Originally Posted by OldCoot
Only problem with (2) 6volts is if you loose one, you are SOL for 12v. Not so with (2) 12volts.
I keep seeing this comment, but I dissagree. First, batteries don't usually just fail out of the clear blue. (With the possible exception of those junk, overpriced Bosch batteries). Second in two battery set ups with 12s,, when one fails, by the time you know it the other one has been dragged down and destroyed too. In our diesel, when one Motorcraft battery failed during warranty. Ford replaced the other too at no extra cost even though it still passed a load test. They said one bad one damages the other badly before you realize that there was a problem. About the same reason we say to replace both batteries so they are the same strength and condition.