Question Regarding Why 12v Alternator Won't Self Excite.


Joined Jan 23, 2018
Often a "fairly dead" wet cell battery will still have enough power left to get an automotive alternator started generating some voltage, which can then get the device eventually producing 12 volts.
BUT there is another factor in play here which is that as all physical alternators (as opposed to theoretical ones) are a bit less than 100% efficient, that physical driving power must be a bit greater than the electrical output power. And usually that also means spinning fairly rapidly.
So the alternator used for battery charging needs a serious "prime mover" to spin it. Most often this is an internal combustion engine, although I have red about both wind power and water power being used.

12 volts and 105 amps is 1260 watts, or close to it, and with 760 watts per horsepower and believable efficiency it will need over two HP for that sort of output. Not a huge engine, nor a small one.