Both the batteries and the charger can get quite warm during the charging and that is where the 15 minute batteries save a lot of power. The heat is a result of low efficiency of the transformer inside the charger and resistance of the batteries. The efficiency of normal chargers is probably somewhere between 60 and 70 pecent(maybe even less), that means 20-40% of the energy is transformed to heat.

The trick of "green" chargers is that they are more efficient. These ultra-fast chargers only charge the batteries to 85% of their capacity because the efficiency drops rapidly beyond this point. If you leave the batteries in, most of these chargers will continue charging the batteries slowly until they are fully charged.

Some time ago, Varta released another type of green batteries that do not self-discharge over time, but that is not related to the topic.