I always form charge Nixx at 0.1C and I have never had a battery get warm at that rate of charge. It is usually less than 1 watt being converted to heat at the end of charge so usually the inherent loss of energy by convection (and conduction to whatever it is laying on) is enough to prevent much of a heat rise. I have never tried to measure the temperature rise so there might have been a very slight one under some conditions. My experience has been that, if there is a heat rise, you are form charging at much to high a rate.