I agree, you need to have the same size and amp batteries for best results. If the batteries are over 3 years old they are most likely ready for replacement. A little pricey but Optima Blue top batteries are the best out there in my opinion. A cheaper batter but good quality are Interste marine deep cycle 850 cca or better. Always use deep cycle for trolling motor use and marine cranking or standard battery for cranking.

Just my 2 cents