I was having this issue with my car. Kept on getting new batteries under warranty. What I found out is that my car only charges enough for what I'm using, smart altetnator and smart charging for emissions on these new cars.
I got my most recent battery checked while at my friends shop. It was 58% charged (after 6 months of driving) but the battery healthy was 100%. I got it on a tender to fully charge and then proceeded to drive it a few more months and tested it again to see. 98% charged and 100% healthy.
That leads me to think that batteries from the dealer are not fully charged leading to premature failure. Secondly the newer cars only charge enough to run what your are using. Its not like our old cars where you sit and rev the engine and it charges.