I had a small solar charge on my main battery for several years. Everybody that got in my boat would ask" "What is that? Does it work?" My answer was always the same, "It's a solar charger and I really don't kow if it works, but my main battery has never left me down and it was never dead and I had my main battery for at least 4-5 years and just replaced it because I wanted a bigger battery."
Also, I never charged my main battery at all. Only while the engine was running. So, I felt it was a good thing, even if it only put a trickle charge back in it.