Yes, great interest.
My dream unit would be an ARB that would do exactly what I think you have done!
I'm almost surprised that there isnt anything like this on the market. The extra cost to manufacture would be minimal and it would be much less hackish than what I've done. ...ok, what I've done could be done much less hackishly than I did but I'm sure you get the point.
The temperature controller was pretty easy to find on amazon. They sell no less than a dozen variations on what appears to be the same device. This
temperature controller is the one I ordered but I'm pretty sure it's not the one you'd want to get. The problem with a dozen of these being available is that they're not all exactly the same. This one seems to be designed to run off of 120v AC but switch 12v loads. A couple of quick goggle searches found another poor soul who also ordered the wrong one but managed to convert it to the 12v. The mods were easy so I did the same thing. If you're going to give this a shot, try this
controller instead. The idea is that it needs to run on 12v and switch a 12v load up to about 10amps (to be safe). Also, make sure the switching is designed to turn on a heater when the temperature drops rather than turn on a cooling device when the temperature is too high. Both of these devices will do either. Some devices do both. Others will do just one of these things. Ok... maybe it wasnt so easy.
The next problem was coming up with a heat source. Amazon sells a variety 12v heat making devices. I really didnt like any of the things that looked like heaters. I wanted something that would heat slowly and have a decent amount of thermal mass. I didnt want to heat the food that was sitting next to the heat source. I felt most of the small heaters would do that. The small heaters werent really all that small either and I wanted something that wouldnt take up much room in the fridge. The best I could come up with was a
Roadpro 12v Hot Pot. The manufacturer claims it will draw 7amps. I've measured mine at less than 6amps. I wouldnt want to wait for it to boil water but the form factor and amp draw were within reason. All it needed was some thermal mass. Luckily I had some
quick setting concrete left over from another project. I mixed up a cup or so and filled the hot pot about half way. Filling the whole pot seemed unnecessary but might make sense. If we have any heat transfer experts in the house, feel free to chime in! When you add the concrete make sure you tap the pot on the work bench a few times get rid of any air pockets. Time will tell if the heating coil hold up to being covered in concrete.
The wiring was simple enough. The controller comes with minimal instructions printed right on the device. It has screw terminals on the back for all connections. Just feed it's control circuit and heating load input with 12v and connect the hotpot to the heating output. The ground from the hotpot can be connected to the controller's ground. I just cut the cigarette plug off the hotpot and used a piece of a 6ft (120v) extension cord I had to extend the plug to the controller.
Here's how it all ended up.
The back side of the hot pot has a small clip that attaches nicely to the wire basket in the fridge.
The seal on the fridge is flexible enough to allow the wires to pass but still seal well enough. I could have fed the wires through a hole in the fridge but decided against it. If the hole was big enough for just the wire I wouldnt have minded much but the connectors would have needed a much bigger hole. Since I only want this thing in during the winter I'd either have a big hole that could pass some kind of connector for both power and the temp sensor or I've to solder/unsolder the wires to install or remove the heater. Luckily the seal is flexible enough.
The controller sits in the cup holder behind the center console of my Tacoma. The fridge is on the "dog platform" where the rear access cab seats used to be.
...and speaking of the temp sensor, I'm still a bit puzzled about the best place to put it. Inside the hot pot might make sense but just leaving it right on the outside of the hot pot seems fine too. The idea, I think, is to keep the temperature of the concrete above freezing. That should in turn keep the stuff in the fridge above freezing too.
It worked great last winter and I expect same this year. On the cold really days, in Boston that's 15F or less, it would average about 1amp/hour per hour which is about the same as the fridge's compressor uses during the hot part of the summer.
The instructions that came with the controller are barely useful but once you figure it all out, setting it to turn the heat on at 31F and off at 33F is pretty easy. It takes some trial and error to figure out the best combination. The controller has a calibration feature so you can try to get it to agree with the readout on your fridge. If your fridge doesnt have a readout you'll need to spend some time watching the readout on the controller and waiting to see at what temperate your compressor kicks in. The most important bit is to simply make sure that the fridge doesnt have to cool the heater down by running the compressor. As far as your fridge contents are concerned it will probably work out well enough but you might kill your battery if you let the kids fight.
