I got 4 Group 27's from Costco for my twin 250 ox66 marlin. I think they were $65 each. I didnt know about the requirements mentioned here, but have had no problems. I bought the same batteries 4 years ago. Good luck
I have a JRC 1500. So simple and perfect for buoys. Price is great as well. Not for bird finding at 10 miles, but the easiest I have ever seen. Nothing ever wrong with Furuno either.