fla question

I have an old industrial battery 520mmx 260mm(20"x10") hard to read but looks like 80??1200 sealed maintenance free type. It will read 13v across the posts but will not arc across posts and when in series in a pack breaks the connection. Can someone explain why? It wont even desulphate with the mig welder because it is dead between the posts. Why does it still read 13v across?