Keep in mind that these alternators have a regulator that internally temp compensates the absorption voltage downward as the alternator heats up. This can lead to chronic undercharging especially with banks that demand lots of current
and GEL that cause the alts to run hot...
I have one Hitachi 60A alt on a customers boat that I can measure pumping out 65-67A when cold, at not much over fast idle. The voltage climbs towards 14.3V. This is pumping out in excess of its face value rating when cold.
However, as this alt heats up, the absorption voltage quickly begins to get reduced to the point where the batteries are seeing less than float voltage and the alt is putting peanuts into the bank due to the lowered target voltage. This internal temp compensation is a self protective feature to the alt but a bank destroying feature to the batteries. It can lead to chronic under charging if you have no other means.
For charging large cycling banks these alternators are often woefully insufficient due to this internal temp compensation feature. They are ideally designed to recharge a battery from a starting load not to charge cycling banks.
With an external regulator it would cut the field current/voltage to cool the alt but the target absorption voltage would remain the same. With these Hitachi alts the voltage gets reduced and less voltage at the battery means less accepted current
to the battery bank. Still cools the alt but goes about it by reducing the voltage the regulator regulates rather than reducing the field or current output. With a reduced voltage the alt pumps out less current the alt can then cool. Problem is you are essentially trying to charge a large bank of batteries on a float voltage.
Very often this little blurb in the Yanmar
alt specs is over looked:
Regulated Voltage 14.4 +/- 0.3V (at 20°C, voltage gradient, -0.01V/1°C)
This means that at 20°C or 68°F the regulator is operating at the target voltage of 14.4V. Great! But here's the rub. The alt WILL NOT stay at 20°C/68°F for more than a few seconds after start up if actually trying to charge batteries.....
These internal regs begin reducing alternator voltage from 14.4V by -0.01V for every 1°C change in alternator temperature above 20°C/68°F. It took me a while to get this confirmed, calls to Yanmar
resulted in NADA, and finally, some calls to Hitachi, answered my question and confirmed this spec.
If not careful one could mistake the low battery terminal voltage for voltage drop but when you measure the back of the alt, and it is only at 13.4V, as I have many times, that answers it...
It is easy to see how with an alt temp of 190°F the alternator is now pumping out 1.22V less than 14.4V or just 13.18V if we go by the specs...... I do find the specs on these alts to be pretty sloppy..... 14.4V +/- 0.3V for example means 14.1V (-0.3V) to 14.7V (+0.3V) is where your initial 20°C/68°F target voltage could