My experience with Battery Tenders is that they do not have the oomph to bring batteries in lower states of charge back up adequately. For that, an older style charger with a amp meter on the front work mush better in my experience. Battery Tenders seem to do best when continuously connected to a pretty well charged battery sitting idle and not asked to deliver a lot of juice.
Just my .02
It isn't broken yet but I can fix that!
2013 550i, BMW F800ST, ML350