The batteries charge faster on the generator or engine. We had new batteries installed in 2012. Going down the road with generator running the it took perhaps hundred miles for the batteries to show full charge on the panel. Older batteries might benefit from the desulfate setting. Read the manual on charge level and have battery compartment door open. However just getting them to full charge might be all that is needed. Check the level in the batteries too. We rarely use our battery bank, of flooded cell T105s. When we do it is by error. We left vent fan on for a month. They say to fully discharge our batteries sometimes, so we did that, and they were dry too. Filled up and charged they seem to be alright, but we will have them checked.