Need help, please. Have 2 sets of 6-volt Trojan T-105s tied in series/parallel. One very definitely is "gone", but believe perhaps the other 3 are good. I'm told, however, that I should replace ALL 4, not just 1 or any less than ALL 4! Is that the concensus, i.e., forget replacing any less than all??? Would greatly appreciate your responses this evening. Many thanks!