By definition, that coil of wire is an inductor. Inductors oppose change in current flow ( reverse emf ). ( likewise, Capacitors absorb/discharge voltage ). The small ( varaible )voltage drop you are experiencing is due to the internal ( very low ) resistance of the coil ( as a side note, a low value high wattage carbon composite or metal film resistor will serve the same voltage dropping purpose- but not all will have a filtering effect.). Not very efficient as a VR, but will provide a decent 'choke' effect for emi interference, and spikes/surges from the generator. Add an electrolytic capacitor( from positive to ground ) before the coil, and after the coil, and also add a ceramic disc capacitor after the coil in parallel with the other electrolytic capacitor. ( You have just made a PI filter, a very good power filter circuit ). If you wanted to get fancy, you could also add a fuse ( prior to this stage in the circuit, and right after the fuse, a zenir diode and resistor combination between positive and negative ( calculate so that 16.5 volts allows the diode to conduct. In this circuit, if the generator fails, and produces over the 16.5 volts, the venir diode would start conducting, thereby creating a short circuit, and then the fuse would fail. Proper fuse value must be determined. ( higher ( I ) than the regular charge rate of the generator).
If we keep on.. these tractors will have some sophisticated electrical systems soon.
Soundguy
""If charge rate is more then above, take a coil of 14 gauge wire and put it in series between alt and battery before any loads.
You then vary the loops on coil (length of wire) to get voltage down to what you want at (13.8/14.8). This cheapo VR will work with a small output alt like yours. The excess voltage and current (watts) is converted into heat in the coil of wire. ""