Thanks that's what i suspected but I just wanted confirmation. I did't want to overcharge the batteries. The boat is in a boat yard so I can't leave it plugged in to a charger unattended. For some reason they get all funny about leaving extension cords plugged int to tightly packed unattended fiberglass boats. The panel is rated for 25 watts, so if it shows 21 volts that are dropped down to 12, it's putting out 3-4 amps? I'll check on it tomorrow and see if the voltage improved. I hooked it up to the house bank, after the isolator. I am hoping that once they exceed 13 or so they will also charge the start battery.