Generators used to charge batteries: the most efficient generator to
charge batteries is one that uses a automotive or similar type
alternator to produce DC directly. Feather River Solar Electric
() is one outfit that builds
these, but they are not intended to be portable, or particularly quiet.
Epower () makes portable units but
these wouldn't be too quiet either.
With a few exceptions mentioned later, using an AC generator to power a
battery charger is inefficient of fuel and space. Most lower cost
battery chargers use a transformer/rectifier circuit to produce the 14
VDC necessary to charge a "12 Volt" battery. Because 120VAC is a "sine"
wave, only the peaks of the waveform are at a high enough voltage to
produce charge current, and then it is a relatively high current for a
short time, until the waveform drops on its path toward the other
polarity. To produce an average of, say, 20 Amps of charge, the peak
current might be two or more times that.
Most small generators are not capable of the high peak currents
necessary unless the charger is very much smaller than the generator
would seem to be capable of running. My Trace DR1512 1500W inverter has
a 70 Amp charge capability, which would translate to requiring about
1100W of power. Testing has shown that, depending on the quality of
generator, it can require up to a 6,000W generator to drive it to the
full 70A output, because on smaller units the waveform "flat tops" under
this sort of load.
One exception, I am glad to report, is the Honda "Inverter" series of
generators. These use a permanent magnet multipole generator to produce
a DC voltage over 200VDC, which is then pulse-width modulated in a high
frequency inverter to produce a very high quality sine wave. The output
voltage and frequency are set by a crystal controlled microcomputer and
are independent of the generator engine speed (which is also computer
controlled). The output of the inverter is monitored many times during
each cycle of the sine wave and the modulator adjusted to maintain a
clean waveform. There may be other "inverter" generators out there but I
have not had any experience with them.
My EU2000i is able to easily drive my inverter to the full 70 Amps
charge at 14.6 volts while barely rising above its idle speed. It is
able to maintain this charge rate even while simultaneously operating my
5000 BTU air conditioner, and, although it does speed up some, it still
has some capacity in reserve. The EU2000i can even drive my Trace SW4024
inverter (for my house) to 60 Amps charge at 28.5VDC, but it works
pretty hard.
Some newer higher cost "switching" type chargers, such as the
TrueCharge, utilize this same sort of high frequency inverter circuit,
and are able to produce full output on smaller generators. Only testing
with each particular generator/charger combination will show for sure
what it is capable of.
I don't work for Honda, or Xantrex (Statpower, Trace, etc.), but I have,
for over 20 years, installed and maintained mountain top transmitter
sites with generators from 4kW to 400kW, battery banks of 12, 24, and
48VDC, and chargers from 2 Amps to 250 Amps, so I have had the chance to
experiment that most folks don't get.
-Jay
