It’s essential to use a cable size that will deliver adequate voltage to the fridge. You need to calculate voltage drop using the maximum current that the fridge draws
- not the average - for Compressor fridges and Thermoelectric fridges this is up to 7 amps and for Absorption (3-way) this is 11 amps. But if the same cable is used to feed other loads or charge batteries, then you need to add this additional current when calculating the cable size needed to keep voltage drop below 0.4 volts. The amp rating stamped on the cable is not adequate to indicate acceptable voltage drop. If the cable feels warm when running at full load, then you are losing significant power due to the cable being too thin. Here is the smallest cable size to use (in Gauge and copper diameter) for 11 amps and 0.36 volts drop over several distances (assuming same size wire is used for the supply and earth leads)
2.5m = 12 gauge - 2.3mm 4m = 10 gauge - 3.2mm 6m = 8 gauge - 4.1mm 16m = 4 gauge - 6.4mm
To calculate how much battery capacity (in Amp hours) you will need, you need to know the current consumption of the fridge (in Amp hours per day, which equals average amp hours times 24) under the conditions that you will be using it, plus the number of days before you can recharge the batteries. For long battery life, you need to plan to use only 50% of the battery capacity and to recharge them within a few days. You would only use up the full battery capacity if there was no sunshine at all for several days
To avoid damage to your batteries you should always plan to recharge batteries fully in one day, although in winter this may mean the equivalent of 2 hours of peak charge current from a panel, depending on whether you are in Tasmania
or Cape York
. For details on solar calculations go to the Solar article.
"Amps aren't amp hours"
Don’t be dismayed if you get confused when you read about fridge consumption - many web pages and magazine articles have errors in the wording used for power consumption. Here’s a quick summary.
Instantaneous current flow - amps - for a compressor fridge this could be 6 amps when working hard, reducing to 2 amps when the contents have cooled down, then zero when the thermostat cuts out.
Average current - amps - for a fridge you need to average out over 24 hours, because the instantaneous current consumption varies so much with air temperature. For a Eutectic fridge, you may need to run
the fridge for a day or two before taking measurements, because the Eutectic banks can store so much energy.
Total Current Consumption - Amp hours - as the name implies, you multiply amps by hours, so if your fridge uses 2 amps average, then over 24 hours it will use 48 amp hours. The daily amp hour usage of a fridge is easiest to use, because it is comparable with battery capacity in amp hours. Also you can add amp hour usage for different loads - e.g. if your Tent light uses 1 amp and you use it for 4 hours a night i.e. 4 amp hours per day, then your total draw from the battery per day will be 48 + 4 amp hours. You should not plan on drawing more than this from a 100 amp hour battery before recharging it