Lack of Recharge on type XXI

Did the diesel engines just not put out enough juice?

Well yes and no, yes in that the diesel engines were powerfull enough to drive the generator, no in that they dont directly recharge the battery's, the Generator does (nowdays its an Alternator). They did indeed charged fine to 100%. Any Rechargable battery does in RL. If it didnt you wouldnt get an Amphour rating out of any battery now would you !!!

Mind you tho, the last 10% or so is always the longest cycle to be recharged.

Now as to the fact that the sub's got the TIME they needed to recharge their batteries to 100% is a differant story. That was all subject to daily events.