Complex sentence: The key improvement in the Type XXI was greatly increased battery capacity, roughly three times that of the Type VIIC with new electric machines, which gave these boats enormous underwater range, and dramatically reduced the time spent near the surface.
Split into simpler sentences: The key improvement in the Type XXI was greatly increased battery capacity, roughly three times that of the Type VIIC. This gave these boats enormous underwater range, and dramatically reduced the time spent near the surface.