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Sounds like you need a wall charger and you alternator controlled on a switch. Other than that you just need a deep cell battery that fits under the hood or do a relocation to the trunk. Think there are a few threads on controlling alternator on a switch, I don't know how to do it but have seen posts about it.
Just double read your post and noticed you want to do a backup battery. Easiest way would be just run a deep cycle alone with no "regular" battery serving primatry duty.
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-Mike
2007 Ford Focus ZX5 - 91k - SGII, pending upper and lower grill bocks - auto trans 
1987 Monte Carlo SS - 5.3/4L80E swap - 13.67 @ 106
2007 Ford Focus Estate - 230k - 33mpg - Retired 4/2018
1995 Saturn SL2 - 256K miles - 44mpg - Retired 9/2014
Cost to Operate Spreadsheet for "The New Focus"
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