Updated : 01/09/2018.

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       Discussion document on 12v only Compressor Fridges in Motorhomes

This web page is relevant to 12v ONLY operation Motorhome and Caravan Compressor Refridgerators. Those without 230v or Gas options.

What is a 12v only Compressor Fridge?

Most compressor refrigerators only operate on 12v, they don't have 230v or Gas options. The compressor electric motor technology they use is similar to a domestic Fridge/Freezer and is more efficient than an Absorption fridge, but still uses a lot of 12v battery power. 
They are being used more and more in PVC's but are they suitable for extended times off grid? 


'How much battery power do they use"  


That isn't straight forward to answer because, if the thermostat is doing it's job properly, the Fridge will only consume power for about 30% - 50% of the time. Also the temperature inside the Motorhome (the ambient temperature) will affect how fast the Fridge warms up in between compressor running times. Obviously how often the Fridge door is opened will also have an impact.

The Waeco Coolmatic MDC65 Caravan Fridge uses 45w or about 4 amps draw from the battery, but it claims that when the motorhome is 'cool' the Fridge compressor will only draw power from the 12v battery for about 30% of the time.
When the motorhome is 'hot' the Fridge compressor may draw power from the 12v battery for about 45% - 50% of the time.

The Waeco manual quotes -
Power Inputs (Watts) : 45w or 4 amps per hour from the battery.
Total consumption in 24 hour period at 100% run time = 96Ah

Typical Daily Current consumption at +32C ambient temperature with fridge internal temperature of +5C :- 45% of 24 hour Total Run time =                                                               42Ah

Typical Daily Current consumption at +20C ambient temperature with fridge internal temperature of +5C :- 30% of 24 hour Total Run time =                                                               30Ah

In a hot climate a compressor Fridge of the above size will pretty much flatten a 90Ah battery in a single 24 hour period down to it's maximum recommend 50% Depth Of Discharge (DOD).

If you add on the typical 20Ah taken from the battery by a days use of the Water Pump, Lights, TV, Laptop charger, Phone charging, etc you can see that the total 62Ah drawn is going to be a pretty tall order to stop the battery being damaged by over discharge, let alone find a power source to put all that back.

In Winter the ambient temperature inside the vehicle will be lower, so the Fridge draw is estimated by Waeco at about 30Ah a day, but the background draw of the heater fan now running all night, longer running time of the LEDs lights, 4 hours extra TV time, etc. is likely to rise from 20Ah a day to 35Ah, so the total motohome draw may still be an overall of average 60Ah a day

Solar Power

'My Dealer says the Solar power will recharge the battery each day, is that true?"

Solar is not a consistent Power source through the Year. The Solar harvest will depend on where the vehicle is in the World, the weather and time of year.
Even in  summer in the UK the Solar harvest can vary by as much as 40% from one day to the next due to heavy Cloud cover.

The Sun angle in the sky, also makes a significant difference.
In the South of Portugal a 100watt Solar Panel can be nearly 3 times more efficient than the UK.
Just moving the vehicle from the South Coast of Cornwall to the North of Scotland can reduce the Solar gain by 30% on the same day/conditions, see UK Solar harvest map here : http://www.aandncaravanservices.co.uk/solar-panels.php

Length of day.
In mid Winter in Scotland the Sun doesn't rise until about 09:00 and it's dark by 15:30. That is around just 4 hours a day when the Sun is also low in the sky. Just enough sun light for a 100watt Solar Panel to generate an average 1 amp per hour, or 4 - 5 Ah per day. 
Throw in heavy cloud and you might not get that even that.

So in mid Summer there can be an abundance of 12v Solar Power, but that is exactly when you need it the least.
The Heating won't be on and it stays light until 21:00 so little requirement for the LED lights, little watching of the TV, etc.

In the South of the UK on a sunny day in June, a 100watt Solar Panel can generate about 60Ah a day in ideal circumstances.
Those ideal conditions include full Sun and zero cloud. 

In the middle of the UK in mid Spring/Autumn conditions, the same 100w Solar panel generates about 45Ah a day which is more than enough to power a 12v Compresser Fridge, but the cooler, longer nights will mean a rise in 12v usage raising the overall daily total figure to close to 45Ah and a potential break even point. 
Obviously those figures will change as the days shorten further, increased risk of Cloud, etc. so the Solar Harvest could be down to 25Ah a day by the end of September. 
As the days shorten, the sun drops lower in the sky and Cloud cover thickens and Solar power generation starts to drop off quite quickly.
So suggest that when calculating the Solar gain allow at least 30% contingency for when conditions are not ideal.

Summary :
So if you only ever holiday in the Algarve during mid Summer, a 100watt Solar panel will give you all you need. But in mid Winter in Edinburgh the battery just isn't going to last even a day.

Even a 300watt Solar Panel set-up will deliver only about 15Ah a day on an Inverness Ski Slope in early January. 

That is why the best Fridge set-ups have always been Gas powered. 

So single operation 12v compressor fridges work perfectly for some from Solar and are a nightmare for others.    

Alternator charging on the move.

'What about Alternator charging? Won't that charge my batteries back up as I move between stops?"

Typically many motorhome Alternator set-ups have lower than expected Alternator charge rates and will charge a conventional 100Ah Habitation area battery at less than 10 amps.

So to put back the majority of 60 Ah taken the day before would require a drive of about 7 hours. 
Doubling the battery bank to 2 x 100Ah batteries can cut charging times by half because each battery may now draw 9 amps each, resulting in only a 4 hour journey being necessary.

The best built motorhome with their more efficient Alternator charging set-up will, roughly,  take a 3 hour drive to recharge the batteries to near full.