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Old 06-09-2015, 11:15 PM   #1
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using a UPS as a power inverter on a food truck

It's me again. I may have mentioned this in another thread. I ran across the idea of using a discarded UPS as a power inverter. The idea is that people often opt to replace a UPS rather than incurring the cost of new batteries. You use a car battery connected to the alternator on the vehicle with a circuit in between to keep from draining the battery under the hood.

Quoting the poster on another forum:
"We use a second 12v battery with a Schottky battery isolator hooked to our alternator, this allows the truck system to charge the second battery without discharging the truck engine battery. We then take an old UPS and use it as the inverter to run a portion of the electrics and 12v led lighting.
You can google ups as inverter to get some info on how to use the ups as an inverter. The isolator diodes are readily available, used in a lot of big car stereo setups.
This seems like overkill, but has proved handy to keep things running and lighting available during setup and changeovers, and its really not expensive, the UPS was free, the schottky diode and heat sink about $40 and a second 12V truck battery." (Generator/Inverter, staying powered up? Food Truck)

I wanted to run this by you good people to see what you think. I plan to experiment with this soon if I can find an old UPS I think I still have or if my brother finds one at his work. I am looking on Craigslist as well. I will keep everyone updated as this progresses. I appreciate your insight.
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Old 06-10-2015, 12:24 AM   #2
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Please keep us posted.

Nat
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Old 06-10-2015, 12:26 AM   #3
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I have run multiple computers for many hours using this method, run off a deep cycle marine battery, very impressed with the results. Now with solar so cheap, I would entertain this again.
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Old 06-10-2015, 04:47 PM   #4
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I did manage to find my UPS unit and the battery inside is indeed dead. To try this out before buying an additional battery, I connected the bus battery under the hood to the terminals that the UPS battery was using. I plugged the machine to one of the battery backup receptacles and it did not turn on. The UPS I was using is only rated at 230W. Does that make a difference if I am using an external battery or a battery bank? Maybe the 230W is a limitation of the circuitry and not a limitation of the battery that is installed at the factory. Of that is the case, I will need a 1000W - 2000W minimum UPS. Not sure if one battery in addition to the battery under the hood would be enough. I am figuring 700W with some headroom for the ice shaver and not sure how much for the chest freezer and cash register along with any fans.
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Old 06-10-2015, 09:31 PM   #5
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Quote:
Originally Posted by The SnoBall Effect View Post
. . . Maybe the 230W is a limitation of the circuitry and not a limitation of the battery that is installed at the factory. . .
That is almost always the case with power electronics. The number and size of transistors or similar devices, the connecting wires, etc all affect how much current the electronics can pass. By "right sizing" the electronics, manufacturers can keep costs down, and market share or profits up.
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Old 06-11-2015, 01:42 AM   #6
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Be aware that many if not most UPSes put out modified sine wave which is quite unkind to motors such as your fans and chest freezer. Further, cheaper UPSes will literally cook themselves at duty cycles outside of their original range.
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Old 06-11-2015, 08:14 AM   #7
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Be aware that many if not most UPSes put out modified sine wave which is quite unkind to motors such as your fans and chest freezer.
Never really understood the sine wave thing, can you explain that? Why is it unkind?
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Old 06-11-2015, 08:36 AM   #8
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A modified sine wave is what's called a square wave, it plateaus on each side of zero, and also "rests" before continuing. A true/pure sine wave is a continuous alternating voltage (positive and negative) wave. Digital chipsets read the voltage and base their time on when the voltage reaches 0.

You can see the differences here-


And this article from Xantrex goes into it further, in easy to understand language.
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Old 06-11-2015, 08:41 AM   #9
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And this article from Xantrex goes into it further, in easy to understand language.
meh, the article does not come close to answering my question, but does say MSW is ok for motors?
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Old 06-11-2015, 11:04 AM   #10
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I always understood it as anything other than pure sine wave is dirty electricity and it will prematurely age some devices, some devices won't even start.
UPS's are designed to sit virtually unused for years so if you start using one constantly it may not last long, same concept as "duty cycle" mentioned above
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