Good questions. I cannot answer them definitively but nonetheless I would like to participate in the discussion.
I am not sure about the hydrophones. On the subject of hydrophones, I am aware that there are some very skilled players who can conduct sound bearing attacks using the in-game hydrophones. I've read that the actual hydrophones on the earlier war subs [the advanced subs at the end of the war had very advanced sets] were not accurate enough to get such precise data, and that therefore this is not a historical method. Hey, by all means play how you wish and hats off to you. I try to play historically as closely as possible so therefore I do not use this method. Not a criticism, just an observation.
Batteries... it would seem logical that the engineers who designed the U-boats would know that servicing the batteries would be a requirement and that it would be likely that damaged or worn out batteries would need to be replaced at some point. So, although I cannot cite a source, I would think that batteries could be replaced. In the film Das Boot you can see the battery access hatches when the Chief goes down to bridge the damaged batteries. That looks like quite a sizeable opening. I would think that in the yard, with the proper gear they could get in there and haul them out. If it were not possible to replace batteries, then the boat's career would be limited at best according to the status of the batteries. I just can't see the Germans designing subs with such a flaw. My guess.
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