The bolded part makes little sense...? That is a VERY SMALL bank if planning to be on the hook for extended periods and running refrigeration. 400Ah and up are pretty standard bank sizes these days especially if running DC refrigeration. A 400 Ah bank of wets will take about what your alt can produce so no acceptance gain with AGM.
The problem with alternators is that all batteries, wet, gel AGM but not LiIon begin drastically limiting "accepted current" at about 80% state of charge. So the area you're concentrating on is between 50% SOC and 80-85% SOC. Beyond that it makes little sense to run the engine for charging. Your 120 amp alt will likely supply about 85-90A when hot....
"Usable" battery capacity gets really small really fast...
You need CURRENT to be able to take advantage of the high acceptance rates of AGM and you really don't have it... With a small bank you could see slightly faster charging between 50% SOC and 80% SOC, but then a small bank is not very suitable for your cruising needs with two refrigerators. You really need a battery monitor and to know what you use in Ah's per day.
Thanks a lot!
But regarding this, that is central to me:
You need CURRENT to be able to take advantage of the high acceptance rates of AGM and you really don't have it... With a small bank you could see slightly faster charging between 50% SOC and 80% SOC...A 400 Ah bank of wets will take about what your alt can produce so no acceptance gain with AGM.
My calculations show that I will have a big difference is recharging times. Maybe I mess it up
. Do you mind to check these calculations? I have used for the calculations data posted by you here:
Battery Acceptance Observations - SailboatOwners.com
According from what I have understood from your article, wet cell batteries can accept 20% charging when they are almost empty but regarding the charging that matters most, between 50% and 85% they can only accept between 13% and 7.5%.
SOC______Accepted Current______% of 20 Hour Rating
53% ____________15.3 amps___________________12.8% Acceptance
83% ____________9.1 amps____________________7.5% Acceptance
86% ____________7.1 amps____________________5.9% Acceptance
88% ____________5.2 amps____________________4.9% Acceptance
90% ____________4.6 amps____________________3.8% Acceptance
93%_____________3.8 amps____________________3.1% Acceptance
Considering an average 11,5% charging acceptance and considering a 360Ah bank the the batteries are accepting 41 Amp.
What I have read about AMG batteries talk about an acceptance two or three times superior. If we consider 3 times, the acceptance will be 124 Amp for that bank, that would be superior of what the 120Amp alternator can provide when hot (about 90) but all those 90amp will be sucked by the batteries and that would give a charging time a bit less than half the time than with wet cell batteries.
If we consider that the AMG can accept only 2 times the charge of wet cells, than they are accepting 81 Amp and the 120Amp alternator is good for that and it is still half the charging time of wet cell batteries.
Mainsail, You are the one that now about this, I am learning and I don't want to make a point out of this, just trying to understand, so please if there is something wrong with these calculations please tell me.
That is a VERY SMALL bank if planning to be on the hook for extended periods and running refrigeration. 400Ah and up are pretty standard bank sizes these days especially if running DC refrigeration...You really need a battery monitor and to know what you use in Ah's per day.
For what I have seeing on this thread I guess you guys are less frugal than the average European in what regards life style aboard and electric consumption on small boats.
On my last boat (a 36ft) I had a 240Ah home battery bank (the boat come standard with 120Ah and 240Ah was with an extra battery) and two small engine alternators to charge it. I used a Link 20 to control the batteries.
With that I could stay on anchor for a week and when I went to the marina was more for water than for charging. Normally I moved around so in most cases I was doing an hour with the engine so if that was the case I did not need to run more the engine for charging. If I staid on the same spot I would need to run the engine for an hour.
Of course this implied a very restricted use of energy: The refrigerator was turned off at night and after 21.00 and it was out of limits to open it. We open it only when it has indispensable, we made a very careful use of lighting, used independent solar charged lighting for most of uses including dinning outside and reading at night. The position light was also solar powered.
That scheme had worked for 7 years and I never changed the batteries, so it was sustainable. I cruised and lived on the boat for 45 days out of the marina in the summer and out of that I used the boat only for weekend sailing, returning to the marina.
On this boat, yes, I have two fridges but If I can only use one on batteries it will be alright for me. The insulation on this fridge is much better than on the other boat, this boat had led lights and a 360Ah bank (more 120Ah) than on the other boat. If I can charge the batteries two times faster, I am quite sure the 360Ah bank will suit my needs. I will be cruising for more time on the summer, out of marinas, maybe 3 to 4 months.
Sure I would prefer to have a 450Ah bank, and it is possible that I could manage the space for that but the max alternator that engine has is a 120Amp and the 450Ah bank would need a bigger alternator.
The alternator that the engine has actually is a 80 amp one. I am going to change to a 120 amp, so I am going to buy an alternator anyway. Do you know if any American or European brand, for that matter, has bigger alternators that fit on the Lombardini LDW 1404M (40hp)?