Base on my experience (I have a Prius) it varies depending on whether one is accelerating or not. When going at a constant speed sometimes just the battery will be providing the power.
When driving at a constant speed, the electric motor may kick in by mistake sometimes, but if you are truly driving at a constant speed, your hybrid is not improving your mileage. Also, you say that "just the battery" is providing power. I don't think this is possible. I understand that the max depletion rate on the prius battery is only 15 bhp for 10 s. This wouldn't even be enough to over come the rolling resistance that your car experiences at highway speeds.
I have a Prius, too. At highway speeds I think both the gas and electric motors are running, but it still takes less gas....