Rather than a second battery I removed the OEM battery and installed a much bigger AGM battery. I had to remove the battery tray and saw the rear end off of it make the AGM battery fit. I still have to monitor the charge level but AGM batteries maintain their voltage to a significantly lower charge level than a wet cell lead acid battery does. If the car will sit for a week, without being driven, I connect a battery maintainer to the "jump" terminals under the hood.
I also have a 7 AH gel cell battery in the glove compartment with a 12 gauge cable with clips that can be used to connect it to the jump terminals under the hood if the car is dead. In over 4 years I have not had to use it because the AGM battery has never gone low enough to not provide 12 volts.
Detailed installation instructions are in the battery section of the Energi forum.