This is all very good theory, but . . .
sizing of the
battery bank is not actually a function of the capacity of
charging sources. They are really kind of separate problems, and if anything, I think one would be more likely to make changes to
charging sources to suit possible
battery bank size, and not the other way around.
In my opinion, the battery bank should be as big as reasonably possible and practical. Other than cost and weight, a larger bank is all good. I don't think there is an "optimum" size, other than bigger.
If your charging sources can't put in an efficient amount of
power (at least 10%, better 15% of C), then they should be increased. It's relatively
cheap and simple to install a larger
alternator, or better, a second
alternator. A large-frame
school bus alternator is a tremendously valuable charging source, and they come in very large sizes, large enough to be 10% of any imaginable battery bank (actually, since main
engine alternators often
work at speeds where they produce less than all of their output, you generally want to size them bigger than 10% of C). Of course you have to take care with the belt drives (double belts are required on the bigger ones) and might have to change some pulleys on the
engine.
If you don't have an alternator that can put out enough
power to feed a battery bank which you could otherwise afford in terms of weight and cost, then you should change it. That means it's a bottleneck in your system. 5 hours a day of running your main engine is ridiculous, in my opinion. That would not be pleasant cruising. That's just a too small alternator.
To give you an example from my usage -- I have a 110 amps (* 24v)
school bus alternator, which is theoretically more than 25% of C for me. If I run the engine at a fast idle, I reckon I'm getting 80 amps (has a very steep charge curve -- made for school busses
), which is a bit less than 20%. And indeed, as the theory would suggest, I can get my batts from 50% to 80% in less than two hours of fast idling, whereupon I have about 100 amp/hours of juice to play with until the next chargin session. I stop at about 80% when I'm off-grid, because the
batteries start accepting less and less power, so run time versus stored power falls off.
Remember it takes a combination of charging power and bank capacity to store a given amount power in a given amount of time -- you are limited by the lesser of acceptance rate of your bank or the charging capacity of your charging source. So to reduce your charging sessions to tolerable periods of time -- and I suggest that 5 hours is way too much, especially if you're using the main engine -- you need to consider both of these factors.
If you're using a
generator, there is another factor to consider -- charging
batteries can be combined with other activities which require the use of the
generator. Hence a lot of cruisers will try to group together their AC power tasks and do it all at once, and the batteries are getting charged incidentally at the same time. This improves the efficiency of the whole process, and also, if you have a lengthy job requiring AC power, like
washing and drying clothes, you can take the chance to get your batts above 80% for once, which they need from time to time for proper
health. Obviously you also need a large enough battery
charger to use your generator runs effectively -- better between 15% and 20% of C (I have 17%). And a large enough generator to run that
charger. My charger at 70 amps and 24 volts nominal, needs about 2.5kW of input power when it's charging at maximum rate.
My charger takes about two hours to get the batts to 80% from 50%; then another hour to get them from 80% to absorption phase, 85% or 90%.