As an experiment, I tried starting a junk car (2005 Malibu) that didn't have a battery using jumper cables: Energizer 30 ft 1 Gauge Quick-Connect Booster Cables. To my surprise, it worked (although, just barely). I was able to start the batteryless car by boosting it from my 2010 Silverado 1500.
Question:
Did that only work because the cables were such a heavy gauge? Would thinner cables not be able to boost a batteryless car, due to reduced current?
Did that only work because the cables were such a heavy gauge?
Correct. Usually when using smaller gauge jumpers with a battery in the car, you are doing two things. First, you have to leave them on the "dead" battery for a while. This puts some charge back into it. Secondly, you'll have some power coming through the jumpers to help get it started.
Would thinner cables not be able to boost a batteryless car, due to reduced current?
It would behave pretty much like if there was a battery in the car and you hadn't left the jumpers on it long enough to give it a little bit of a charge back. It may sluggishly turn the engine over, but most likely would just cause the starter to rapidly click, or just click and thunk, either way the engine wouldn't start (most likely). There would be a point when the jumpers would not be able to provide enough power to get the car started, whether that's 2 or 4 or lower gauge wires, I'm not sure. There is the risk, as Weather Vane stated in the comments of burning out a smaller cable. If you put too much of a load on the cables for too long by continuously trying to start the vehicle with too small of cable, the amount of power going through the thinner cable could cause it to literally melt the wire. This could cause a catastrophic mess.
You weren't boosting the dead car, you were powering it totally via some longer leads. The longer leads have losses, so shorter is better. You're right that thinner leads also have higher losses. So short&thick cables are best.
Once the car is started, it should be able to continue running off the output of the alternator even without a battery. Assuming the alternator and wiring is working correctly, etc.
The battery is required only for starting the car, else you'd need a crank handle or some other way to spin the cold engine over. Imagine a rope-pull on a car, like a lawn mower or chainsaw, or a kickstarter like a motorbike.
The other purpose of the battery is to provide power when demand is high. Sometimes the alternator cannot provide enough power to run all the lights, heater motor, electric fans, and options like heated seats or large audio amplifiers, and the battery helps bridge those load-spikes.
In theory you could drive the running car without a battery, but protect the positive battery lead so it doesn't short-circuit onto any part of the chassis. Also, don't stall!
