A $5 transformer wouldn’t have enough capacity to run anything except a small radio anyway. Why do you think jumper cables are so thick? A tiny little transformer would likely not even run the interior lights in the car.
I had a 1953 Chevrolet that never wanted to start below zero. In those days the mechanics all said to use, I forget, I think 10W30 or something like that. I kept a 12v battery in the house on a trickle charger, and when it was below zero, took that out and jumpered it, using a screw driver across the starter. And, when it started, I immediately jerked off one of the jumper cables.
But, the only real electronics was an old tube radio, which I would shut off when I shut off the car. The ignition was the old style.
When I gave the car to my brother, who was smarter about oil in those days, he put in #5 and it started all the time.
It is hard to say what 24v would do to a modern, all electronic car, but it could vary from nothing, to everything in that car, totally fried. Everyone here is correct. The battery is not the issue, though it could also be blown up if things went sour.