On a routine check of the batteries in the truck, we discovered a slight problem. The students had learned how to use a refractometer to check the specific gravity of the lead acid cells to determine the state of each cell in comparison to the rest of the pack. The results found the 14 month old US Battery US145xc batteries were within a few tenths of each other except for one cell was considerably lower than the rest. Upon checking it out further, we noticed the water level was lower as well, even though we had topped them off the day before. We also noticed that there was a considerable amount of oxidation underneath the battery box below where the battery in question was.