"Do I have this correct?? Batts must be matched ie: AGM batts in the same system with tire store "cranking amps" junk car batts a big no no..?"
Chemistry matching is the vital point. Just like Single Malt Scotch and Vermouth don't mix, different types, brands, construction hate each other and take cross pot-shots across the gully.
A significantly older or abused battery has significantly different chemistry than it's newborn brethren. Not the easiest thing in the world for non-techhie types to determine when "different" means "significantly different"
