We all have rules of thumb. I used to tell
people that a 12 volt compressor camping fridge set at 1.5 on
the dial (beer and milk setting) in normal weather conditions
will use about 25 Amp Hours a day. When the nice people at
Manson Engineering HK asked me to test their version of a Watt
Meter, I decided I had to test my assumption. The meter is
available from Jaycar in two versions. One uses an external
shunt (MS6172) the other has an internal shunt (MS6170). Using
the external shunt allows you to locate the meter up to several
metres away from the high current cable runs. You can also use
the one meter in several places by adding a $15 shunt in each
circuit that you want to monitor, under the bonnet, in the rear
of the vehicle or in the camper trailer.
Not many people know it,
but these meters are data loggers. They log most of the data
that they show on the LCD every 3 minutes for up to several
days. Other than the better quality (in my opinion) this
functionality explains the extra price when compared to the
other watt meters. To extract the data from the meter you need
to purchase an optional interface and software kit (MS6174). The
data from the Java software supplied can be saved in a
proprietary format or a .CSV that you can open and manipulate in
Excel. The data can also be seen and exported in a generic but
cluttered graph. Below is an example.
Anyway, Back to the fridge
When my mates go away
camping for a restful weekend they take fishing rods, beer and
magazine. Well I take a multi meter, wire, heat shrink and crimp
connectors. Here is a photo of the testing panel behind the rear
passenger seat of the Prado. (Warning the image may distress
some OCD types.
I logged the use, power
and environment of our 35 litre Engel for two days. From when I
put it in the Prado warm until we got home the second evening.
Both days were over 35 degrees celcius with no wind. The car was
parked under the trees on the bank of the Edwards River in NSW
in dappled sun light with one door open for ventilation. The
Engel is in a transit bag and when the full sun was shining on
the rear of the Prado, a white towel was put over the windows.
The temperature shown on the graph was air temp in the rear of
the Prado about 100mm above the fridge. My son Josh and I
accessed the fridge every few hours as we normally would.
Starting with a warm fridge on a 35 degree celcius day, I added
twelve ‘cool’ 375ml cans of drink and 2kg of cold meat to the
fridge. The fridge used a total of 43 Amp Hours in 24 hours.
This was a 37 degree celcius day. I added 600grams of warm food,
six warm cans of drink and a 750ml bottle of warm water. The
fridge used 34 Amp Hours in 24 hours.
My new rule of thumb will
be 25 Amp Hours minimum in 24 hours with normal use and 35 to 40
Amp Hours per 24 hours on hot summer days. It looks like 1 Amp
Hour per 24 hours per degree celcius … more testing required.
thanks to Jeff Pethybridge