I have several of the 3 amp versions of this charger/maintainer, and wanted another with a little more oomph. This is a great product to charge and maintain batteries in equipment that is not regularly used.
Had it on for over 16 hours said evaluating then charging then stayed there. May have worked on a smaller battery
Both the positive and negative clamps had to be attached to the battery in order for it to charge and the owners manual does not address this situation (not sure how many people will use it for a Rolls Royce though). That aside, it *fully* charged a *completely* dead accessory battery (it has two, one for starting and one for accessories) that had been dead for months, AND, it did it within a few hours. In addition, it is *way* too easy to use. Just start hooking the clamps up to things and when the combination is right, it’ll automatically begin charging!