Celebrating the independent kiwi spirit of invention.

Brass Monkey 50 L dual zone 12 V fridge/freezer:

By Ian Mander, 5 & 9-15 January 2020


Testing at Jaycar

On my two testing visits to Jaycar, attempting to run the 50 L dual zone fridge from my Eneloop 12 V battery pack gave mixed results. On my first testing visit the compressor tried to cycle on and off, but the fridge gave up each time the current exceeded about 6.4 A. (I think this corresponded with about 60 W.) The staff were impressed with my battery pack, though.

On my second testing visit I couldn't even run a 22 L fridge without low voltage errors, but the shop 12 V supply was also giving low voltage errors. The 50 L fridge ran without problem from my 12 V SLA battery pack, then ran without problem from my Eneloop pack. I saw 6.41 A momentarily, with the voltage a little over 9 V. There was less voltage drop on the 12 V SLA, so it was clearly more comfortable powering the fridge. At times there seemed a disconnect between the voltage the fridge was showing and the voltage the battery pack meter was showing. The 12 V power cable was printed with "1.31mm²" which would give about 0.62 V drop at 6.41 A.

Turned off (but connected) the fridge was drawing 0.1 W. Turned on, the results were "a bit more varied".

Test Run Fridge
Operation state
Duration (min:sec) Energy (Wh) Average Power (W) Capacity (mAh) Calculated Average Voltage (V)
1 50 L Dual Zone
low voltage errors
14:10 0.371 1.57 53 7.00
2 22 L,
low voltage errors
12 V SLA
7:20 0.935 7.65 147 6.36
3 50 L Dual Zone
12 V SLA
6:40 1.728 15.55 317 5.45
4 50 L Dual Zone,
4:29 0.017 0.228 1
5 50 L Dual Zone
3:07 0.665 12.80 145 4.59
6 Home: Thermoelectric cooler 5:00 2.187 26.24 191 11.45
7 Home: Thermoelectric cooler (50% duty cycle) and fluorescent light 5:00 1.342 16.10 120 11.18

None of the compressor fridge results make sense! If the capacity in test 4 has just been rounded down, and is actually as high as 1.49 mAh, then it produces a sensible average voltage. The current figures shown on the meter were jumping around all over the place. Is it possible that current draw was very spiky and that the meter couldn't cope with it? Very likely.

The fridge will run on 12 V or 24 V so there is almost certainly some sort of DC-DC converter powering the compressor. It would be quite informative to have a data logger recording the voltage and current several times a second, or an oscilloscope for viewing the actual current draw.

I added a couple of tests with my Peltier cooler to see how the meter coped with that device; perfectly, as it turned out. P = IV worked just fine. This supports the idea the fridge uses a DC-DC voltage converter powering the compressor.


Testing at home

The joule (J) is the base unit of energy.

The second (s) is the base unit of time.

The watt (W) is a unit of power derived from joules and seconds. Power is energy per time, so a watt is a joule per second.
1 W = 1 J/s.

The kilowatt-hour (kWh) is an alternate unit of energy, since it's power × time.
1 kWh = 3,600,000 J.

1 J = 1 W·s
1,000 W = 1 kW
1 hour = 3,600 seconds.

Amount of energy removed from 4.25 L water dropping from 20 °C to 4 °C = 16 × 4250 × 4.18 = 284,000 J = 0.079 kWh. The fridge will take less energy than this to extract that energy, but the fridge itself needs to be cooled down.

Precool the fridge and contents!

One of the most useful things to know is how much energy does the thing use in 24 hours. This will vary greatly, depending on the fridge settings, how much inside it needs to be cooled/frozen, and the ambient conditions (temperature, wind, sunlight falling on the fridge).

To that end, I'm doing some testing of various configurations using a mains watt meter (my 12 V compatible watt meter is incapable of accurately measuring the presumably spiky power draw of the fridge).

  • Using the mains power adapter the fridge uses between 45 W and 80 W when the compressor is running, and 0.1-0.5 W when it's not. The mains power adapter is probably about 90% efficient, so I've taken 10% off to calculate the 12 V values, for likely energy use when operating from a 12 V battery.
  • I need to get a no-load baseline for the mains adapter with nothing attached.

I started each test with one 3 L bottle and one 1.5 L bottle of water positioned against the centre divider (when the divider was installed), not touching any actively cooled walls. The cover was closed and fully zipped up, and indoors in the shade. I started the fridge, set it to the desired setting, and let it run for a total 24 hours, measuring the power consumption with the mains watt meter.

Test 1: Minimum energy.

  • Left zone off, right zone (with both bottles) set to 4 °C, ECO. Fridge and bottles initially at 20 °C, 11:10pm. This is the minimum energy it will use as a practical fridge.
    • It was down to the set temperature within half an hour (actual time unknown), but this doesn't mean that the fridge itself or the contents were down to temperature.
    • After 11 hours: 0.176 kWh. This is not on track for 0.25 kWh, but a lot of this will be cooling the fridge and contents.
    • After 20 hours: 0.268 kWh. Pretty stable energy use since the 11 hour mark.
    • Final result: 0.309 kWh/24 hours. This is 0.278 kWh at 12 V.
    • Using the last 13 hours as typical energy consumption gives 0.064 kWh cooling the fridge/contents and 0.245 kWh/24 hours running it. On 12 V, 0.220 kWh/24 hours.
    • For 10 hours, that lower rate would be 0.102 kWh, or 0.092 kWh for 12 V.

Test 2: Typical fridge/freezer.

  • Centre divider in, left zone (with 3 L) set to 2 °C, right zone (with 1.5 L) set to -20 °C, MAX. This is the energy it will use as a typical fridge/freezer.
    • The fridge was precooled for this test. A bit over 10 hours used roughly 0.440 kWh. The 1.5 L bottle is well on its way to being completely frozen, but not there yet.
    • After 12 hours: 0.466 kWh. Extrapolation 0.932 kWh/24 hours. This hour's energy use was the same as the first.
    • After 18 hours: 0.666 kWh. Extrapolation 0.888 kWh/24 hours. After 12 hours, there was a consistent reduced energy draw (maybe the bottle had frozen).
  • Final result: 0.849 kWh/24 hours. This is 0.764 kWh/24 hours at 12 V, 64 Ah.
  • This is a disappointingly high amount of energy, but might be OK during a sunny day. The 1.5 L bottle is frozen absolutely solid – excellent.

Test 3: Reduced power fridge/freezer.

  • As above, but left zone set to 4 °C, right zone set to -10 °C, for 10 hours (overnight use) and 24 hours (full day). This should greatly reduce power required overnight without thawing anything.
  • At 10 hours: 0.205 kWh/10 hours. This is 0.185 kWh/10 hours at 12 V, just over 15 Ah.
  • This should be quite doable with my deep cycle 22 Ah SLA if it doesn't have too many other demands such as evening lighting. I'll face the solar panel east to catch as much early morning solar power as possible. The 1.5 L bottle is still frozen solid – excellent.
  • At 24 hours: 0.502 kWh/24 hours. This is 0.452 kWh at 12 V, about 38 Ah.
  • This is right in the middle of the amount of energy the fridge will use just cooling the right zone as a fridge, and running as a cold fridge/freezer. Nice to have options, but it's considerably more than the 0.2 or 0.25 kWh/24 figures claimed.
  • The unit was then switched off for the following night to see how it would fare.
  • After 10 hours switched off overnight: Left zone was 10 °C, right zone was 6 °C. The water felt cool, not cold, and the ice bottle had melted quite a bit. This would not be particularly good for the contents of either, but could probably be survived (unless it was ice cream).


  • The fridge/freezer works very well but uses more power than claimed (of course). In order of energy use, when powered at 12 V:
    • Right zone only, 4 °C, precooled items: 0.220 kWh/24 hours at 12 V, ~18 Ah. (Last 13 hours of test 1.)
    • Right zone only, 4 °C, cooling items: 0.317 kWh/24 hours at 12 V, ~26 Ah. (First 13 hours of test 1.)
    • Left zone 4 °C, right zone -10 °C: 0.452 kWh/24 hours at 12 V, ~38 Ah. (Test 3.)
    • Left zone 2 °C, right zone -20 °C, precooled: 0.689 kWh/24 hours at 12 V, ~57 Ah. (Last half of test 2.)
    • Left zone 2 °C, right zone -20 °C, cooling: 0.839 kWh/24 hours at 12 V, ~70 Ah. (First half of test 2.)
  • Both the fridge and freezer zones need to have the unit running overnight to keep them cold.
  • There are several options for reducing the energy requirements for night while still running the fridge. For example:
    • Run the fridge/freezer very cold during the day and turn up the temperature setting a bit at night.
    • If the freezer zone is not specifically needed it can be run only during the day and ice bottles transferred to the fridge overnight. This is using ice as thermal mass.
    • At night take out the divider (or turn it sideways), thereby running the whole thing as a fridge with ice in it.
  • If maintaining a separate freezer at night, it might be best to not put warm items in the fridge late in the day – let stuff be cooled by solar energy instead of the battery.

Further proposed tests.

  • Test: Maximum energy.
    • Centre divider in, both zones set to -20 °C, MAX. This is (maybe) the most energy it will use.
    • When I tried this earlier with the divider out it stabilised at -8 °C. I suspect this is because the right zone was doing most of the cooling. When I put the divider back in the left zone was -9 °C and the right zone was -24 °C.
  • Test: Left zone off, right zone 4 °C, MAX. This is a repeat of test 1, but set on MAX.
  • Test: Centre divider in, both zones set to -20 °C, MAX. My guess is it won't get to -8 °C.
  • Test: Centre divider out, both zones set to 10 °C, MAX, cover left open, fridge opened periodically. This is the energy which might be used for just a bit of cooling, like cooling drinks for a party.
  • Test: Centre divider out, both zones set to 4 °C, MAX, cover left unzipped but flap closed, fridge opened periodically, items put in and taken out. This is the energy which used as one big fridge, no freezer.
  • Test: Centre divider out, both zones set to -20 °C, ECO. This is similar to test 4. My guess is it won't get to -8 °C.


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