Engines typically have alternators fitted which produce DC charging current
. Alternators are approx. 35-40% efficient. This means that in order to produce 100A charging current
at 14V (1400W or ~2 hp), the engine must develop around 5 hp at the pulley.
Generators produce AC current and typically are more efficient. The Honda
2000 for example uses a 3 hp engine to produce 1,600W so it is 70% efficient. Bigger generators are just as efficient but only at Max rated power. Now, this AC current needs to be converted to 14V DC charging current, so you lose another 10% in this conversion. Rounding up, we can say that you need 3 hp to produce 100A DC charging current with a generator but you also need a powerful and heavy charger.
So what are the options?
If you have a relatively small engine (up to 21 hp, smaller boat), the best option for me is to install a heavy duty alternator, say 160A, a Balmer or equivalent charger that allows flexible setting of charge current. If running at anchor
, you go to fast idle, produce up to 160A, loading the engine with 7-8 hp which is a considerable load (reduces glazing). If underway and you need the power for propulsion
you use the small engine mode on the regulator
to reduce the alternator power takeoff. I have a similar setup on my boat
, 13 hp engine, 80A alternator (capped at 80 since my batteries cannot take more).
If you have a bigger engine (40-60 hp) then no matter what you do, you cannot load the engine in a meaningful way with the alternator in fast idle. Either you need to charge under way or accept that glazing will occur. In this case the generator option becomes highly desirable.
Still, generators are typically used to meet household AC needs. In order to use a generator mostly for charging one needs to have a similarly sized charger and batteries. A setup that may work
would be a 5 kW generator, 4 kW charger (300A+), 1000 AHr battery bank to accept the current... seems daunting to me. Of course you can use the generator below it's rated output but then the efficiency goes down (unless it is an inverter
Last point, glazing is not so bad after all. It can easily be fixed if you know how to work on your motor
and can be done in place. So one way to deal with glazing is that every few years you take the head
off, inspect the cylinders, grind them with the glazing tool, may be do a valve job on the head
, then reassemble. I did this on my Yanmar
engine and it is not so bad, less than $500 if you do it yourself. On a bigger engine it will be more but no where close to the cost of a new diesel
generator plus install. Not for everyone though