I have a trawler with twin Yanmar 4JH4E, and a battery bank of 12 DEKA GC15s in a 6 x 2 configuration, 1,290 AHr capacity.
After burning up two Balmar 100 A units, two Balmar 150 A units, and two Electromaax 180 A units, I finally threw in the towel on marine rated small case alternators.
You simply had an incorrect set up for any alternator, not just small case, unless you are talking an Electrodyne or a large brushless alt with external rectification..
I installed heavy duty aftermarket Delco Remy 24SI alternators, rated at 160 amps continuous, 200 amps cold. I have about three hundred hours on them and have had no issues. They run hot as hell, but are built to take the heat as they are designed for heavy duty diesel trucks.
You will cook the 24SI's too. Even at 320A you're barely breaking a charge rate of .25C and bulk will last approx 1 hour. I would strongly suggest setting the belt manager to at least level 3 and running the absorption time out. Deka's do best at 14.6V in a PSOC situation..
My engine room is hot, running at about 35F above ambient, but not as hot as the under-hood temperature of a dump truck on a hot summer day.
A dump truck on a hot summer day is not asking for full output for for more than a few minutes at a time which is exactly what these alts are built for...
They charge the battery from 50% to 100% in about four hours.
A Deka AGM is not full until amperage at 14.4V - 14.6V is 0.3% of Ah capacity or less. This is less than 4A for your bank at 14.4V - 14.6V not float voltage. You can not take this measurement at float voltage as it will be grossly misleading. Unless you have carefully programmed your Balmar regs and gone into the advanced settings you are suffering from premature float.
It is not physically possible to charge a Deka AGM from 50% SOC to 100% SOC in 4 hours. Even in ideal lab type conditions, where absorption voltage is held until amperage declines to 0.3%, before dropping to float, this takes in excess of 5.5 hours with basically brand new batteries. As they age and sulfate this takes even longer. Your Ah counter is simply not programmed correctly and is misleading you as to when you hit 100% SOC. If you keep this up the capacity of the bank will continually walk down.. Try to get to a real 100% SOC
at least every 4-7 days..
If the regulators have not been programmed using the custom settings your performance could be made significantly better.
They put out 125 amps at idle.
Idle is the most damaging output range for many alternators because cooling fan speed is greatly reduced.
My regulators are MC614 with a Centerfielder. At $145 each I can afford a spare alternator.
The factory 24SI has a thermal protection built into the regulation system which folds back charge voltage based on alt temp and this in-turn reduces amperage output. When you convert a 24SI to external regulation you lose this thermal protection.
FWIW I have been charging a LiFePO4 bank now for over 4 years with small case Delco CS-130 based alts (same as Balmar 6 series & E-Maax).... The secret is in proper set up and
not expecting the face value rating. From my bottom SOC the small case alt is in bulk for upwards of 3 straight hours, if I were to run the engine that long. Unlike an AGM bank my alt never gets an "absorption break". I can do this thanks to the use of
belt manager with temp compensation as an added insurance policy...
A few weeks ago I ripped out the rectification and made it externally rectified, because I can not fit a larger case alt on my engine without serious engine room mods.. Based on test runs I am seeing significantly cooler temps and at about 25-30A more than I had been running it at in the past.
The 24SI is an excellent stock alt but the best versions, really wired for high performance, cost a bit more than $145.00. Mark Grasser and American Power Systems both build some really solid 24SI conversions. I have an APS alt that runs 190A at idle feeding a massive Genasun LiFePO4 bank, bump it above idle by about 100RPM and it is kicking along at 225A.... Still, even these alts need proper set up and need to run belt manager.
The big mistake most folks make is thinking their alt can run at its rating all day long.. Buy bigger, dial it back and accept where it can run at optimal healthy temps..