These things are HUGE! I picked up a group 31 yellow top yesterday at Interstate ($168 after tax with a military discount) that I'll use as my secondary battery for the dual system until I get another group 31 to put in in place of my Exide Orbital starting battery. I also asked about the blue tops, and (just in case anyone here didn't already know) apparently the only difference between those and the yellow tops is that the blue tops have both automotive posts, and the 3/8" threaded holes. With the yellow tops you have to choose one or the other. I'm happy though because now the most expensive part of the dual battery setup is taken care of. All I need now is to make my cables (got all the supplies), buy the second tray, the Hellroaring isolator and figure out how to do up a coolant overflow bottle (I won't be able to use my stock bottle because of where my cruise control actuator is mounted).