I've found you will get a true fuel consumption with a journey over 10 miles. You'll see the display gradually improve between 0 and 10 miles as it all warms up. Engine starts immediately from cold as it runs to get a minimum heat level, then it will cut on and off depending on Battery and driving demands.
It will not make a difference to how fast the 12v battery charges - stationary, in traffic, or belting down the motorway, are all the same to the 12v battery. As long as the 'system' is 'on', the traction battery and inverter is in control of 12v battery charge regulation. The engine obviously only runs to keep the traction battery topped up . 96 243 97 494 29 351 285 229