Electronics Forum / Basics / October 2006

Need more facts. AC or DC power? How much can you draw?

Charging your batteries from an 18 volt power supply might not be such
a good idea.  For charging batteries, you need a battery charger.  If
you can find one that works at 80 volts, you're more than halfway there.

Is the refrigerator AC, DC or both?

If AC only (uses a compressor), then you're looking at storing power
in some BIG lead-acid batteries ($$$) and using a big inverter ($$$)
to run the refrigerator.

If the refrigerator is DC or AC/DC capable, then it probably uses 12
or 24 volts DC and your backup power source should be designed to
provide the correct voltage (lead-acid batteries $$).  Note that this
is a less expensive option, as the direct heat-cycle refrigerators use
much less power than the compressor-powered compression/expansion
cycle refrigerators.  These refrigerators are usually only available
in smaller sizes - you won't find an 18 cu. ft. (or equivalent metric
size) unit unless you have nearly unlimited funds ;-)
Sample AC/DC/LPgas unit is $425US:
http://www.lehmans.com/shopping/product/detailmain.jsp?itemID=4453&itemType=PROD
UCT&iMainCat=816&iSubCat=896&iProductID=4453
There are also LPgas (only $1300US) and kerosene powered ($1800US)
units:
http://www.lehmans.com/jump.jsp?itemID=896&itemType=CATEGORY&path=1%2C2%2C676%2C
810%2C816

If your cooling needs are small (space) and the temperature
requirements are limited (44F below ambient temperature), a
thermoelectric (peltier effect) cooler might work.  This is basically
a picnic cooler that uses a thermoelectric cooling module instead of
ice.  Typical units are $80US - $150US depending on size and features.
Sample unit (can hold 40 soda cans, or 4 or 5 two liter bottles):
http://www.okto.com/sports/product_info.php?products_id=156075

When charging batteries from an unreliable power source, you'll need a
charger that can operate from the reduced voltage supply and still
provide enough output to charge the batteries in the limited amount of
time that power is available.  The charger must also prevent
overcharge in the event that power is available at higher voltage
and/or for longer periods.  A charger that meets these specs is not
likely to be readily available (except at the off-grid suppliers, such
as the solar power vendors), nor will it be inexpensive.  

Since the batteries are likely to be subjected to deep discharge
cycles, you should plan on getting deep-cycle batteries - in a
capacity that can provide at least three days worth of power (so the
batteries will not be discharged below 50% on a typical day).  This
will be more expensive initially, but less expensive long-term because
of the increased battery life.

If this is a short-term need (less than a year), then automobile
batteries *might* be adequate, but they are NOT designed for the type
of deep discharge that your description implies.

You might also consider solar panels to charge the batteries - the
solar panels do work most of the day ;-)

John