I bought 3 new deep cycle group 31 batteries in May, 2016. They are about 9 months old. Recently, one of therm tested weak, so gonna get it checked. Will it be ok to replace just that one battery? I've always heard that you shouldn't replace just one battery in a 24 or 36 volt system. I sure would hate buying 2 new batteries, if they exchange this one if it's bad. Thanks.