[Corsair]

I agree with Ducimus... the way the battery reload works, it basically takes one engine off from propulsion of the sub to action the alternator reloading (you can see the right shaft is not turning). I don't see why this engine would use more fuel running the recharge compared to actioning the screw shaft.
Of course if you want to maintain the same speed with one engine instead of two, then you have to have them run faster and use more fuel. But there is nothing that says that you have to. I just run slower on one engine while recharging for a few hours, nothing to hurry about.