I like using battery isolators when using different types or ages of batteries. When I added a new battery to my system I knew I needed to add an isolator. The NVX BIR200 has been working great for me for three years now. Zero difference in voltage from one side to the other and it protects my batteries from pulling each other down and shortening their lifespans.
Only four stars because the studs are a little soft and I don't recall a listed torque spec so I messed up the threads a little. Also, the plastic in the center between the studs doesn't allow normal copper lugs to fit without either shaving the plastic down, shaving the copper down, or adding a washer or two.
I'm getting new batteries again soon and I'll be pulling more than 200a so I'll be retiring this product after three and a half years. It'll be sitting on a shelf for when I need another one though because it's nowhere near worn out.