Well I guess that pretty much hit the nail on the head. I was a little skeptical of that style charger. Guy at graham marine said my Johnson 90hp only puts out like 40 amps or something. And that's at wide open throttle. So pretty much useless for lake fishing. Thanks!
You would have to look at the charge curve for your alternator. At cruising, it is probably 25 amps.
A rule of thumb is 10 percent of the battery capacity for charging with most batteries being in the range of 100 amp-hrs. So the charger rate should be 10 amps for a longer battery life. For gel cells, it is 5 percent.