Beery - some more links that I have found searching some infos about batteries recharging time:
http://www.pashnit.com/roads/cal/USSPampanito.htm (8 hours full charge)
http://www.geocities.com/gwmccue/Doc...port_1898.html (8-9 hours)
http://www.ibiblio.org/hyperwar/USN/...ubs/index.html (6 hours - 1.5h to 50% ??)
http://en.wikipedia.org/wiki/German_Type_XXI_submarine (less than 5 hours)
It seems that german boats recharge batteries faster?
I hope this helps.