8D AGM is a good battery, no question there, but it is HEAVY over 2x the weight of each of those six volts he has and over 2x the price too. SO what he has gives far more "Bang for the buck" and far less likely hood of medical problems
(The joke I say is that with two six volt GC batteries in series you get one big 12 volt battery,, With the same size (it is between a 4D and an 8D) 12 volt battery you get ... A hernia.. That said, I can indeed lift an 8D with no danger and have carried the equivalent more than once).
And the alternator very likely can not provide that much charge to the batteries to make that big a difference..
Finally, Xantrex recommends the same max charge rage for "Generic" AGM as they do for Flooded wet cells.. LIFELINE is the only one I have seen who recommend more and then only for their AGM product.
