Here's some data I collected last year. The batteries were 2 Group 31 Firefly batteries with a charge acceptance rate of .4C. The alternator is a Balmar 165a controlled by a 618 regulator with the field current limited and a temp limit of 100°C. Over the course of an hour the SOC was raised 30 percentage points. At 20 minutes you can see the acceptance rate begin to decline, over the hour it declined from .36C too .26C as the internal resistance increased. Like wise voltage was increasing and approaching the absorption voltage of 14.4. After an hour I decided to stop wasting diesel and let the sun take over. It had been a couple of very overcast days on anchor in December.
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Nice table - I also read through Rods article you posted the link to, while my question was not answered, it is a very interesting write-up!
The exact reason I've gone away from lead-acid chemistry is the difficulty (time/energy) to get back to 100% fully charged. Fighting with sulfation is a never ending battle and actually getting a fully charged and then equalized battery is very difficult to do as a live-a-board. Lithium ion batteries do not have this problem.
But I digress from the OP's original query.
His question was has anyone put on a dual fuel generator as a backup power source. To answer that, yes I have.
I put the generator on my rear platform and hook it directly into my shore power plug on the outside of the boat every time I am going to use it. I would not have considered hooking up the 12v output (mine doesn't have one, but wouldn't consider using it unless it matched or exceeded the power output of the AC side).
I would not use propane for this, I would use gasoline. I like to keep my propane installation as simple and small as possible - it runs only my stove.
I store the generator inside my boat when not in use and take it out and tie it onto my rear platform for use. Three reasons for this - noise, to avoid theft and to keep the unit exposed to the sea environment as little as possible. Time will tell if I like this set-up or not. I will use the generator for backup charging of my batteries and if I want to use my heat pump or electric hot water element. The heat pump and hot water I won't run off my inverter, they are too close to maximum capacity as single units, and together - they would destroy my inverter... Not a good thing... My hot water tank is dual heated, engine or electric. My heat pump - only runs off shore power (generator).
dj