The "Black & Decker" unit does a good job. There's some kind of circuit in there that can detect when you've got it connected wrong, when a correct charge is underway, and when the charge is complete. When status is "complete," the unit is not delivering current to the battery (I checked with an ammeter in series with a clip lead)This unit further can detect when it has become time to top off the charge, automatically switching back on again to restore a full charge.I use it on a trailer boat parked in the garage.  The outdrive has an electric circuit to prevent corrosion in salt water, and it will drain the battery in about a month unless I disconnect the battery or use this maintainer.  It works well.I've watched how the LED will turn solid green indicating a full charge, and, after a period of time, go back to flashing green to restore a charge as described above.The connection safeguard is pretty cool -- brushing the adapter across the carpet in the boat when it's slightly moist triggers the LEDs to indicate a bad connection. That's sensitive.I like that they include three ways of feeding the battery. Each method connects to the output cable of the charger.In particular, the 12V power connector (cigarette lighter plug) is an excellent way to backfeed a charge to the battery. For the boat, I don't have to lift the engine bay lid, un-do the plastic retention strap and plastic lid to the battery box to get to the terminals. For a car or truck, it saves having to put the clips on the battery terminals and perhaps leaving the hood slightly open.Just remember to make sure your accessory jack is always on, and not disconnected when you turn off the key to the vehicle.In the Olde Days the auto supply houses like Pep Boys sold little boxes called trickle chargers. They had a transformer and a set of diodes to take household current and convert it a voltage just a little bit higher than a fully charged battery.  This was a good match, because leaving it connected for a reasonable amount of time would typically not boil the battery and ruin it like a charger with higher current capacity would do. You also would check the level of battery acid every so often, and top it off with distilled water to compensate if the charger did cook off some electrolyte.With Digital Technology (insist on including the hype word in there, regardless of whether there's any digital circuit involved) the contemporary "charger/maintainer" can deliver more current than the old trickle type, but also can sense, electronically, when enough is enough and shut down. This means you can leave it on there for prolonged periods of time and not risk burning out a "maintenance free" battery whose cells are not easily checked for level anymore.