Smart battery charger for free-vented lead acid batteries. Modern chargers, so-called primary switched, which is the same technology as e.g. computers use to reduce dimensions, to increase performance and perhaps most importantly, get an interference voltage and current. The clean current means that the batteries do not get so hot during charging and they last longer. A clean, well-controlled voltage means that the battery charger neither interferes or destroys sensitive electronics in the car or caravan. They are packed with features and can charge a car battery overnight while being able to maintain a boat or motorcycle battery over the winter fully automatically.