Determining the safe max. output of an alternator can be complex or if you are comfortable with using the alternator temperature sensor found on most external regulators, simple.
First let's conclude that your existing alternator has no ability itself to actively reduce output as a function of temperature, although some do in an effort to save themselves even though they are internally regulated.
You need to determine from the alternator manufacturer what is the max allowable temp. and where on the alternator is that temp. measured.
According to Delco, my 40si is rated at 257º F at the center of the rear air intake. I actually questioned the location and had it confirmed in an email from the Engineering Dept.
The simple way is to set the temp. sensor of the external regulator to something less than the rated 257º, and mount it on the assigned location on the alternator. Plug it in to the regulator, program the regulator and you should be running at the max. allowable output at all times based on alternator temp.
If you are only using a DC to DC charger things get a little more complex and subjected to a bit of trial and error.
First we need to quantify what the max. load on the alternator is without any charging going on other than that required to support your running loads like Nav. lights and equipment, radar, sounder just the regular stuff. Let's say that load is 25 Amps.
You have an alternator with a name plate rating of 105 Amps.
So that is 80 amps of potential charging, but some of that is unusable long term due to heat.
Victron makes a 50 Amp DC to DC charger that will likely do quite well in your situation.
I would start with the charger programmed for the charge parameters of your new AGM inverter bank and the output of the charger limited to 25 Amps. That loads the alternator at a total of 50 Amps or about 50%.
With a depleted inverter bank go for a cruise at your normal speed and monitor alternator temp. with an IR gun or better yet a thermistor and a meter. Run for 2 hours with the alternator outputting 50 Amps to establish a stable alternator temp.
If you have headroom on the Temp you might want to reprogram the DC to DC to output 35 Amps and retest.
You could continue to step up the DC to DC's output current (which is a load on the alternator) until the 50 Amp capacity is reached which would put a 75 Amp load on the alternator.
If your alternator is still cool (doubtful running at 70%) you could then add another DC to DC charger and add more load to the alternator.
How close to cooking it you want to run is up to you.
As for me, I run my 257º rated alternator at a 185º max temp and have the regulator derate the alternator as required. This derate does not happen too often as I happen to have a cool running engine room with lots of cool air flow directed at the alternators.
I am not a fan of running electronic things at anywhere near their max temp, but that is just me.
I quickly looked at the Prolso which is some kind of progressive combiner/isolator and don't see the usefulness of the product.
