Building Uninterruptible Power Supply for Medication Fridge

[Steve Demuth]

To be reliable and long lived systems, including the batteries, need to be designed for the charge and discharge cycles they follow. A typical car battery is designed for frequent relatively shallow discharge cycles, and is probably not a bad match for the way your refrigerator uses power in that respect. But it's going to be the equivalent of starting a car a couple dozen or more times a day (assuming your fridge has to run for a few minutes every hour, at minimum). The battery life will affected accordingly. You would also need to consider the charger = it should be regulated to only charge when the battery is discharged, and to throttle off when the battery is fully charged. Overall, unless you're into designing charging circuits and regulation, just hooking a charger and inverter to an automotive type battery and letting it run is not an optimal solutio Yourn. If you're looking at any other type of battery, same considerations apply, but with different parameters. Your case, e.g., is somewhat similar to what the drive battery in a hybrid car experiences in stop and go traffic. There is a reason for the elaborate electronics and software that manage those batteries.

On the other hand your battery + inverter + transfer solution isn't perfect either, but it's close to the way a lead acid powered UPS would work. As long as you check the battery periodically for ability to sustain a discharge, and top it up as needed, it's a pretty simple solution - not quite fire and forget, but minimally complicated. If it were me, I'd have gone out and bought a real UPS, but short of that, your solutions with the ATS is pretty sound.

[Rick Potter]

Back to what Leo suggested....battery backup with a battery MAINTAINER instead of a regular charger. Deltran Battery Tender is the best known, and what I use to keep car batteries up. They, depending on model, can charge a low battery, but normally cycle on and off as the battery starts to get low, charging at a slow rate to maintain the car battery, keeping it ready for your power outage. After the outage the maintainer will automatically recharge the battery.

I have gone as long as six months without starting an old car, and it they have always started. Got four in use right now. About $40.

[Leo Graywacz]

You can put a solar charger and a outlet fed charger on the battery at the same time. If the AC one dies the solar will continue to charge.

I have a solar/inverter setup in my truck. 3KW sine inverter and a 125AH AGM battery. It'll make almost anything go. I've run my house on it during power outages. Obviously the essentials. But the solar chargers are pretty reliable and you could likely get away with a 50w panel. At least that way there is a redundancy.

[Alan Lightstone]

Back to what Leo suggested....battery backup with a battery MAINTAINER instead of a regular charger. Deltran Battery Tender is the best known, and what I use to keep car batteries up. They, depending on model, can charge a low battery, but normally cycle on and off as the battery starts to get low, charging at a slow rate to maintain the car battery, keeping it ready for your power outage. After the outage the maintainer will automatically recharge the battery.

I have gone as long as six months without starting an old car, and it they have always started. Got four in use right now. About $40.

Is that better than the one I'm presently using? It's this. Seems to be a battery maintainer also:
https://www.amazon.com/Automatic-Mai...08L4BGJLC?th=1

[Alan Lightstone]

You can put a solar charger and a outlet fed charger on the battery at the same time. If the AC one dies the solar will continue to charge.

I have a solar/inverter setup in my truck. 3KW sine inverter and a 125AH AGM battery. It'll make almost anything go. I've run my house on it during power outages. Obviously the essentials. But the solar chargers are pretty reliable and you could likely get away with a 50w panel. At least that way there is a redundancy.

Solar isn't an option. The fridge is in an interior room 35 feet off the ground.

[Rick Potter]

Same type, different brand, Alan. Sorry, I must have missed where you mentioned you had one.

[Alan Lightstone]

Well, really glad I put the system together. We went out of town for 5 days, and the utility cut our power twice for over an hour as they are replacing many electric poles. I had finished and hooked up the system the night before we left town (and made a deep cut on my finger needing me to test out the Nexcare I had just bought). Really glad I did it.

The backup power, RV transfer switch, charger, battery worked great. When I got home, I checked history on the temperature disc in the fridge and the system kept the medication at optimal temperature the entire time with ease.

Thanks for all the information, guys. Really helped and saved us from a real mess.

[Leo Graywacz]

A hour outage shouldn't have caused much issues as long as the fridge is kept closed. But glad it worked out and the feeling of not having to worry really makes it worth while.

[John Lifer]

For those suggesting a UPS, at 80 or so watts, it could run 30 minutes on that most expensive UPS ($230 one listed) And if the house is in FL, and the AC was off for several hours, it most probably run way more than what you think. And if power off in August for say 48 hours, that UPS would be long dead. Bad solution for what you are looking for. I like the switch and you can ALWAYS put two batteries in line or more if you wanted longer time. (I'd sure test it to see the time it would keep things cool) I would for sure add just a battery maintainer, heck I use a $15 option from Harbor freight on my tractor in the winter. Has worked for 5 years.
As long as you are charged when you leave, you would be fine.
And fwiw, you can get alarm to a smartphone if your power goes down. Not to expensive an option to at least know it's down.

[Bert McMahan]

Three pages in and only one person replied to your original question- "can you just hook up the relay directly to the AC line voltage"?

Yes, you can. I think you must've misread the datasheet for your relay, because 75 ohms is WAY too low for a 120V relay. That's a reasonable value for a 12V relay, but not 120V. Check out page 3 of this datasheet:

https://cdn.automationdirect.com/sta...s/78relays.pdf

I just picked the first one in the list. Their 120V coil relay has a resistance of 4.43 kOhms, meaning it consumes under 2 watts of power. That's going to be fine to be left on all the time. If you add a power resistor, you'll reduce the voltage across the coil and it won't work.

Also, I doubt you can measure the resistance of the coil with a normal multimeter. Maybe you can, but I suspect the "resistance" they're quoting is the AC impedance at 60 Hz. (Basically, the AC version of resistance is impedance, so the math still works with Ohm's Law). If you measure an AC relay coil using a normal multimeter, you'll get the actual resistance of current to flow to DC voltage, which isn't what you want.

I'd do your original plan- hook up the relay to mains power, then if mains drops out it connects in the battery/inverter solution. Make sure to test it a couple times per year to make sure the battery is still working. This is a better solution (IMHO) than letting the battery run it full time.

If your house power dies, you will know it immediately. If the battery dies and you don't have an extra alarm, you won't know it until your fridge stops working.

Basically, the "critical failure" point here is for the fridge to lose power. With your relay solution, you reach critical failure with *two* things going wrong- your house power dies, AND your battery dies. With the "let the battery run it all the time" solution, you reach critical failure with just *one* thing going wrong- the battery failing.

Now, in any given case you could argue that having option two means that you can detect the battery failure *while you still have house power* and can therefore fix it. With the relay solution, there is potential for you to not notice the battery failure until both items fail.

You can only plan for so many contingencies, but I think I'd still go with the relay solution and put an alarm on the battery itself. Then you're not worrying about the inverter and battery providing fridge loads 24/7, and will notice when the battery dies.

Either solution will work- you can just decide which failure scenario seems less risky from your point of view.

[Alan Lightstone]

Three pages in and only one person replied to your original question- "can you just hook up the relay directly to the AC line voltage"?

Yes, you can. I think you must've misread the datasheet for your relay, because 75 ohms is WAY too low for a 120V relay. That's a reasonable value for a 12V relay, but not 120V. Check out page 3 of this datasheet:

https://cdn.automationdirect.com/sta...s/78relays.pdf

I just picked the first one in the list. Their 120V coil relay has a resistance of 4.43 kOhms, meaning it consumes under 2 watts of power. That's going to be fine to be left on all the time. If you add a power resistor, you'll reduce the voltage across the coil and it won't work.

Also, I doubt you can measure the resistance of the coil with a normal multimeter. Maybe you can, but I suspect the "resistance" they're quoting is the AC impedance at 60 Hz. (Basically, the AC version of resistance is impedance, so the math still works with Ohm's Law). If you measure an AC relay coil using a normal multimeter, you'll get the actual resistance of current to flow to DC voltage, which isn't what you want.

I'd do your original plan- hook up the relay to mains power, then if mains drops out it connects in the battery/inverter solution. Make sure to test it a couple times per year to make sure the battery is still working. This is a better solution (IMHO) than letting the battery run it full time.

If your house power dies, you will know it immediately. If the battery dies and you don't have an extra alarm, you won't know it until your fridge stops working.

Basically, the "critical failure" point here is for the fridge to lose power. With your relay solution, you reach critical failure with *two* things going wrong- your house power dies, AND your battery dies. With the "let the battery run it all the time" solution, you reach critical failure with just *one* thing going wrong- the battery failing.

Now, in any given case you could argue that having option two means that you can detect the battery failure *while you still have house power* and can therefore fix it. With the relay solution, there is potential for you to not notice the battery failure until both items fail.

You can only plan for so many contingencies, but I think I'd still go with the relay solution and put an alarm on the battery itself. Then you're not worrying about the inverter and battery providing fridge loads 24/7, and will notice when the battery dies.

Either solution will work- you can just decide which failure scenario seems less risky from your point of view.

I essentially went with the relay thing (bought ready made RV transfer switch. I also thought the resistance of that relay couldn't be that low.

Is there a power outage monitor that would tell me if the power was out (my utility, Duke Energy, does send out texts, and their outage map, while usually not terribly accurate does give the big picture of the area)? I would have to be able to receive the message with no power and no internet, so that seems difficult.

Also, how would you know if that battery fails? I can monitor that in the house with the inverter (it has a digital readout of battery voltage, as does the charger). I know my solar inverter measures battery voltage and if you are close to it you can see a reading via Bluetooth, but it's also very inaccurate.

The system is done now, and seems to be working great, so this is more intellectual curiosity, and possible help to others.

[alan weinberg]

thread drift—backup power is great but don’t forget temperature monitoring. My SensorPush sends me a text and email if wine cellar temp or humidity rise over set numbers. There are other systems but this one is reliable and cheap.

[Alan Lightstone]

thread drift—backup power is great but don’t forget temperature monitoring. My SensorPush sends me a text and email if wine cellar temp or humidity rise over set numbers. There are other systems but this one is reliable and cheap.

I use SensorPush sensors also. Didn't realize it could send me a text and e-mail. Never used that function. Wouldn't work here with a power failure as we'd lose internet access and wifi, but their sensors have been great for traveling with portable insulin fridges for medications.

Actually, just looked on their website. You need their wifi gateway and I think their higher-end sensor, but again a non-starter for me with the prospect of losing internet with a power failure, but a nice option.

[ChrisA Edwards]

So back to he RV stuff, we also use a device called Waggle. It's a small device that uses a battery and communicates via cell(Verizon, $20 per month). In the camper it stays plugged into an outlet. If the power goes out, it sends me a text message. When power is restored, I get a text message.

It also has high and low temperature limits that also cause it to send a text message.

We use this mainly when we leave our dogs in our camper for a few hours. We are never far away, but if camping in a hot State and our RV has a power outage, we can either rush back to the camper or call the campground to let our dogs out.

[Alan Lightstone]

So back to he RV stuff, we also use a device called Waggle. It's a small device that uses a battery and communicates via cell(Verizon, $20 per month). In the camper it stays plugged into an outlet. If the power goes out, it sends me a text message. When power is restored, I get a text message.

It also has high and low temperature limits that also cause it to send a text message.

We use this mainly when we leave our dogs in our camper for a few hours. We are never far away, but if camping in a hot State and our RV has a power outage, we can either rush back to the camper or call the campground to let our dogs out.

That Waggle is interesting, though pricey, especially considering monthly coverage. That's for my usage in a fridge. To protect your pet in an RV - worth every cent. https://mywaggle.com/

I wonder if the temperature alerts can be set to the needed range for a medication fridge (36-46°F). Can you look at your Waggle app and see if that's possible?