Actually all Nickel batteries, be it Cadmium or Metal Hydride, have a condition, commonly known as the "memory effect", just that it is much less suceptible in the case of Metal Hydride as compared to Cadmium batteries. A discharge of the Nickel Metal Hydride batteries once in a month or two should give the batteries optimium performance.
I suppose the batteries get pretty hot during charging using the 15 min charger, do they? Good thing it has a fan.