After reading several topics about batteries and chargers I decided to find out what I had. After much effort to get the unit out (see photo) I found I had a Parallax 775, a 75 amp ferrroresonant converter/charger. This unit is very simple - a ferroresonant transformer and a diode bridge but their documentation claims that it will trickle charge a fully charged battery. I don't know if I trust them. Are these any good?
John, I changed mine out and replaced it with an Iota 90 amp smart charger. It was under $300 and ended my battery issues. The Parallax was still charging and my son-in-law is still using it.
This will ruin your batteries. It is not a multi-stage smart charger.