No gas? No problem!

[Chardok]

...Or is it?

So, I'm coming back from a marginally successful patrol off Honshu, sinking three merchants and dropping James bond off in Tokyo, and a couple times I had to go balls-to-the-wall to get away from some destroyers (Couldn't submerge, or I'd have sunk...all I could do was go flank to keep a little distance until nightfall).

I get about 250nm out of Midway and speed drops to 0kts, totally 100% out of Diesel. So I decide to ssee how far I can go on batteries. Do I dive to 30ish feet, go a few nm, then surface. Thinking I'm borked, I sit helplessly on the surface for awhile, then I get a notification. batteries to 25, sir!

WHAT?!

My batteries are charging without running the diesels? HOW IS THIS POSSIBLE!

not one to complain, I porpoise to midway, top off and head on in to pearl. So how is this possible? and is it possible to radio for help or something if you run out of gas?