There's dead and then really dead.
Most modern cars also have an ECU and an electric fuel pump, those need power too otherwise no bueno. Maybe bumping and residual magnetism will get them going again, maybe not. I bumped my old Rabbit GTI several times while I had a bad battery but was waiting for next paycheck... but I don't know if I'd count on it with a 100% dead battery or a significantly more modern car.
nate
I've never tried starting a car with a really dead battery but what you say makes sense. I once drove my old Scirocco off the street when the fuel pump wasn't getting any power. Just put her in gear and turn and hold that key in start. I just came up with that while my car was at a red light and the engine died. That starter moved the car just fine. That was my favorite car even though the engine died at random times.
Mopar put Alternators in the 60 Valiant along with the new Slant Six. I think there were some years before that where you could get a large, heavy French Alternator as special order, like for police use.
Google,,, History of Alternators (allpar website)
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