Well, my results with the fix Cdr Gibs posted are excellent. Note: I also did something he suggested that wasn't in the original 'list' of what to change that he posted. That is going down near the end of the basic.cfg file to find the Battery upgrades, and set the energy on the battery to 1000. While it doesn't truly 'fix' the long recharge problem, it does work. You will probably be unable to drain your batteries after setting that, and since I already hunt in a pattern of 12hrs hunt, surface to recharge a bit...(or snorkel), then back to the depths to hunt more, it doesn't bother me to essentially have limitless batteries. And I still have to deal with oxygen forcing me to surface as well, so it's not a big realism hit, to me.
Thanks again, Cdr Gibs, that fixes the XXI enough that it's become my favorite boat. I switched last night, and had one of the best patrols of my career!