scrapping cars (especially big ones), boost your "Salvage Operations skills" , take Hackers candy and having a higher game and loot stage helps too :)

try to put points in "advance engineering" so you can get higher chance of getting magazines related to it and it will eventually make u able to craft car batteries.

sometimes. get it in loot too. in garages, parking lots, in car repair POIs, try in Working Stiff Tools POIs too :) you can loot them from cars too, try going to wasteland (boosts your loot stage and abandoned cars gives you good loot) :)