Make sure you allow for temperature. If the batteries are cold, you may need higher voltages. Watch for gentle bubbling (gassing) while charging... champagne size bubbles every one to three seconds... not a rolling boil. Like others have said, gassing is required.
Keep in mind, when your converter drops to 13.6v, or whatever, the current also drops by ~ 60-80%. So if you have a 45a converter/charger, it suddenly becomes a ~ 9-18a charger once in abs/c.v. mode, and amps quickly taper from there. Then amps will level out. This results in much longer charge times. I charged the other day, and it took several hours at 15v just to get from 6a to 5a tapering (near full, but certainly NOT FULL). Now, imagine charging at only 13.6v... it would take days to get the batts truly full.
