Building Uninterruptible Power Supply for Medication Fridge

[Alan Lightstone]

So, here's my latest project. We get occasional blackouts here, and certainly have the risk of them with hurricanes (although no power loss with the last hurricane). We had one yesterday for two hours when they were replacing the poles and someone must have screwed up because 1900 homes lost power.

We have a fridge that we keep some expensive medications in. When the compressor is on, it consumes 80 watts, and otherwise near zero, cycling on and off. So really small electricity usage which is nice for this, and does a great job controlling the temperature in the safe range for medications.

I just took a marine deep cycle battery and hooked it up to a pure sign wave inverter and plugged the fridge into it. It works great, and keeps the medication at a safe temperature. Should run about a day on its capacity.

Now, if we're away from the house, there is no way I'm going to be able to talk someone over the phone into unplugging the fridge from the wall, plugging it into the inverter, and turning the inverter on. In the dark. So, I figured, lets build a homemade uninterruptible power supply. Far, far more capacity - run time than a typical computer UPS (battery is massively bigger, about 90AH).

Still a little iffy about how to wire it, but I think I've got it. I do have some questions for our electricians.

I bought a relay with a 120V AC coil. I haven't measured the resistance on the coil yet, but have been wondering how much energy it will consume just in standby, plugged into the wall, waiting to flip if the power fails. Just looking online, it looks like some of these relays have a 75ohm resistance. If that's so, the relay will consume 192 watts. That seems like a ton. Would it be helpful to add a power resistor inline to reduce the energy consumption? Would the relay still work?

Also, is the correct method to essentially just plug the coil into the wall AC current, or is that a short circuit that will fry the relay, tremendously heat up, or flip a breaker?

Lots more questions to come, but let me start with those important ones.

[Frank Pratt]

I would just buy a suitably sized UPS. Done. No need to plug and unplug anything.

[George Yetka]

Ive purchased some refrigerators for medical jobs that have these features. But at the price of those you can put in a whole house generator and reduce this issue.

Something like this may be a better purchase (im not sure of the temp your looking for)
https://ivyxscientific.com/products/...Y4cbeFQigSmjS0

[Alan Lightstone]

Ive purchased some refrigerators for medical jobs that have these features. But at the price of those you can put in a whole house generator and reduce this issue.

Something like this may be a better purchase (im not sure of the temp your looking for)
https://ivyxscientific.com/products/...Y4cbeFQigSmjS0

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.

[Doug Garson]

Not an electrician but how about if you run the fridge off the inverter powered by the battery full time and keep the battery charged with a charger plugged into the wall? What am I overlooking?

[ChrisA Edwards]

In the RV industry, i.e motorhomes, 5th wheel trailers etc, we have transfer switches that detect being plugged into the electrical grid.

If not plugged in, i.e. when we are traveling, it transfers to an inverter which powers the fridge from the battery.

Expensive; I have a 270AH lithium battery that would probably power that fridge for several days.

[Alan Rutherford]

Reply deleted

[Alan Lightstone]

Not an electrician but how about if you run the fridge off the inverter powered by the battery full time and keep the battery charged with a charger plugged into the wall? What am I overlooking?

May be nothing, and I do have all the equipment to do that so an interesting idea. Does that wear out the battery quicker though? Certainly uses more power, as less efficient.

[Alan Lightstone]

Something like this?

Sounds like the solution.
https://www.amazon.com/AC120V-Automa...lbWF0aWM&psc=1

[George Yetka]

Something like this?

Alan your battery would be DC, so you'd also need an inverter before going into this and have a way of charging the battery

[Lee Schierer]

Why not purchase a RV fridge that works off of 120 volts or 12 volts and propane. Ours maintains temperature pretty well as long as you don't open and close the door frequently. It will operate for days off a 12 volt battery and a tank of propane. Norcold makes a wide variety of sizes.

[Alan Lightstone]

Alan your battery would be DC, so you'd also need an inverter before going into this and have a way of charging the battery

I already have both, and they are already wired up. Right now I have to switch over manually, and the inverter needs its power switch turned on to work.

[Alan Lightstone]

Why not purchase a RV fridge that works off of 120 volts or 12 volts and propane. Ours maintains temperature pretty well as long as you don't open and close the door frequently. It will operate for days off a 12 volt battery and a tank of propane. Norcold makes a wide variety of sizes.

Already have the fridge. Interestingly, I have a very accurate Bluetooth temperature disc/thermometer I use to look at the temperature of the fridge (I use this when traveling to monitor the medicines temp in our insulin cooler). This fridge I bought is great at staying in range. Our full size kitchen fridge - terrible at it.

[George Yetka]

I already have both, and they are already wired up. Right now I have to switch over manually, and the inverter needs its power switch turned on to work.

Oh you really are there then. That should work.

[Jim Becker]

One "modern" solution is one of the ever-popular and growing products from the "battery generator" industry with solar charging as an option in addition to from the main power supply. They are basically similar to a UPS, but have more smarts and can even talk to you about their status. There are a whole bunch of brands.