presume the car battery connectors are lead alloy becasue the plates in the battery are lead alloy... usually lead with some nasty chemical in it like antimony or some such to give it some rigidity.
worked for a car battery recycler in my youth.."acid proof gloves n boots and a space suit, blood test every 3 weeks" and the vast majoirity of end plates in a lead acid car battery were created with the top contacts for the battery as an integral part. may no longer be the case.
battery tech has come a long way in the last 30 years
i wonder if its hard to solder to lead and easier to just drill and rivet or put a self tapping screw through to make a connection
Dave