regen vs regenerative braking

regen

noun
  • A regenerative radio receiver. 

  • Regeneration. 

verb
  • To regenerate. 

regenerative braking

noun
  • A form of braking used on some electric vehicles in conjunction with the main braking system, where kinetic energy from braking is converted into electricity and fed back into the power supply by using the traction motors as generators. 

How often have the words regen and regenerative braking occurred in a corpus of books? (source: Google Ngram Viewer )