Fridge Hacking For Fun & Profit*

* Actual fun & profit levels may vary.

This is was an ongoing side-quest in the programme of continuous improvement known as Land Rover ownership.

As part of the Mk1 Camper Conversion we bought a 3-way camping fridge as fitted to basically every caravan & camper van ever. Ours is an Electrolux RM212 but they are also known as Dometic and few other brands over the years & around the world.

But first, let's examine the options for the overlander in need of a cold beer, and venture into…

Or “why didn't you buy an expedition fridge, are you mad?

* Actual excitement significantly lower than intimated.

If you come from the rough-tough hairy chested world of 4×4's and “overlanding” rather than the world of either caravanning or boating you'll know that the only fridges worth a fart are made by Waeco or Engel and cost about the same as the amount of gold bars you can fit inside them with diamonds stuffed in the gaps and crack cocaine dusted over the top. And that's 2nd hand prices.

If you come from the world of caravanning or camper-vanning then a fridge is a tastelessly wood-veneered unit capable of running on mains, 12v, or rather confusingly on gas.

If you come from the world of boating you're used to everything costing as much as a gold bar dipped in cocaine so we'll leave you to consider your life choices.

1. Compressor
2. Absorption
3. Peltier

So what's the difference here?

Your fridge / freezer at home (and indeed your air conditioning unit and/or dehumidifier) work by using an electric motor to compress refrigerant and pump it around - the vapor compression cycle. It's simple and efficient, and can achieve a very high level of refrigeration (it can get very cold even when working in hot conditions).

The expedition type fridges use a similar basic concept with some added electronics to allow them to run from different voltages (EG mains & 12v) in one unit, plus presumably a gold bar hidden in the case somewhere to justify the price.

Camper fridges, however, use the vapor absorption cycle. It's not quite as easy to get your head round and it's not as efficient. Its major advantage, though, is that it is powered by heat. Yes - you heard that correctly - a fridge that's powered by heat.

Sounds crazy but it's quite useful when you're off-grid, vapor absorption fridges (let's call them VA to save time) can run from a small flame from a bottled gas supply, lasting many times longer than if you tried to run any sort of fridge from batteries. This is a dead giveaway - almost every caravan & camper van has a fridge that can run on mains, battery, AND gas, and you're advised to run them on gas whenever you're parked up without a mains connection. We'll see why later.

A secondary advantage of VA fridges is that they have no moving parts (although they do still have circulating refrigerant) - no electric motor whirring away. Not a big deal in your kitchen at home, but in a camper where you are sleeping inches from the damn thing you really might appreciate the fact it can work utterly silently. In the olden days, making a small, reliable, affordable compressor fridge for caravans would have been almost impossible, hence how the technology cornered the camping market even as domestic fridges all became compressor-driven.

If you look closely at most hotel minibar fridges they often turn out to be absorption-type fridges, presumably as they're a good way to get a small, silent fridge.

Portable 12v coolboxes and a lot of mini-fridges or “desktop” fridges (often sold by beer companies during world-cup season) use Peltier coolers, which for want of a better explanation work by magic. They are a small lump of magic stuff that, when you connect it to electricity, “pumps” heat from one side of itself to another. Even more awesome, if you swap the electricity connections it pumps heat the other way!

Although this is undoubtedly groovy physics type stuff, they do have several limitations: They are not very efficient, they need a heat sink & fan to help transfer heat, and they can't get very far away from ambient temperature compared to a compressor fridge - if it's hot out, they just can't keep your Ben & Jerry's frozen. On the plus side, they are super-cheap, and can be used to keep stuff warm as well as cold if needed.

So to sum up the technology side:

Technology is not the full story here - VA fridges have pretty much ruled the roost for campers, caravans and a some boats & trucks since the year dot with only really cosmetic changes, so they're ubiquitous, common 2nd hand, and pretty much have the market sewn up. Compressor-type (let's call them VC) mini-fridges have limited market, mostly as aftermarket units for camping & overlanding, and hence seem to have huge price tags. Peltiers (let's call them PC) are the preserve of portable coolboxes and cheap “novelty” single-beer desktop units.

It's also common (for mobile fridges) that VA fridges are most like a home fridge with a front-opening door whereas the VC type are often top-opening and PC are usually coolbox style. There's a few reasons for top-opening: It's easier to get to stuff when the thing's on the back seat, jammed in the boot between luggage, or in a footwell, it stops things falling out when carrying it around, and it prevents all the cold air from falling out when you open it.

There are a few reasons we ended up with the fridge we did:

• It was cheap - 65 quid on ebay, less of an issue if it turns out to be not what we wanted
• It's front-opening - We'd have had to make a slide-out rail for a top-opening type, costing space & convenience, or pay even more for a front-opening one which is firmly in the luxury yachting category and even less common 2nd hand.
• It's silent
• It's got enough space for stuff

We don't actually care that it can run on gas, it will probably never do so - more of that later.

I actually own a genuine Land Rover Discovery (Waeco) compressor fridge which had some ridiculous price tag when new, it's an excellent little unit for a weekend away with a car & tent and has seen me through more than one music festival with cold beers on tap. However, it is small (~20L) and top-opening, so not a candidate for permanent mounting in a camper.

More detail on the practicalities and workings of a camper fridge.

VA fridges come in all shapes & sizes but the controls and the bits round the back that do the work all look almost identical:

• A switch to select battery, mains, or gas operation
• A thermostat which controls the temperature when running from the mains (more later)
• A gas control knob which sort-of controls the temperature when running on gas
• A spark ignitor for the gas

The back will look more or less like this:

You can sort of see how it's working here:

A learning experience!

With a bit of a tight schedule and no time to shake-down the Mk1 camper conversion we bolted the fridge into it's designated hole in the kitchen, hooked up the 2nd battery (one of two that came with the vehicle) and set off for Europe. We had no mains hookup and no gas connection to the fridge, this was a mistake.

We very quickly discovered two things:

• In 12v mode, the fridge thermostat is out of the loop - the fridge runs flat out all the time
• The 12v mode draws LOTS of power (~100W)
• It has no low-battery shutoff

These three mean it draws LOTS of power ALL the time. More than our quite-small 2nd battery could happily provide for any length of time. In fact, if you work it out, it's drawing ~10A constantly: our battery was 100Ah (100 Amp-hours), so the fridge will flatten it in 10 hours (in a perfect world with a perfect battery) or less (stone cold luke warm reality). Also, 10A is double the C20 rate of the battery, reducing the available capacity by perhaps 10%.

We then discovered that the fridge needs a LOT more ventilation than we'd given it. In the hurry to get things done, we'd basically just plonked it on the floor with a gap behind it where hot air could rise and flow out from under the sink. However, in “normal” caravan or camper installs, the fridge has two ruddy great ventilation panels cut into the side of the van (which we do NOT want to do). I'd assumed they were predominantly a slightly OTT measure because of the gas burner but warm beer taught me otherwise.

A properly ventilated fridge.

We solved the power problem (temporarily) by nipping out to the nearest supermarket and buying a 25m mains extension reel and a couple of commando plugs, then hacking it up so we could plug in on site and run the fridge on mains most of the time. It mostly worked, although absorption fridges take aaaages to get back down to temperature once they're warmed up. However, this was not before the fridge had run our 2nd battery down way below 10.5v, permanently crippling it and ultimately writing it off.

The ventilation problem was harder, in the end we left it alone for the rest of the trip and resolved to investigate when we got back.

So, after a poor performance on the first outing, we had a few issues:

• The fridge used too much power in 12v mode
• The fridge needed much better ventilation
• Potentially, our battery wasn't big enough

First off I discovered the fridge has no thermostatic control in 12v mode, it just runs flat out all the time. The wiring diagram shows the issue:

Wiring diagram - note thermostat only switches the mains side.

I thought maybe the 12v side was lower power than the 240v side and therefore running flat out all the time was just they way it had to be, but on investigation I discovered both the 240v and 12v heating elements were rated to the same power (about 90W), a bit like having your headlights on 24/7. No wonder the battery suffered. Basically it comes down to a design decision by Electrolux: Thanks to Ohm's Law, 90W at 240v is a very small current, around 0.4 Amps. Easy to switch with your cheapy thermostat. 90W at 12v however requires 7.5 Amps of current, much harder to switch with a cheap switch and needing bigger wires. They expect 12v would only be used when the vehicle was on the move and that on-site the fridge would be run on gas or mains, so why fit an expensive extra thermostat or a complex changeover circuit? You could do it with a relay or two, but mixing the mains & 12v sides is risky business from a manufacturer's point of view.

So, thing #1 to do: Make the 12v side thermostatically controlled.

There is also the issue that, running flat-out, the fridge has NO cutoff if the battery voltage drops too low.

Thing #2: Add low-voltage cutoff

Which leads me to the possibility of adding a bit more intelligence to the fridge: Maybe we could add a relay or switch that runs the fridge “flat out” when times are good (engine running, battery charging) and drops back to thermostatic control to conserve battery power when parked up. Likewise, if the vehicle is plugged into the mains, the fridge could automatically switch over to mains-power and run colder.

What I don't yet know is if I can run the fridge with a low-power element (maybe 25W) some of the time and it still have some effect, or if the physics of the absorption cycle require a specific temperature to work properly.

Anyway, you can see there's a few avenues to investigate power-wise.

Next issue is ventilation or lack of, here you can see the recommended ventilation:

And here you can see what we did, ventilation up the back but no airflow from below:

Basically, fail.

In order to work out how bad things were, we needed to do some science.

So, I grabbed a Raspberry Pi, some ds18b20 temperature sensors (actually, presumably they were knock-offs as they came from eBay for peanuts), a bit of Python code from googling the terms ”ds18b20 raspberry pi“ and voila! I had a 4-channel temperature logger. I also added a GPIO input to tell me when the fridge was running (thermostat closed) and another to stop the script using a button.

As I was only using 12v, I hijacked the mains-side thermostat on the fridge and wired it to a 12v relay so it switched the 12v heater on & off, that way I could see if running with the thermostat would improve matters. The Pi would log the on/off state of this via a 2nd relay wired to its GPIO pin and the Pi's own ground (a basic hack to avoid putting 12v up a 3.3v pin accidentally).

Finally, I would need some sort of calibrated test loading for the fridge to represent normal operating conditions. Perishables like milk, cheese, or the blood of my enemies would not do as they'd quickly spoil between tests. In the end, I found a highly suitable test subject:

Commence the trials!

This was done with our original setup - so the fridge is running flat-out, sat on the floor with no help, starting from warm:

You can see the heater (yellow) ramp up - the ds18b20 maxes out at 130deg so we can't see the true picture but I'd guess it gets pretty warm. Then, as the cycle gets going the heat-sink of the condenser (green) heats up, indicating it's starting to do its thing.

Some time after that, the inside temperature (red) starts to drop, and very gradually works its way down below zero.

Given that this graph is over at least 24 hours, the time taken to get cold is, well, frickin' ages. In fact it takes over an hour for the thing to even start thinking about working. Not an auspicious start.

For the 2nd test I decided to try giving the fridge a bit more ventilation, so I raise it up by ~50mm off the floor to allow some flow of air.

Nah, not helping. The heater is still running 100% flat out.

For this one, I decided to add a cooling fan to the heat-sink to force air over it, surely this awesome turbo-charging would improve matters!

Not really.

This time round I decided to bring the thermostat into play, as fridges are supposed to run around 5degC rather than heading for freezing as it was currently doing. Let's watch:

Well, you can see it working, but it's still spending a lot more time running than not - averaging ~70% on / 30% off which means it's drawing an average of 70W all the time.

As a final act of desperation, I decided that since we're not using the gas, I could block the chimney off and prevent heat from the heating element escaping up the chimney.

Yeah, that's not really helped. Drat.

It rather looks like no matter what you do, a 3-way fridge will draw an average of 7Ah. I'm sure they're much more efficient on gas, but we're not going to cut holes in the vehicle for that.

Unfortunately, our plans include events where we'll be parked up for ~5days at a time without a mains hookup and without opportunity to run the engine, so this is not going to work.

The upshot of all this faffing about can be seen in our Mk2 electrical system write-up.