There's lots of discussion around lower FE on short trips because the ICE needs to warm up. But what about the batteries? Traditionally, batteries have higher internal resistance at lower temperatures. I'm guesssing this is also true for NMH pack in the Prius. Perhaps it's a question of numbers: What percentage of the Prius' energy gets stored in the batteries (as opposed to going directly from ICE to wheels)? And what is the efficiency drop in battery storage between, say, 80F and 30F? In my EV (a converted Geo Metro) I put battery warmers to improve economy/range (when the car was plugged in/charging, the batteries were kept warm).