Back around 1978, my wife and I bought an old Fiat 124 sedan. It was at the owner's house and the battery was dead. I disconnected the battery terminals and connected the jumper cables from our car directly to the battery terminals and the car started. After the Fiat was started, I connected the cables back to the dead battery, and I could tell the Fiat's system suddenly had a heavy load on it but continued to run. It was starting to get dark and I got the Fiat home as quickly as possible without turning on lights, radio, etc. Made it home fine and got a new battery the next day.
My questions are: can modern cars be started this way? Will it damage the electrical/electronic system?