Thanks Faster and Med sailor. That was just the information I wanted, via Mainsail. But Faster I have no doubt that a second alternator can give a faster charge if the batteries can handle it. On my previous boat I had two alternators and I was complaining to the guys from Bavaria that there should be some problem with my 220 charger because the engine could charge the batteries in almost half the time.
They checked it, saw the amperage and say that it was normal. I had an 340 Amp bank and according with those 20% of the total capacity for the size of the alternator it should not happen, since that gives a 64A alternator and that should be about the amperage of the 220 charger (maybe 50A) and about the size of the engine alternator (maybe also 50 A).
Now if we consider what Bamar says regarding the size of the alternator, 20% to 40% of the total bank amperage, then 20% is just a minimal value and that explains Why I had much better results with the two alternators (maybe a total of 110 A), about 30% of the battery bank.
There is also another important thing to consider: on a Alternator the stated amperage is only produced at the operating RPM. So if you go at cruising speed you will get close to the given Amp, but if you are at anchor running the engine a bit up idle (1200/1300RPM) to charge batteries you are only getting half of that if that much.
If the objective is charging a 4X120A bank of batteries at 1200rpm (while at anchor) than you need 20% x 2 of the total battery bank. For 480A that is a 192 Amp alternator, or 240 alternator if we consider 25%.
In what concerns using a dedicated DC generator the problem of operating RPM is not there because it can be run at operating speed with not much noise and little fuel waste.
Some can even vary speed regarding the needed charge and cut off and connect themselves when needed:
"The new DC alternative from HFL is based on a regulation system
whereby the operation of the diesel charger is automatically controlled by the state of the battery.
Whenever 12 or 24v consumers are used the battery voltage will drop dramatically. Once the batteries have reached a preset voltage, the diesel charger will automatically cut in, replenishing the batteries up to 14v or 28v respectively, then switch off again automatically.
A DC generator is capable of providing output via its alternator over a wide set of operating conditions, with the engine speed being variable offering fuel economy and lower service requirements."
A typical small generator will waste also much less diesel than the alternators run by the engine, maybe 3 times less.
Thanks to Mainsail numbers I can now calculate what I need in what regardes the power of a DC generator for my boat that has a 400 Amp battery bank : considering 80% efficiency and considering 25% of the size of the bank it will give 125 Amp.
Bottom point, that is about the power small DC generators provide in amperage. I guess that the ones that are doing them know what they are doing
D.C. Traveller | HFL
But if I was getting that juice from alternators connected to the boat engine, while at anchor, I would need to get the same (considering half efficiency at slow RPM), a 200 to 250 Amp alternator, I mean to get max efficiency on the battery charging, assuming the boat engine a bit over idle. That's is also why boat engine manufacturers provide a second alternator that with the first can produce that kind of juice, I mean, in the case of the 40 hp Yanmar, 210A.
Nice to know that the guys on Yanmar also know what they are doing