Muskeg,
Several things.
Replace the diodes with higher voltage rated diodes. Do it externally on a good heat sink.
Want to keep diodes cooler, 150C is their Max rating, do not want to operate there long term, will fail given enough time. Diode case temperatures while mounted to a heat sink are lower than internal temperatures, 150C case temperature the junction temperature is much higher, your way over the temperature rating of the diodes. I worry about electronics above 100C long term. Removing diodes will help rest of alternator run cooler... can't hurt. Careful shopping not too expensive.
Normal battery at 14v the AC peak voltage is 14 x 1.4 = 19.6v your at 40v.
The alternators output will be higher, hot diodes and wiring will push the internal voltage above 45v as seen by the diodes. The voltage rating of the diodes varies from 40v on up. Many alternators are 55v.
Some alternators use load dump zerner diodes, which start clamping the output above 40v peak voltage. I do not know the alternator your using.
100 amp alternator is the max rating, not intended to do this for hours. Most vehicles battery systems are charged within 15 minutes... Your over taxing your alternator. Suggest dropping down to 60 amps or less.
"While automotive units are not efficient they are easy to get.."
Since easy to get, use several, Your engine is rated at 15 hp, like you said it hardly notices it.
With two at 60 amps each your engine should notice that. Use separate diode banks for each alternator.
The field and resistors may be wasting power, the over all system is so inefficient, not going to give you any pay back to fix it. Energy is energy, 100w of 2,000 is 5%.
The more you can load the 15hp engine the more efficient things will be. Looking at ICE their efficiency typically peaks at 80% of rated load. 4 alternators at 60 amps each will give it a good load, specifics about your battery are missing, can it take 240 amps!? By wiring the fields in series parallel can improve the efficiency of powering the fields. (No switching regulators). 3 alternators looks about right in series (based on 7v from your notes). Notice I only suggested 4 alt. Automotive alternators are 54% efficient, the four will be fully loading your engine. Most small ICE are rated on thermal power, since 25% efficient, can rate it 4x larger than the HP output rating I call that a fantasy rating, like my lawn moor, rated at 6.5hp, the bigger engines seem to give a more realistic rating based on actual output HP.
Battery charging
Complex subject, will hit on couple points. High current charging the battery voltage will start coming up above 80% state of charge (SOC). Current must be reduced to let battery reach a fully charged state. Not necessary to fully charge a battery every charge or even weekly.
Less generator runtime = some success..
Less propane!
Have fun,
Scott.
