So, to summarize this advice:
1. If you are going to home-build a generator, then it should be a DC generator, since small home-built AC generators are not really practical. For reasonable battery charging
performance, you will need an external regulator
for the alternator, like this: Balmar Multi-Stage Voltage Regulator
. Adverc is another good maker of these. Here's yet another way to do it: Sterling Power ProAlt C Alternator to Battery Charger
2. If you are going to home-build a generator, take a little time and engineer
it properly. You will be wasting your time and money
if you don't design it for sustainable demands on both engine and alternator. You don't need to pee on the electric
fence yourself, so to speak -- others have gone before you and the data is available. The engineering is not rocket science.
3. If you go with a cheap
store-bought generator (which I recommend if your own time is worth more than $1 per hour), it should be an AC generator. It should be AC because (a) there are no mass-produced cheap small DC generators, since 99% of buyers need AC power; and (b) store-bought DC generators, unless they are very expensive, do not usually have proper regulation for charging deep-cycle batteries. So if you don't already have a battery charger, you will need a small multi-stage battery charger like this: Victron Blue Power 12V 17A 230v Battery Charger
. Bigger or smaller depending on the battery capacity (amps of charger should be 15% to 20% of nominal amp/hour capacity of the batt).
It is important to keep in mind that car-type alternators will not do the job without external regulation. They are made to run a car's electrical
loads while the engine is running, and to replace -- very inefficiently, but efficiency is not required -- the very small amount of battery power used up in starting. They do not produce enough voltage to recharge a battery which has been deep cycled -- that is, a battery which has been used as a power source without an engine running. As a result, if you attempt to recharge a deep-cycled battery with an unregulated car alternator, it will stop putting much power into the battery long before the battery is well charged, leaving it chronically undercharged while requiring unnecessarily long generator runs. This is especially important in your application, where you will be hauling the generator in the dinghy
on board in order to charge, and will not want to charge longer or more often than you possibly can.
If you are severely budget
limited, then I would go with a Kipor suitcase gen and something like a small Victron multistage charger. That is probably about as cheap a solution as you will find; the parts
for a home-brew DC gen, once you figure in a regulator for the alternator, will probably cost more, not even getting to the value of your own time. However, if you want to built your own genset just for the fun of it, and not because it will be cheaper -- that will work perfectly well if you engineer
I forget whether or why you rejected solar
; this might be better than any of the above solutions, without really being more expensive. You would need only a pretty small panel.