Using a UPS as an inverter

The basic question is:- will the UPS run forever at full output? The UPS is made to run for .5 to 1 hour at rated out put, but it might not run for 1000 hours with a huge battery. It might heate up and fail.

To make the UPS run constantly you will need to beef up parts or derate the maximum.

Testing will determine this. In addition a carava used in summer heat might break, even it runs at 20C forever.

In theory a UPS will work as you wish it, but you need to take the limitations I mentioned into consideration.

Thanks for that,

I am sure the final duty cycle will end up at 25% of the total rated max power rating, I will fit a 90mm PC fan to assist in cooling and do some measurement checks.

Regards

Andrew

1. Battery can not be discharged to ground zero, depth of discharge not more than 70%. Rate your battery Ah accordingly.
2. According the replaced battery you need to increase charging current; C10 rate.
3. Some computer UPS do not give pure sine wave out put. Hence you first check the UPS out put wave form.

I am using a CLF lamp for 5 to 6 Hrs on a computer 650VA UPS during power outrages. CFL can work with lot DC component from a computer UPS.

The Smart UPS is sine wave output (the BACK-UPS is a "modified sine wave", or square wave style).

I have been considering exactly the same thing, with a SMART UPS 700 (120 volt version) but I would attach it to the RV battery source. Usually electronics reach thermal stability within minutes, so I think it is worth a try. The UPS is free after all. I would really like one over 1500watts continuous. It would get the hair blower going for the wife.

Have you checked the fridge and freezer and ascertained if either have and onboard converter? I ask because often equipment is D/C inside.

The key problem is the cooling. You need to install forced cooling otherwise the UPS will overheat itself.

Being a Caravaner myself I am surprised that you need to run the fridge on mains voltage. All the ones I have seen run on 12 volts, gas or Mains. If yours is one of these, then you should be running it on 12 volts when driving and gas when stopped (if no mains available!).

The same goes for the freezer (in a caravan!), but if its a normal home freezer, then it will require mains. Get it really cold on mains at home first and just use the inverter to keep it cold.

The battery should be far bigger than you think, one reason is that inverters are often not very efficient, and efficient ones are only that at one particular power. reckon with around 50% efficiency and you will err on the good side.....

Make sure its a "Leisure" battery as these can accept being "over" discharged better, a car type will die quite quickly in such an environment......

Charging from the vehicle is good or from a portable generator, but make sure that the charger is intelligent and does not overcharge (cause the battery to gas).

Also make sure that the battery cannot gas into the vehicle, it should gas "outboard" and never close to a source of ignition. Never check the battery with a cigarette in the mouth for example.......

Trying not to be negative, but I would be very surprised if this inverter could supply both units at the same time......especially if of the compressor type as motor starts generally take 6 times the running current. In such a case it may not even be "man enough" to start even one fridge..........now if they are of the absorption type, you might just get away with it, and the unit is very, very efficient, but I doubt it......just one probably.

Fridges and freezers work much better with a small fan blowing air over the cooling coils. On my caravan I have 2 x 220VAC fans and 2 x 12 volt DC fans, so that I can keep the coil cool no matter where I am......it gives 5 to 10 degrees lower temperature in my fridge (really!) when used and improves the efficiency as well.....

Why do I have two? Simple, with a simple switch (Center off, 2 pole changeover) system I can run them in parallel or series, this gives me a high and a low speed for either voltage, the low speed is quiet enough to run at night!!

Never rely on convection alone to cool the fridge...

Great post,

"Being a Caravaner myself I am surprised that you need to run the fridge on mains voltage. All the ones I have seen run on 12 volts, gas or Mains. If yours is one of these, then you should be running it on 12 volts when driving and gas when stopped (if no mains available!)."

My caravan fridge is a dometic and runs on 220V and Gas, although a dual 220 and 12V element is available, it is a major b!tch to fit. This may however be an unfortunate necessity / reality. The losses in the inverter may would probably justify this. Years ago caravanners used to travel with the gas flame burining but this would make me nervous.

"The same goes for the freezer (in a caravan!), but if its a normal home freezer, then it will require mains. Get it really cold on mains at home first and just use the inverter to keep it cold."

The freezer is a standalone unit and runs on 220V or 12V or gas. The freezer is actually extremely thermally efficient, a 5 hour trip with no cooling will thaw the top layer of food very slightly, not enough to be a problem

"The battery should be far bigger than you think, one reason is that inverters are often not very efficient, and efficient ones are only that at one particular power. reckon with around 50% efficiency and you will err on the good side....."

This is a problem for me especially in that the inverter requires 24V, 2 batteries will be a little inconvenient. If I were to power direct from a battery I would need a circuit to drop the element out at a certain voltage threshold in order not to over discharge the battery.

"Make sure its a "Leisure" battery as these can accept being "over" discharged better, a car type will die quite quickly in such an environment......"

It will be.

Charging from the vehicle is good or from a portable generator, but make sure that the charger is intelligent and does not overcharge (cause the battery to gas).

"Also make sure that the battery cannot gas into the vehicle, it should gas "outboard" and never close to a source of ignition. Never check the battery with a cigarette in the mouth for example......."

OK

"Trying not to be negative, but I would be very surprised if this inverter could supply both units at the same time......especially if of the compressor type as motor starts generally take 6 times the running current. In such a case it may not even be "man enough" to start even one fridge..........now if they are of the absorption type, you might just get away with it, and the unit is very, very efficient, but I doubt it......just one probably."

Not negative at all, there are may ways to kill a cat, more on that later.

"Fridges and freezers work much better with a small fan blowing air over the cooling coils. On my caravan I have 2 x 220VAC fans and 2 x 12 volt DC fans, so that I can keep the coil cool no matter where I am......it gives 5 to 10 degrees lower temperature in my fridge (really!) when used and improves the efficiency as well....."

Agreed. I have a a special cowl fitted to my outlet (it is better to suck than to blow in this instance). The cowl has a 12V circuit controlling the fan, it kicks in at ~25 degrees C and steadily increases in speed to max speed at about 35 degrees C. This avoid the one / two fan scenario. I have also especially ducted the fridge enclosure for maximum drafting). I managed to get 8 degrees C based on tests.

"Why do I have two? Simple, with a simple switch (Center off, 2 pole changeover) system I can run them in parallel or series, this gives me a high and a low speed for either voltage, the low speed is quiet enough to run at night!!"

You may wish to have a copy of the circuit?

"Never rely on convection alone to cool the fridge..."

Will take issue on this to some degree, I wrote an article for our local caravan magazine on absorption fridges and did a months testing in the Kruger National Park in December which tends to be hot as hell (it is about 5meg but you are welcome to a copy). The single biggest impact on fridge temperature in my case was the mechanical controller that is fitted. The one on mine had a hysteresis of +-7.5 degrees C. This is typical for these gas / mechanical devices. The controller when set to give an internal temperature of 3 to 4 degrees would freeze overnight and rise to 11 degrees at times in the day. It was however very erratic.

I have made an electronic contoller with a pre-settable hysteresis that functions perfectly, I will be fitting this in the next few nights. That will keep the fridge to within a degree of required temperature.

The gas setting is less sensitive level of the flame is constant for a setting on the dial, it only become a problem with extreme changes in ambient.

Am revising my thinking and am considewring fitting an 12V element to power both, straight off either the car or a battery.

Thanks for the detailed post.

Andrew

Great detail in your answer!

I am now sure that you will get your fridges working just fine for you.

Let us know the results.

Have a great day.

Well over two years ago, I submitted a similar question to this site. "Can a UPS be used as a continuous inverter?" As now, there has been mixed comments. I continued my quest and have been using two UPS's for over a year and a half. I started with an APC 1400 and powered my refrigerator as well as a number of house lights with the only problem that I encountered being overload conditions when the ice dispenser/crusher was used.

Now I employ an APC 3000 (purchased on eBay for $62.00) and have added house lights, my entertainment equipment and have experienced no problems what so ever.

The only modification to the units was removal of the sonic transducer to eliminate the annoying beeping. If I could find schematics for the APC 3000 and work around the relays, I could reduce the small power drain I have encountered.

Hope this helps.

I am not surprised.

Many opinions, few facts.

Send me an email and I'll give you what I've found. Email to beatle.ex at gmail.com

Ah Nick, you make my heart happy.

"Well over two years ago, I submitted a similar question to this site. "Can a UPS be used as a continuous inverter?" As now, there has been mixed comments. I continued my quest and have been using two UPS's for over a year and a half. I started with an APC 1400 and powered my refrigerator as well as a number of house lights with the only problem that I encountered being overload conditions when the ice dispenser/crusher was used."

Are these as backups or continuos operation, any measure of the duty cycle?

"Now I employ an APC 3000 (purchased on eBay for $62.00) and have added house lights, my entertainment equipment and have experienced no problems what so ever."

Great and affordable power, well done.

"The only modification to the units was removal of the sonic transducer to eliminate the annoying beeping. If I could find schematics for the APC 3000 and work around the relays, I could reduce the small power drain I have encountered."

Did you simply unsolder the transducer? Goes right through me ... gotta kill it.

"Hope this helps."

Plenty

Nick a couple more questions if I may? I am trying to etablish the losses from 24V to the 220V side.

When I switch the UPS on, without the mains connected and only the battery connected the current draw is 180mA. When I hit the push button until it switches over to the battery power, without any load, the current draw lifts to 750mA. This appears to be the innefficiency, 750mA @ 24V = 18W. Sounds very low, right?

Next check, connect a 220 - 240V 5W energy saver bulb, rated current draw 35mA. When the light is switched on the 750mA lifts to 950mA only 200mA not the 35mA rating.

So it takes 950mA @24V = 22.8W to run a 5W bulb???

Obviously the losses are a one off and measured against a small 5W bulb the losses appear greater proportionately.

Any idea of my losses? Do I need to provide other information?

Regards

Andrew

heers

Andrew

The rated 35mA is at 240V, or 8.4VA, (about a .6PF).

The Bulb is rated 5Watt, so at 24V with 100% efficiency the current draw would be 5W/24V=208mA.

You saw an increase of 200mA, and that is very close. The background draw of 750mA didn't change. The unit is inefficient at 5W output.

A very interesting test would be the efficiency at rated output.

We are back to the age old problem of lightly loaded equipment being inefficient.

Inverters generally only run at one particular load for best efficiency, anything else (generally less) and efficiency is lost completely.

Even best efficiency is generally only around 76-80% on a well designed version. A badly designed one may even at best only be 50% efficient, less when lightly loaded!!

This is perfectly normal.

Many mobile homes with 120/240 mains equipment buy small standard wattage DC to Ac inverters, and run them at about their full rating. You can buy them in varied wattages. That way they are running at their max power = maximum efficiency.

And they buy a few 12/24 volt deep cycle batteries of large size to run things. Ideally, you can buy dual voltage fridges etc thst have built in inverters for their motors.

Thanks,

Right now I am experimenting with 2 x 7.2Ah 12V batteries. I tried a 500W globe but this is a bridge too far for the batteries and the UPS shuts down. I was hoping the batteries would hold up for me to take a quick reading but no luck.

Will try organise two bigger batteries purely to test with.

Thanks for all the help guys.

Andrew

PS: I was in the process of building a 12V to 220V 500W modified sine wave inverter. I have completed and populated the PCB. Before I continue I would be curious to establish if the 12V input could be connected directly to my car's battery? Would the alternator have any effect on the 12V?

Lots of guts in an auto battery. Fuse it or you can have a nice fire (done that).

12 v could be as high as 16volts.

Don't depend on driving if you run the inverter overnight!

Thanks for that.

I am not sure I understand your question fully or correctly about the alternator. Please expand.

Remember, a car's electrical system is almost never at 12 volts, its usually higher....while the engine is running, it will generally be around 14.4 volts.......a 12 volt battery usually is around 13 volts.......etc etc etc....

Hi

I can understand the alternator delivering 14.4V but after that surely there must be a 12V regulator somewhere feeding the rest of the car electronics from which supply can be taken?

Cheers

Andrew

There are regulators for certain sensitive equipment, but at what voltage I cannot say......

Remember, there is no logical reason to be sure that they are at 12 volts, most likely is 10 volts or less as a battery could easily sink below 12 volts, but unlikely to sink below 10 volts.....