I have an old charger from the 60's (or even 50's), the type that has a 6v - 12v switch, use it on everything including the bikes and had no problems ever, guess I'm lucky!
Throw it on the bikes during winter every four- six weeks or so for 4-hors or so if they're not being used much or just sitting, as they do when the rain is cold and the roads icy.