In the old days when automotive batteries were of the type that had to have water added periodically, there was no problem other than dealing with the acid. The allow form battery plates only became a problem when they went to "no maintenance" batteries. The lead alloy used for those plates contained Calcium (I believe, but not sure now...too many years), that would result in the lead alloy when melted, to expand too much and when the alloy "froze" in the mold, would contract and shrink away from the mold walls resulting in misshapen bullets. I do not know if the posts of the new maintenance free batteries also contain that contaminant, but internally the plates attach to the posts and may be cast as a unit. It is not worth the risk of contaminating your melt to find out. Once contaminated, diluting with enough good alloy may not be successful...ask me how I know.