Electronic – 3 Phase alternator to charge batteries

Okay, lots of stuff to talk about.

The thing about battery charging is that the battery has its own voltage, so resistance is only half of the story. If the charger is at 12 volts, and the battery is at 11.6, then the .4 volts difference is all that is pushing the current into the battery. This small voltage divided by whatever resistance it finds in the cables and battery determines that current. You might see a 100 amp peak, say when starting an actual car. That causes a rather large voltage transient.

And then, that isn't exactly right either. The voltages that you see while charging will be really very close to each other, and what's actually happening is you are replacing electrons in the battery chemistry to make its reaction go backwards. Its just a lot easier to model this mathematically as an ideal voltage source behind some internal resistance.

But the resistance is a real thing. Professional tools actually place a small value of AC current through the cell and take an AC voltage reading across the cell. This and Ohm's law gets you the value of the internal resistance for the pack or cell, wherever you can get terminals to check it. But, generally, this won't matter to you.

Hooking it all up together will work, with the caveat that diode Vladimir mentions needs to be in the line so the battery doesn't discharge through it. It may be built into the alternator already, maybe not. Do check that. What will happen is that when the battery voltage and the alternator voltage are the same, with no other loads connected, no current will flow.

What happens at the indicator terminal is that there's a trio of diodes from the winding phases that provide the current for the field coil. When the alternator starts up, some current is needed through the field winding (on the rotor) for it to generate enough magnetic field to produce an output, so that's why you needed a low resistance to replace the bulb. Once the alternator is producing an output, the trio brings that terminal up to the same voltage as the battery, since the drops across the trio and main diodes are similar, so there's no potential across the bulb, and the trio is providing the field current. With the alternator not turning, the bulb (or bias resistor) current is still being dropped across the field winding, which is a few ohms so the voltage at the terminal is not zero. You could try putting a couple of diodes, (or a zener) in series with your 4k7 and another pulldown to ensure that the input is below the low state threshold on the MCU input.

Other alternators have a regulator that switch the field internally based on the output of the windings under the residual magnetism of the rotor, these just have a transistor (usually a FET) that pull the indicator output down. These would function the same - they don't need the bulb or any other current source to aid starting up.