Why hasn't this ever been in to practice?

[badassyas]

Ok. So you have your vehicle which with just over 12 volts cranks the starter motor happily and fires your car in to life. So there's ample oomph available...
Why aren't there self propelled alternators?? They have less rotational resistance. You'd switch the ignition on, a 12 volt, 2500rpm motor starts spinning your alternator bringing charge to 14.4v. Hence you'd always have decent cranking power, less drag on the crankshaft reducing emissions and increasing performance. And freeing up engine space as it could be relocated to wheel arch areas Etc.

[Speedtouch]

Don't forget, the alternator has field terminals (to generate the electromagnetic field) which actually need a +12V feed before the alternator will actually generate any power, so if you're having to use a 12V motor to drive it as well, that's quite alot of load on the battery to even get the charging to work - all well and good if the battery is fully charged and strong, but hopeless if you're dealing with a weak, partially charged battery in the depths of winter.

What you're proposing is some sort of perpetual motion/free energy machine. Consider also the frictional losses and losses due to resistance in the cables/windings, and it becomes an impossibility; you would need friction-free bearings and super-conductors that only operate at very low temperatures (less than -273 degrees Celsius) to make it possible.

Energy is neither created nor destroyed, and you can't get something for nothing!

[BATTS91]

We try a very small quick similar mock up of somthing similar we found we had to find quite a large 12v motor to produce enough revs to constantly run without burning the brushes ect , this was an attempt to produce a 12v self propeling generator type device , = no running cost free electricity . The thing is ud also still need a battery too :/

[rich318i]

it needs a world war to bring that tec on the market, stop start was made by VW in the 80's but was put on the shelf due to the fact starting the engine uses more fuel. and GM own the right's to magnetic engine they paid millions for the rights that was in the 70's.