Will I damage my alternator by using it to charge a flat battery?
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"Bad batteries take a lot of current trying to pretend they are okay. As they

get weaker the battery voltage deteriorates, from failure to hold a charge

and sometimes from shorted cells. Either way, failing batteries (or dead

batteries) cause the alternator to put out maximum current for progressively

longer times in order to bring the voltage up to the regulator level.

"Alternators are odd things - by design, they will only put out so much

current for a given field excitation. The ratings you see on alternators are

those maximum ratings for about 14 volts on the field. The way that works is

that the voltage they produce is proportional to rpm, and the frequency at

which they operate internally is proportional to rpm. Since the stator

windings are inductors, their reactance is proportional to the frequency (in

turn proportional to rpm) so the maximum current the alternator will put

out, even into a short circuit, is limited.

"In the '60s, the alternator could put out the full rating (usually 35 amps)

indefinitely. They were big, Tim Allen devices that scarcely ran warm at

full output. Those days are long gone. Modern alternators are much smaller

and are called on to put out much more current - often 60 to 100 amps. With

a good battery, the electrical systems are carefully designed not to burn up

and still keep up with demand. With a bad battery, the alternator runs too

hot and cooks the insulation and the diodes. Eventually too much damage is

done and the alternator fails."