I just bought my minn kota terrova and need some batteries now. Its a 36 volt system which I know needs at least 3 batteries. My question would be how much more on the water time would I get out of group 31 batteries opposed to group 27. And where can I find the group 31 batteries I've looked all over Walmart and Costco and the only ones I can find are group 27, thanks for your help in advance.