T&T: DC Gensets

[David&Joan]

More thoughts on DC gensets:

If all of your loads are DC, ie battery charging, then a DC genset makes a 
lot of sense. A large frame DC alternator of 150-200 amps or so at 12V, 
driven by a modern 3 stage regulator, can put a lot of amps into a battery 
bank quickly. These regulators are optimized for just that purpose, whereas 
shorepower based chargers may not be.

But if you have any significant AC loads then an AC genset makes more sense. 
On a prior boat I had a 5 kw Northern Lights AC genset. Each day or so I 
would start it up and run it to supply AC to the Freedom 25, 100 amp 
charger/2500 watt inverter. This loaded the genset to about 2 kw with 
charging loads. Then when the charger started cutting back to about 1 kw 
after 45 minutes, I would flip the water heater breaker on and load it back 
up to 2 kw or better for another 45 minutes until the water heater 
thermostat tripped. At that point the batteries had gotten about 100 
amphours and I shut the genset down. I repeated the cycle the next day.

I could do the same while running a 16,000 BTU A/C, but only if I started 
the A/C first. The AC would not restart with all of the other loads on the 
genset.

If I were doing the same thing with a DC genset, the first 45 minutes or so 
would charge the batteries at 150 or so amps. Then after the charging 
current dropped, I would turn on the inverter to supply AC to the water 
heater and it would draw about 100 amps to power the water heater coil. 
Charging would be a bit faster, because of the first 45 minutes at 150+ amps 
(assuming I had the right battery bank to accept it). But I couldn't run the 
A/C at the same time that I was heating water. The A/C draws 16 amps and the 
water heater draws at least 10amps and that is the more than most inverters 
can supply. Plus it is more DC amps than some DC gensets can supply.

Efficiency of either scheme is about the same. DC gensets and DC power 
distribution is less efficient than AC (high current and alternator 
rectification losses), but you don't have the losses in the battery charger 
to deal with on an AC system, but you do have them on the inverter side of 
the equation. Again if most of your loads are DC then it probably is a wash. 
If they are mostly AC loads then the advantage is with the AC genset.

So, I guess that is why 99.9% of all trawlers with a genset, have an AC 
genset. Actually I have never seen or heard of a DC genset in a trawler and 
have only heard about them in a sailboat.

David