[Soekris] Mobile power for a Stompbox-like setup

[John Lingate]

Tried to post this once already, but it didn't go through.

Happy New Year to all,

I am looking to build a Stompbox-like device.  I am often out on the road, and am sometimes in areas where a normal wireless card doesn't quite work.  I am hoping to create a semi-portable solution that is normally residing in the car, but can be removed for travel (plane->rental car, etc).  To increase range, I intend to use a Wilson cell amp and a good external antenna.  Someone did a Stompbox like this:

http://www.richins.net/stompbox/

My intention is to do something similar, integrating the cell amp and any other stuff.  The idea came to me that I would like the ability to use the service with the car off, without draining the car battery (northern winters), and it would be even more interesting if it could be used without an external power source at all.

Adding a battery seems like it should be simple, but what I would really want is to design it so that it was as automated as possible.  I would like the unit to always come on when the car is on (easy enough), where it should charge the Stompbox battery.  I would like the Stompbox to remain on if I've pushed a button before turning off the car.  I would also like the Stompbox to come on if I push that button when the car is off.  It should then run on internal battery power until the batteries exhaust, or possibly a signal from the Soekris to shut off.

The electronics for controlling power are within my grasp, but the question of charge circuits and battery life come to mind, which I have no experience building.  I know it is Very Bad to treat batteries poorly.  I began looking for pre-existing solutions to the problem, and did come across

http://www.powerstream.com/DC-UPS-1212.htm

but I have not been impressed with the lifespan of lead acid batteries.  I believe I could make such a circuit work, however, the $50 price tag is on the high side of what I'd want to spend, I'd prefer something with a better battery technology, 
and it seemed like someone might have solved this problem already.  Bonus points for any extra filtering/conditioning of noisy vehicle power.

My expected power requirements are 12VDC, under 3A.  I am looking for a solution of up to 3A just in case I decide to add something else later.  Has anything like this been done, or does anyone have any helpful links, pointers, suggestions, or the like?

Thanks!!
       
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