The Diesel is very much only an emergency backup. They are not expected to "charge batteries" under normal conditions during a trip, for all energy for board systems and engine is not coming from batteries, but directly from the fuel cells, the traditional diesel-battery-engine-concept cannot be compared to in this case - this is NOT a normal diesel sub with an added fuel cell. The engine is directly powered by the fuel cells. The energy in these fuel cells can only be replenished at harbour, by replacing and/or loading the cylindrical modules that store these ressources (they do indeed look like giant AAA battery cells,
) Normal operation of the the engine and screw is energized by the fuel cell only - always. Top secret propeller and fuel cell makes for so little noise at up to medium speed settings that the sub is described to be "undetectable by contemporary passive sonar even if sensors are just a "Katzensprung" away" (quoting a leading engineer of HDW in an interview with a defense affairs journalist at the beginniong of this year, a guy I once talked to). The same source told him that the "212 is easily the most "non-existing" sub in the water today."
The limited hull size (still much more room than in the 206 and 209) is referring to the roots of the project in the cold war. The 212 hull design was once meant for exclusive Eastern sea operations.
The duration of the sub beeing able to remain submerged without snorkeling or taking ressupplies is top secret, and various sources indicate various time frames from up to 3 weeks to up to 4 months. Since the sub is Germany's top intel gathering platform is is now meant to serve in that role globally, if wanted, and that requires long endurance capabilities. time and again German medias have pointed out that this sub can operate completely indepedent from surface (snorkelling, ressuplying) for "many, many weeks". That'S why some tend to think of it as "a nuclear sub without a nuclear engine."
but whatever - it looks damn sexy.