Refrigerator Battery Backup: Progress Report

It’s been five months since I completed my battery-powered insulin refrigerator, and I’ve discovered a design flaw that might require some rework soon.

Refrigerator battery bank
The “finished” product.

The other day, the fan on the inverter started kicking on for no apparent reason. The Cobra CPI1000 inverter in the battery bank has been a solid performer, but the fan on it is horribly loud. It’s a tiny fan that runs incredibly fast and sounds like a little siren when it runs. Luckily it only runs when the inverter’s gets get hot, which has only been when the inverter is running under load. But now the fan was running without any significant load on the inverter. What gives?

Turns out that even though there’s no real load on the inverter, simply having it powered up on a 90° day is enough for the fan to need to run occasionally. And that’s exactly what we were dealing with on that unseasonably warm May afternoon. It’s never been above 70° in the house since I put the bank into service, and suddenly it’s 80° plus upstairs. Problem solved. Well, diagnosed at least, if not quite solved. That fan noise is an added nuisance to Grace, since the fridge is right outside her room. The compressor generates enough noise for her to deal with; no need to add fan noise to it. So I need to solve that problem.

IMG_20141207_142552_266I suppose the techno-fix would be to add a relay to turn the inverter on only when grid power is lost. That would be pretty simple in theory, but probably difficult in practice – the circuit box is already jammed kind of tight as it is. Plus I’d need to figure out the remote control interface on the inverter so I could wire up the relay. Not impossible, but time consuming.

But the biggest drawback to turning the inverter on and off would be that I’d lose the benefit of knowing the inverter is working. That was a design feature – the inverter would always be running off the batteries, and a small 120V pilot light on the output would confirm that fact. I’d lose that if I switched the inverter on and off.

In the end, I think the best solution will turn out to be the low-tech approach – manually turn the inverter off on hot days. Temps upstairs shouldn’t get that hot too often; despite my reptilian wife’s considerable powers of persuasion, I generally win the thermostat war when it gets really uncomfortable. For the rest of the time, a simple push of the power button and a sign on the fridge reminding me that the inverter is off should suffice. There’s a little risk that the inverter will be off in the event of an unattended power outage, but we’ll just have to be careful. After all, you can’t tech your way out of every problem.

And I think the underlying message here is an important one for preppers: Don’t piss off your family with your preps. Yeah, keeping the insulin fresh is the overriding concern here, and it would be tempting to just tell Grace to take one for the team. But that’s not fair, and the last thing I want is to turn anyone off to the prepared lifestyle and mindset, especially a young adult soon to be making her way in the world. I want to leave her with a positive impression of what we’ve tried to accomplish here, so that she’ll at least keep preparedness in mind as she builds her own life.


  1. Brother Harold

    Here’s a thought. They usually put the cheapest, crappiest fans they can in consumer-grade electronics. I know you’ve already had one warranty claim, so I’m not sure how open you are to cracking the case, but have you considered upgrading the fan in the inverter? It’s probably a 40mm case fan. For less than $20 on NewEgg you can upgrade the fan to something that will likely move more air more quietly, and be more reliable.
    Don’t forget: any manual process is subject to human failure. If you can’t harden the inverter against higher temps, or automate the process of turning it on/off, you run the risk of pilot error. And a sign on the fridge likely won’t save you from that. Murphy’s Law says you will forget the inverter is off and will be away from home when the power goes out. Well, at least that’s the kind of luck *I* would have….

    1. APB

      I just so happen to have the inverter I torched right here on my desk, opened up – my son says it’s just for geek ambience, but I was actually working on it – and it does indeed have a 40mm brushless 12VDC fan from China. I don’t get anything in English when I search for the part number, though, so I can’t get a spec on the noise level for the stock fan so I can compare to potential replacements. It wouldn’t be too hard to replace, but I’m not thrilled with the idea of going through such an operation with the possibility of not improving the situation. Other than optimizing blade design, I can’t see how any resonably priced replacement could be markedly quieter.

      Still, as Murphy and I are old acquaintances, it bears investigation. I really don’t like building a failure into the system, but if I can’t quiet the fan, I don’t see that I have much choice. I suppose I could go full tech-tard and Arduino up a monitoring solution that sends reminder texts to my phone every hour when the inverter is off, but I’d still need to be here to close the loop. Taking the Arduino further, I could use it to close the loop by being able to turn the inverter back on remotely, but if I can do that, then backing off to the simplest solution makes the most sense – just add a relay that kicks the inverter on when the grid power dies.


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