I have also had new batteries that fail within days.
Do both batteries have to be good to start the 6.6 ? or can one be weak/undercharged ?
2009 3500HD 4wd blk GMC DMax CC SLT LB,window/hood /sunroof deflectors, GM black side steps, A.R.E. cap, cargo rails, GMC bedrug
2008 2500HD 4wd Chevy DMax CC SB Z71/LT (traded)
1957 WD45 wfe factory p.s. Grandpa bought new
1954 WD45 factory p.s.
1941 WC, sat for 29 yrs, started and dynoed 27h.p.
1989 V3500 4wd blk GMC(25K new) CC Dually,454,4 spd ,all options except cargo light.(sold)