'B' is for engine braking. It's like trailer trucks' jake brakes - the engine spins to bleed off your speed. Under most conditions, you actually get LESS battery recharging when in B mode (so it is NOT B for battery, but actually B for braking).
When the hybrid battery reaches its upper charge limit (shows as 100% on the screen, but in reality it's only around 80% of full charge on the battery), the excess generated electricity is used to spin the gasoline engine faster. You'll probably hear the engine still spinning once you get down to flat land and cruise along on surface streets in electric-only, while the battery tries to get rid of that excess charge and get back to its happy medium.