MS recommends a higher output alternator (120+) so that your alternator will provide the maximum power that your batteries will accept without putting undue stress on the alternator.
We need to remember that the vast majority of "high performance" aftermarket alternators are
based on "small case" automotive alternators. The Balmar, Electromaax or my own CMI alternators are all
based on a heavily modified CS130D frame. While this frame is ooodles better than the older 10DN frames, that used to be used, they are still limited due to cooling capabilities.
These alts are capable of delivering a huge portion of their total output at a very low RPM. Low RPM = low cooling capability. High output at low RPM is part of the high performance design criteria for a cruising boat.. However this frame capable of spinning at upwards of 18,000 RPM
continuously. At 18,000 RPM you'd get a
LOT more cooling, yet we turn them very, very slowly in comparison.
While these alts are
redesigned and
hopped-up in numerous ways, they are still
small case alternators.
A mistake many installers and DIY's make, due to what I do consider quasi-misleading marketing, so not entirely their fault, is to believe that a 70A, 100A, 120A or 150A high performance small case alternator can run at this output rating, in bulk, all day long. This = FAIL!
Nothing could be further from the truth. Even modified super-duty large frame brushless alternators, such as those based on a 28SI frame etc., can't run at full bore for hours on end. They do better than a small case, sure, but they still can't cut the mustard into a LiFePO4 bank unrestricted and non-temp protected.. I have seen numerous large frame alternators cooked, when folks believe incorrectly, that a large frame alt can run at full bore for hours on end. Even these very, very expensive large frame alternators require temp protection.
The only way I know of to get full output, or
close to it, from any small case alt, is to remove the internal rectification and remotely rectify the alternator. This is what I have done on my own vessel but I am driving a LiFePO4 and do not really have the space for a large frame alt.
Bottom Line: If you desire a steady 100A output you buy a 120A or 140A output small case alt, and de-tune it in Belt Manager. If you desire a steady 150A output, with a small case alt, you buy a Balmar AT-200 and de-tune it...
Good advice regarding regulator location;
Yes the Balmar regulators should not be installed in an engine bay. Especially not up high where the concentrated heat is.
but, my problem was that the alternator temperature, as measured by the alt temp sensor, was exceeding the Alternator Temperature Threshold (AL1) (pre-programmed in the MC614). When the alt temp rose above the AL1 set point of 104C the MC614 would reduce the field output by approximately 50%.
This is the job of the Alt temp sensor. Balmar however has since changed the way it cools the alternator. I call it "
Adaptive Temp Sensing" and have really been pushing Balmar to market it because no other regulator maker has it and it is significantly better than 100%>50%>100%>50% or 100>0%>100%>0%. It was a change made due to field requests and one Balmar did make while the engineer who designed it was still alive.. With the new
adaptive temp sensing field output moves up and down in 5% increments and essentially finds the max output sweet spot.
I still prefer to use the alt temp sensor as an
insurance policy and limit the max output in Belt Manager. This means a much longer life for the alt.
This would cycle until the batteries reached a sufficient charged the alt output would drop and subsequently the alt temperature. Once I got some serious air flowing across the alternator and adjusted the belt manager, the alternator temperature seemed to be happy.
This is why limiting the alt in Belt Manager yielded significantly better performance than depending upon the old temp sensing circuitry. Sure you made a 5-20% decrease in max output but it is steady. With the new temp sensing there is no more ping-ponging but I still don't like bouncing an alt off it maximum all the time hence Belt Manager as primary limiting then alt temp sensor as an insurance policy..
In my article
LiFePO4 On Boats (LINK) I have an entire section devoted to alternators. Ignore the LiFePo4 stuff and skip to the alternator part.
Also one area folks routinely screw up on (including the voltage regulator manuals), for optimal performance, is proper voltage sensing.
Alternators & Voltage Sensing - Why It's Important (LINK)