I was driving on EV mode with the battery well charged. All of sudden I saw the EV sign went off and heard the loud noise of the engine running. On the screen for vehicle energy, I saw a straight green line, indicating that engine was running to charge the battery. The whole episode would go away in a minute or so and the the EV sign would come up again. First it happened only once a week or so, but today it happened three times in my commute home. Does anybody know what this is?