Just to throw something in from "left field", my understanding is that the main failure mode for batteries in these systems is sulphation, and it is the solar power community that led the way to the use of de-sulphator units.
I've owned one of these for
several years now, and I'm quite convinced that they do have a good effect on old
sulphated batteries. Of course, there are other factors that cause batteries to decline, especially in car use, and de-sulphators are of limited benefit in those cases.
The de-sulphator which I have came from this company
http://www.courtiestown.co.uk/. The fact that they have been in business so long, says something about de-sulphators for those who are doubtful (no connection other than satisfied customer)
I understand that the International Union of Pure and Applied Chemists (IUPAC) has now agreed that everyone should use the term sulfation, but I don't want to do that... and also, I still regard Pluto as a planet.
B