>> If you can be electrically creative, a cheap alternative is a PC UPS,
>> (Uninterruptible Power Supply). Most of them use an internal 12 volt
>> sealed battery. Remove the battery and connect to your 2nd battery.

I once interviewed with a UPS maker (Exide).  We got to talking about this idea and I was told that a lot of low end UPSes are designed to overheat just as the battery goes dead.  If you increase the size of the battery, the unit could overheat if run anywhere near full load.  With a light or intermittant load, most units should be ok.

Bill