Building Uninterruptible Power Supply for Medication Fridge

[Doug Garson]

You also need to monitor the battery to make sure it is charged when you need it. We have very infrequent power outages, (last one to last more than a few minutes was over a year ago) and last time we had one at night I connected my CPAP to my backup battery and dead . I need to remember every few months to plug it into the charger. In your case you might also consider keeping a few ice packs in your freezer as a second backup method.

[Brian Runau]

Would solar with batter backup work?

[Paul F Franklin]

Lot of replies here and I didn't see an answer to your questions...So, ahem...

If it's a 120 volt ac coil on the relay, you just connect it directly to 120 volt line. Even a fairly large relay will draw less than 100 milliamps current from the 120 volt line to keep the coil energized, so less than 12 watts. Smaller relays will be even less, and a solid state relay, even less than that.

I'd probably put a 1 amp 120 Volt rated fuse (so not an automotive fuse) in series with the relay coil so in the very unlikely event of the coil shorting out, the fuse will blow.

I assume you are switching the DC to the inverter, so you need a relay rated to switch DC. It should be rated to switch DC current of at least twice what your inverter draws fully loaded to ensure long life of the contacts (although it won't switch very often, so not much of an issue).

[Ed Aumiller]

What Doug said.... buy a 12v charger that stops charging when battery is full..(almost all today are that way)... put it on a quality deep cycle (marine?) battery ... connect a small inverter (300watt??) to it.. plug refrigerator into inverter..
The battery will always be close to full charge and the charger will use minimum current except when the refrigerator is running and then only about what the refrigerator is using...
I did this for years to make an UPS for our house wifi modems and laptops... the battery never wore out !!!
With the modern batteries you probably would never have to replace it!!

Just remembered that my house alarm system uses same setup, except it uses a small motorcycle battery and it is almost 25 years old and still working !!!

[Steve Demuth]

I think you're looking for one of these: https://www.amazon.com/Switch-Batter.../dp/B07RS48WTH.

[Keegan Shields]

For our business, we use off-the-shelf UPS to cover power interruptions for critical equipment.

The cost depends on capacity needed and the battery needs to be replaced periodically, but they take care of the battery charging and switch over to battery power when grid power is lost.

Something like this:

Server UPS

[Alan Lightstone]

I think you're looking for one of these: https://www.amazon.com/Switch-Batter.../dp/B07RS48WTH.

Just looked at the specs. The voltage of the power source, battery, and charger must be equal. All DC.

[Alan Lightstone]

Would solar with batter backup work?

That would work out great for our whole house solar array. But for $37K or more, have to pass on that.

Also, no way to have solar wired to the room it's in.

[Alan Lightstone]

Lot of replies here and I didn't see an answer to your questions...So, ahem...

If it's a 120 volt ac coil on the relay, you just connect it directly to 120 volt line. Even a fairly large relay will draw less than 100 milliamps current from the 120 volt line to keep the coil energized, so less than 12 watts. Smaller relays will be even less, and a solid state relay, even less than that.

I'd probably put a 1 amp 120 Volt rated fuse (so not an automotive fuse) in series with the relay coil so in the very unlikely event of the coil shorting out, the fuse will blow.

I assume you are switching the DC to the inverter, so you need a relay rated to switch DC. It should be rated to switch DC current of at least twice what your inverter draws fully loaded to ensure long life of the contacts (although it won't switch very often, so not much of an issue).

Thanks, Paul. Was wondering about that as a safety issue.

Actually, since the particular inverter I have won't automatically turn on when energized (stupidly, they have an solid state on/off button on it), I'll be switching the AC output from the inverter.

It sounds like the RV automatic transfer switch will be the way to go. It will switch the AC output from the inverter when the AC line power fails. Since the inverter will always be getting 12V DC from the battery, I'll just turn on its switch, and it will stay on, but producing no power as it has no load until the transfer switch closes the contacts. I'll keep the charger hooked up to it. Presently I have it on a timer to trickle charge it once a week. It sounds like I can just keep it on, and it will stop charging when the battery is full.

[Steve Demuth]

Just looked at the specs. The voltage of the power source, battery, and charger must be equal. All DC.

Sorry, my bad. I was looking for DIY alternative to an off the shelf transfer switch, and didn't read the fine print. This is the link I was originally looking for: https://www.amazon.com/AC120V-Automa.../dp/B0BRQ7KQQ7. Understand that it's probably a bit pricy compared to what you're trying to do.

[Alan Lightstone]

Sorry, my bad. I was looking for DIY alternative to an off the shelf transfer switch, and didn't read the fine print. This is the link I was originally looking for: https://www.amazon.com/AC120V-Automa.../dp/B0BRQ7KQQ7. Understand that it's probably a bit pricy compared to what you're trying to do.

I purchased one. On paper looks like a perfect solution. I'll hook it up in a couple of days and see how it goes.

[Leo Graywacz]

Looking for a relatively inexpensive solution. Since I already own the battery and inverter, it's now just about the electronics to make it switch over to the inverter when the power fails.

The fridge is to keep the medications 36-46°F (2.2-7.7°C). So while that freezer is really interesting, what we need is for insulin and biologics.

Do you just hook up the relay coil directly to AC current? That's my biggest question. The rest of the wiring I think I can figure out.

Don't do any of that. Hook up a charger to the battery and that's it. The battery charger keeps the battery up to snuff and the inverter powers the fridge. When the power goes out the charger no longer charges the battery but that's it, the battery keeps the inverter working so the fridge keeps working. When the power returns it will charge the battery back up again.

[Alan Lightstone]

Don't do any of that. Hook up a charger to the battery and that's it. The battery charger keeps the battery up to snuff and the inverter powers the fridge. When the power goes out the charger no longer charges the battery but that's it, the battery keeps the inverter working so the fridge keeps working. When the power returns it will charge the battery back up again.

An interesting, simple approach. My concern is which approach is more susceptible to failure, when we're not at home to notice it (or at home for that matter, and the battery/charger fails and we don't notice that).

Of course, in the outdoor scenario here in Florida, car batteries tend to die in 2 years. The brutal heat takes its toll. This battery, while it did live outdoors for a few weeks, lives in a constant 74°F, so it has an easier life. But if/when it fails, there is no backup if it is the usual power source for the fridge, and the medication is ruined. Unless the inverter can carry the load of the refrigerator on its own, and it's not designed to do that.

Which is the most likely part to fail? The inverter (which will not restart if it loses power)? The battery charger (not the most expensive one, but not garbage either)? Duke Energy providing energy to the house? The RV transfer switch (which is a cheap part, but how likely is it to fail when power goes out and its coil stops the contacts from staying attached to the AC part of the circuit)?

With the RV transfer switch, the fridge runs off utility 120V AC for 99+% of the time. In case of a blackout, the transfer switch needs to flip over to the battery system, and the fridge stays powered. My hunch is that this is the most reliable setup (it is, after all how typical UPS systems work). Essentially, this is replicating a UPS system.

What do you all think? Which is likely to be most reliable?

[Leo Graywacz]

So you need a back up for your back up?

I understand you overthinking this but there are going to be situations beyond your control. How far do you plan on going? The inverter can fail, the refrigerator can fail, the charger can fail. If you put in a UPS it is essentially what I described. An inverter working on a battery with a charger so that when the power fails the battery just continues on providing power until the battery is depleted. The bigger the battery the longer the protection.

Put a power alarm on the device so when it fails the alarm sounds. If you aren't around to notice it's going off it won't really matter.

You want to do this with items you have instead of going with a specialized dedicated system. Well dedicated systems are specialized to protect and have other safety features, but they cost an arm and a leg. You want the shortcut to reduce the cost. I'm not sure you can have it both ways.

So it's up to you to decide the risk factor. Use a LIFePO4 battery with a charger and hook it up to your inverter and it should provide you with good protection. Not perfect, nothing is unless you have countless redundant systems as backups.

Good luck.

[Jim Becker]

That would work out great for our whole house solar array. But for $37K or more, have to pass on that.

Also, no way to have solar wired to the room it's in.

Just a program note that this is different than I was suggesting...which was something like a Bluetti or Jackery portable "battery backup generator" and power supply that can be recharged with solar panels if necessary or desired.