Huh, 3rd time in 3 days I've had the opportunity to give this advice regarding batteries...
Add 2 oz of mineral oil (3-in-1 oil) to each battery cell, even if you have to turkey baster remove some water. This helps prevent water from being boiled out too quickly and reduces the corrosion present on the tops of most wet cell batteries.
If your converter is going bad though, make sure you get it fixed or replaced. Meanwhile, the oil will help get extra life from the batteries.
Mineral oil has been used for this purpose in wet cell batts for over 100 years now.