Hello Mr. Tore and Katuna,
That last post makes a lot of sense now... The batteries in parallel would allow a high current charge to happen while the U-Boat was in port. When out at sea and high current charge of 1660 amps with the battery in parallel (E-motor Armatures always parallel when charging) and driven by a Diesel engine at high speed with the prop un-clutched. Then at 830/415 amps, the battery in serial and driven by a Diesel at the appropriate lower speeds.
My internet translator and my German - English Dictionary just ain't as good at Mr. Tore's experience...
Regards,
Don_