Well, I didn't do any big in-depth studies on the matter, but I do know that by doing it my way in a type IID I did the Wilhelmshaven/Iceland/Ireland/Brest patrol, and was at sea continuously for 30 days. I still had some fuel to spare when I finely arrived in Brest. All battery charging was done at 2kts, which is practically at idle. All I know is: IT WORKED!