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 power usage.....

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.

day one	day two

summary

Day 1 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.

Day 2 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.

conclusion

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