I get asked this question quite often and the answer is almost always a resounding, yes. There are always caveats to everything however..
I finally had the opportunity to set this up for demonstration with a small 12W panel and two banks. Both banks were charged with regulated solar until full then the controller was removed. Panel was left flat, not angled towards the sun, just like it would be on a boat. This did not happen over night but the small 12W panel eventually brought both banks over 15V!!! This panel is 10% of the 125Ah bank in watts and only 5.5% of the 220Ah bank in wattage..
Bank #1 = 220Ah Lifeline 6V AGM
Bank #2 = 125 Ah Marine Maxx Group 31 Flooded Lead Acid
Solar Panel = 12W
As you can see even a diminutive 12W solar panel can push a 220Ah bank of Lifeline batteries well over 15 volts!!!! Do you need a solar controller? The answer is almost always a resounding YES!
For a more in-depth look at this topic please see this article: