For anyone doubting what I said about signal levels affecting battery consumption even when on Wi-Fi, take a look at the far right of the image below. I blew up dezy's chart above and moved it so that the right edge could be used as a ruler or straight edge. You'll need to tap on the pic to see it clearly. At the top you'll see a straight drop in battery power for a short while, indicated by the green line starting high and suddenly continuing a bit lower. At the bottom you'll see the signal going to red. The two are directly connected time wise.