Quote Originally Posted by jwatte View Post
That's actually pretty crazy. When a car battery is low, the alternator will have to produce a fair amount of current for a fair amount of time; I wonder if you can kill the alternator in your car by doing that on a hot day?
Typical car batteries are ~40Ah/5hr and only take a minute or so with the alternator at higher speed after starting to recover most of the energy/charge used during starting (my CR-V starts at ~1500RPM then to drops to ~1300RPM then eventually idles ~800RPM when warm). Even a weak/dying battery will get a couple engine starts without any recharge. When my <6 month old defective battery went fully flat overnight (~9V and wouldn't even turn on status lights to crank) only took ~15 minutes with a Imax B6 50W charger to get back to a voltage that could crank up again (<400mAh total recharge).

In almost all alternators, the 'internal regulator' is just a circuit to control the rotor excitation voltage to adjust rotor field strength which adjusts the 3-phase diode-rectified output current of the stator to match the load to achieve the desired voltage (no linear regulators or filtering of the output). If there is no internal regulator, then an external system has to continuously provide the rotor excitation voltage needed to keep the rectified output at the desired voltage. The lower the RPM, the higher the excitation voltage required to achieve a given output power. Higher excitation voltage means more heat produced in the rotor and more stress on the rotor brushes.

If the alternator follows automotive standards, then it will have three numbers stamped on it somewhere (e.g. "50/100A 13.5V") which indicates current ratings at 'idle'/1500RPM and 6000RPM as well as test voltage (https://www.lifewire.com/understandi...ratings-534785). If the engine and pulley combination is spinning the alternator below its 'rated' RPM, then the actual current output will be severely limited (voltage will sag) and trying to draw more current for an extended period will possibly damage the rotor.