Double Conversion UPS, Part I

Most people are familiar with the idea of an Uninterruptible Power Supply (UPS), such as used to help keep a computer running throughout power outages. Intended for only seconds to minutes of use, long enough to get past a short power glitch, or to give the user time to save files and shut down before power fails completely, a home or office UPS is typically small and inexpensive. Plus, a typical UPS will be a square wave or modified sine wave (essentially the same thing with gaps), which is fine for a computer, but we would prefer a pure sine wave output. See our previous ground solar inverter article for more details about these different waveforms.

On the other end of the spectrum, commercial or industrial uninterruptible power supplies will use a huge supercapacitor, plus a backup generator. The idea is that the supercap will supply power for the short time it takes the generator to come online, and then the generator takes the load from there. These systems, intended to keep critical facilities such as hospitals operational, tend to be expensive, and beyond the needs of many people who need more backup power than a small computer UPS, such as to operate a freezer or refrigerator.

Slightly below those very expensive backup systems are alternate energy inverter-charger combinations. Rated into the kilowatts, these systems are also more expensive than what we’ve put together here. By coupling an inexpensive inverter, a small deep-cycle lead-acid battery array, and an inexpensive AC charger, we can create an inexpensive solution. The AC charger keeps the batteries topped off and supplies power while AC is available. Meanwhile, the inverter supplies power to the load from the battery array 24/7, and doesn’t care whether AC power is interrupted. But, if AC power is interrupted, the battery array, as large as you care to make it, can supply the load for hours or days, unlike a computer’s UPS.

An advantage of this system, known as a double conversion UPS, is that if power goes out, the AC charger can be swapped out for a solar charger, a generator, or any other charging source you wish. Planned for ahead of time, switching out charging sources can be seamless and easy; the load will never see the difference. Plus, you have hours to get the work done instead of a mad scramble.

An example of such a system, which we’ve been running for about four months as a longevity test, is shown below:

In the center are our two Duracell 29HM batteries from our ground solar work, purchased from Sam’s Club, and arranged in series as a 24 volt array. Our regular readers may recall that we paid a total of about $200 for both of these batteries.

Upper right is the Microsolar 24v, 1000 W pure sine inverter, also used during our ground solar work (see the bottom of this post for links to those articles), as described in the inverter article from that series. You may recall that this inverter turned out to only be good for about 600 to 700 watts, but has a really nice sine wave output. We paid $189 for this inverter at Amazon, but it looks like it is available for about $30 less at this link. The power that comes out of this system is cleaner than utility power, so we have no concern about using this with inductive loads.

To the left is a new item for this setup, a 24 volt Samlex Power AC charger, capable of a 25 amp output. Coincidentally, this is a 600 watt output, about the same as what the inverter can realistically produce. A closeup of this charger is shown below:

As you can see, this is a no-frills device, with analog meters instead of LED or LCD display. There is a certain charm and sense of robustness about this option. There is another 25 amp Samlex charger in a blue case with no meters for about the same price ($329 as this is written), but we wanted some visible feedback about what was going on with the system. In the above picture, the system is using three amps while running our 70 watt freezer (the white appliance to the right in the first photo).

Tying it all together is 6 AWG THHN cable we purchased from Lowe’s. In this case, we are using all red, with the negatives marked with black electrical tape near the ends. We highly recommend black and red for a more permanent installation. The cables are terminated with 5/16″ crimp lugs, described in detail along with the crimper in this post. Completing the system is a DC breaker, shown to the upper left of the charger in the first photo as a tape-protected mess. This item is a 150-amp automotive Cooper Bussmann CB185-150 breaker which can handle up to 42 volts, perfect for a 24 volt system with above 28 volt charging, and costs around $25. We’ve also evaluated some slightly less expensive alternatives, and they all seem fine.

So, what if AC utility power fails and we have to switch in a solar charger or generator? Simple. Just trip the DC breaker, isolating the charger from the batteries. Then, remove the cabling from the AC charger, route it to a solar charge controller, and reset the breaker. Or, more easily, plug the AC charger directly into a generator’s 110 outlet. The load will continue on the inverter for as long as the batteries last while you get this done.

This kind of system has another nice bonus, particularly when powering sensitive inductive loads. Theoretically, the AC charger, provided it has solid state rectification at the front end, can swallow many low-quality generator outputs that might damage a motor, such as a freezer’s compressor. The pure sine inverter then generates clean power which these appliances require. As a result, this system not only provides continuous operation, it can filter out power glitches from a variety of input sources, including small, inexpensive generators, increasing your backup options.

As mentioned previously, this system has been in continuous 24/7 use for about four months. This week, we decided to kill power and take some measurements. The next article in this series will discuss those measurements, and update our readers on some lessons-learned about battery life and discharge characteristics. We’ll also talk about electronic failure modes and what this means for 24/7 system usage. In future articles we’ll also perform some live swaps to other power sources, and discuss a much larger array that has also been running here for months.

Bottom line, this system, as shown, and its larger cousin, has been performing admirably for these four months, and we are comfortable keeping it in full-time service.


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3 Comments on "Double Conversion UPS, Part I"

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4 years 1 month ago
We used a very similar setup recently in order to maintain communications and control at a remote location at the top of a hydro power penstock. Power to the tunnel location runs along the exposed penstock that is routed partially up a cliff face. In case of penstock breach, it’s necessary to close a supply valve at the top of the penstock, but without power, it’s impossible to gain control of the valve. We use a small PLC that runs on 24VDC, running from two 12V lead-acid batteries in series which are kept in constant charge from a 120VAC charger.… Read more »
Mark Dunning
Mark Dunning
4 years 1 month ago

one consideration might be cooling fans in the DC > AC inverter
does it have one?
What’s the projected lifespan of the moving bits?
You have achieved 4 months lifespan so far, be interested to know how long this lasts…..

(ed. Great questions! Yes, there are cooling fans in all the chargers and inverters we’ve tested so far. Reliability is in the queue for the next article in this series.)

4 years 1 month ago
I’m currently planning out a system similar to this to power a fair bit of computer equipment that I run at home. The one issue I haven’t found a good solution for so far is how to monitor the batteries when the AC power is out. I’d like to be able to have some mechanism to automatically watch the power level of the batteries and when they start to run low I can have the systems begin to shut themselves down in an orderly fashion. Do you have any suggestions for how to accomplish this? I believe that with my… Read more »