

["Hank Eskin" <henry@xxxx>]

I have a small electronic challenge for anyone who wants to help me out.

I have a Sears "Security" Die-hard battery which shut itself off if its
voltage drops below a certain level (around 10 volts), in order to preserve
enough juice to start the car when I return (in case of the low current
drain many of us experience). It's a great feature - I only drive my car
once a week or so, and the battery is always charged enough to start the car
once I reset it with the key-fob remote.

I also have a new Alpine radio. As you can suspect, each and every time the
batter turns itself off, I lose all the settings and presets, and there are
alot of them on this head unit.

I'd like to install an small, auxiliary 12 volt battery to keep the radio
powered when the main battery cuts out. A system with a relay will most
likely not work, since any momentary loss of power causes the radio to reset
itself. Is there a way to wire a second battery to the radio that will power
the radio, but not also feed whatever is draining the charge of the main
battery? If someone can describe the schematic and required parts, I can
build it. As a bonus feature, I'd like this circuit to allow the aux
battery to be charged like the main battery when the car is running. I
figure some diodes would do the trick, but I don't know enough about what
kind or how to build the actual circuit. Is it as simple as wiring a second
battery in parallel with a diode or two to prevent current from flowing from
the aux battery back to the main battery, and then feeding the power from
the aux battery to the radio? That's about the limit of my electronic
knowledge. This is where I need help.

Also, I'm not looking for suggestions on solving the low current drain, I've
tried many many things and still can't locate it. Thanks.

-Hank Eskin