An AGM battery is different than a wet cell if your putting wet cells in the trunk your asking for trouble. What you fail to mention is that your caps up to 2 farad only have the potential to hold as much as a 9 volt battery. There is no difference between a .5 farad and a 2 farad cap until the voltage is increased past 24 volts. A capacitor is a marketing gimmick always has been. Their used to make newbs feel like they did something to help there alternator when if fact they only hurt the performance of the electrical system. A surface charge on a wet cell still has more capacity than 10 2 farad caps. By adding a small yet efficient AGM battery you help stabilize voltage so the alternator can do its job if you add a large less efficient battery you will need a high output alternator.
