This setup just charges the battery as long as the resistor is connected to the field coil. There is no regulation of the charge and it can overcharge the battery. To properly regulate the charging, the alternator should have a regulator instead of the resistor. The regulator will shut off the charging when the battery is fully charged.
The alternator’s metal body is connected to the battery negative with a heavy wire. The alternator’s large, high current terminal (it might be red) is connected to the battery positive with a heavy wire that can handle high current. The pulley of the alternator is connected with a belt to the engine to turn it. To activate the field coil of the alternator a resistor of about 33 ohms is connected from the battery positive to one of the alternator’s smaller terminals. When the right terminal is getting current from the positive, through the resistor to the alternator’s field coil, the alternator will get hard to turn and will put a load on the engine and the high current from the alternator’s positive will charge the battery. The 33 ohm resistor will let about 1/2 amp flow through the field coil. That may be enough for a slow charge. If the engine is powerful like a car engine then the resistor might be lower, maybe 16 ohms. The field current will then be about 1 amp and the alternator will be a heavier load on the engine. A typical alternator may put out about 35 amps continuously, maybe more for a short time. For the best monitoring of the charging current, the heavy wire from the alternator to the battery should go through a zero to 50 amp ammeter.
