Larry wrote:

Does someone make a 12VDC compressor? I've never seen one, here.

You're pretty much admitting that you've never seen a custom marine
refrigeration system and know nothing about them. Most of the small
boat systems in the last 20 years have used DC Danfoss hermetic
compressors. The larger systems use a belt or direct drive from a DC motor.

I do see
AC inverters built into fridges driving standard AC-powered compressors
that have relays or electronics to switch to shore power if shore power is
available to run the compressor straight off shore power, however.

Perhaps the AC/DC units on powerboats do it this way.


Inversion and synthsized AC power creation from battery power is now in the
range of 98-99% efficient with switching power supply technology using
pulse-width-modulation to accomplish a near-perfect sinewave output to
drive loads. These powerful synthesizers are very cheaply constructed and
very profitable.

It can be done, but if you buy a general purpose inverter it will likely
run at about 80%,


The AC compressors synchronize to power line frequency (50 or 60 Hz), which
gives them a steady power output regardless of condensor pressure loading
which would drive a DC motor crazy trying to maintain counter EMF.

It makes little difference in efficiency running a switching inverter
outside......or building one inside for another $1500 to sell at the boat
store. The price difference is phenomenal!


OK, I just checked the energy efficiency of a GE compact fridge, 5.7
feet with a tiny freezer. It uses 360 kWh's a year, or 1 kWh per day.
Supplying this with an inverter would take over 100 AmpHours. However,
a similar sized fridge built with a small Danfoss will only use about 35
Amp-hours. My system includes this size fridge (bigger, actually), plus
a 5 foot deep freeze which keeps a summer's worth of burgers and steaks
at about 5 degrees only uses 55-60 AmpHours in New England, maybe 75-80
in the tropics. (Mine is a belt driven 12V compressor with holding
plates and water cooling. In the tropics, if I had to do it over, I'd
be considering two small Danfoss air-cooled systems, one for the fridge,
the other for the freezer.)

I'm a fan of faster charging, either with a high output alternator or a
Honda 2000i pushing a 100 Amp charger - either way I can do most of my
charging at 75-95 Amps, even though its only a 440 AH bank. But still,
adding 65 AH a day to save some money doesn't work, even assuming the GE
fridge would fit in a boat. Frankly the cost is irrelavent; there's no
way I would double my charging time to have less capacity. Since you're
claiming you can't charge at over 25 Amps, running the commercial fridge
would take an extra 2 and a half hours, and a system like mine would
require even more hours.

Of course, you're saying that a marine fridge *could be* built for on a
few dollars more than a commercial unit. Possibly true, but the market
is tiny by comparison, and that's what its all about.