The chemical reaction that occurs when charging heats up the battery pack, if the computer detects the batteries are already too hot, charging is restricted to protect them. I've heard of this happening if the vent to the battery pack is obstructed by something, 120 degree weather could very well do it, as well. This is one of the noted design flaws of the Honda Hybrids -- the fact that the battery pack is air-cooled from the cabin-air, rather than cooled directly by air-conditioner refrigerant, as I believe the Toyota Hybrid system does it.