I put the diode on the positive lead and installation direction is important. WIth the output of the little 6W panel under load, voltage is not too high for the batteries. For a larger panel, I would agree that some type of regulator would be necessary.

I use an inexpensive Harbor Freight 6W solar panel and it charges 2 batteries on the boat without issue. What you are likely experiencing is the panel discharging the batteries overnight or on cloudy days. WIthout sunlight, the panel can present a load on the batteries drawing some power from them w...