I'd like to add some new infos here about recharging batteries.
The recharging technique used in SH5 (and other SH series probably) is not related to your sub speed. Your Engineer will decouple a diesel engine from the propeller and run it full speed to recharge the battery, this is true wether you run at full speed or wether you stand still on surface. The recharging speed should be the same in all cases, although you will have reduced sub speed when chosing speed sets with the dials, but that's of no importance because you can set a specific speed, with a VIIa/VIIc you can run 14 knots while batteries are recharging, not more I think.
You give the order to recharge batteries by selecting the Engineer icon (first from the left), chose Propulsion and click the battery icon, this will start to blink, it takes a couple of minutes for the crew to decouple the propeller and initiate the recharging procedure. This happens automatically when your sub returns to surface, but it's necessary to give that order in the case you want full propulsion for your sub and then return in battery recharging mode.
It's important that you don't go in high TC until you read 'Batteries recharging' in the message box and also hear the voice message,
otherwise the battery will NOT recharge during high TC.
A nuisance is that if you are at decks awash depth, the battery recharge order in the propulsion menu will not work (at least in my modlist, TWoS I don't know) and giving that order directly to the engineer NPC will cause that he will return to standard propulsion soon after, so you need to be fully on surface for recharging batteries. Also I still don't know if recharging is possible when running with the Snorkel deployed at 7 knots (max speed with Snorkel), probably not.
So the only 2 things to remember are: To recharge batteries, be fully on surface and be aware that your max. speed will be 14 knots.
I hope this helps!
XS