I have had several hybrids (Sienna has not yet arrived), 2 Toyotas, 2 Hondas, and all of them would charge the battery when going downhill. I dont think you have a need for power when coasting downhill, mostly. Key word "when going downhill". Your car is slowing down on exits, charging the battery. My opinion is dont worry about it, the hybrid system is great, but not perfect.