 A series of designs, starting with the VK 3001 (P), the unsuccessful VK 4501 (P) heavy tank prototype (which became the "Elefant" tank destroyer) and concluding with the heaviest armored fighting vehicle ever prototyped, the Panzerkampfwagen "Maus" of nearly 190 tonnes in weight, were just two examples of a number of planned Wehrmacht "weapons systems" (including the highly-"electrified" subsystems on the Fw 191 bomber project), crippled in their development by the then-substandard supplies of electrical-grade copper, required for the electric final drives on Porsche's armored fighting vehicle powertrain designs.The regenerative braking system, a core design concept of most modern production HEVs, was developed in 1967 for the American Motors Amitron and called Energy Regeneration Brake by AMC.This completely battery powered urban concept car was recharged by braking, thus increasing the range of the automobile.