Is this possible? Automatic transfer switch on a PC between battery/AC power ?


Old 10-10-17, 11:01 PM
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Is this possible? Automatic transfer switch on a PC between battery/AC power ?

Here is what I'm trying to do.
I want to run some PC on a bank of car batteries(powered by solar panels) and when the voltage drops(power running out of batteries) have like an automatic transfer switch that will switch on the main AC power supply for the computer.

Almost like a reverse UPS that instead of switches from mainpower to battery when it detects a power interruption, this one will switch from battery to main power when the batteries are failing.

This is an expansion of another project I was reading about " how to battery power a desktop pc"
" Pick your battery, make a good solid 12V, 5V and 3.3V regulator circuit and your set. But they have to be very good and stable regulators. I would suggest pulling apart an existing PSU. Isolate the AC side from the DC side. You'll have all the regulators and protection needed there. Double check how much DC voltage the PSU has coming from it's bridge rectifiers and then connect a battery with the same voltage."

If a car battery runs out of power, it can ruin the battery(unless deep cycle), I want to switch off the car batteries when they start dying and automatically switch on the AC power supply kind of like a automatic transfer switch/UPS in reverse.

Any ideas on how this can be done?

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Old 10-11-17, 07:06 AM
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If you can convert solar power to electricity, why do you need the batteries at all? Connect the converter directly to the PC. For example, I see solar powered emergency phones on the side of the highway.
Better yet, use a small windmill instead of solar. A friend of mine told me about someone who converted all the lights in his house to 12 Volts & used wind power.
Old 10-11-17, 09:20 AM
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Your starting point is to get the power supply built to run the computer directly off the batteries. Once that is built you go from there.

Solar powered emergency phones have batteries in the pole.... otherwise they couldn't work at night.
Old 10-11-17, 06:31 PM
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You can get a replacement power supply for your computer that runs on 12 volts.
Old 10-14-17, 09:28 AM
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All of these suggestions solve the issue of running the computer off of solar power but don't address the issue of having it switch to conventional AC power when the batteries fulltext diminishes. That's going to take some kind of a voltage (current?) sensing device to trigger a relay to switch the power source to AC.
Old 10-14-17, 01:36 PM
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Is it possible? Sure. That's what they make electrical engineers for (if it's IMpossible, that's what they made DARPA for).

Is it expensive? Absolutely.

You don't mention what your objectives are but the cheapest, lowest-fuss and IMHO most practical way to approximate what you're asking about is with a grid tie inverter.

You connect one side of the GTI to your PV cells and the other side plugs into any wall outlet in your house. It converts the solar-generated DC into AC with a frequency to match what's on your home circuit and with slightly higher voltage, and it "injects" the current into your house's electrical circuit.

It's not as glamorous as if the PC/UPS were getting exclusive use of your solar watts but it is more practical because (presuming your PV array is only sized to power your PC) it assures that none of your solar watts ever would go unused. Which otherwise could happen if the PC is in a low demand condition and the batteries all are topped off. With a GTI and a modestly-sized PV array, what your PC doesn't use, your refrigerator will.

I suspect a nominally PV-powered UPS that switches to commercial power when the batteries discharge to some strategically determined working limit would have you paying through the nose for batteries, one way or the other, because that plan guarantees you're going to have at least one discharge/charging cycle during every day with significant sunshine. Some days more than one cycle. The batteries in consumer-grade UPSs do well to last two years, commercial UPS batteries more like 5-7 years, but in both cases that's with infrequent significant discharges. In this application you're probably looking at something on the order of 20-30 times more frequent significant discharges (more in Tucson and less in Seattle). You could always use a bank of several batteries along with a shallower discharge limit, which would extend battery life by lessening the impact of each charging cycle, but then you have a substantially higher initial cost.

Just use a GTI instead and you avoid the expense and misery (and explosion hazard) of a bunch of extra batteries, a battery charging controller, a switch to control changeover to commercial power (which you probably would have to build yourself), and a battery charge level monitor to control the power switching. Plus it's a "greener" solution because nobody pollutes in the manufacture of the batteries you don't buy and electrical circuits like this are loaded with exotic materials.

Plus it's only one device. You take it out of the box, connect it to the leads from the PV cells and plug it into an outlet, Two minutes and you're done and dusted.
Old 10-17-17, 03:33 AM
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Those grid tie systems are pretty expensive too.

I saw a commercial grid tie system that was $100,000, I wonder how that compares to a battery bank.

So computers/servers naturally run on DC power when you look past the power supply which converts the AC power into DC power.
Solar panels naturally produce DC power.

So how much power is loss in the inverter process to get the DC-to-AC, then again how much power is loss in the computer power supply to go from AC-to-DC ?

The bank of batteries wired to a bunch of servers seems like maybe an interesting idea. I know Google Servers are all already tied into a battery backup which go right in the server.

I wonder why it has never been done before especially for people with server rooms.
Old 10-18-17, 08:22 AM
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Those grid tie systems are pretty expensive too.

I saw a commercial grid tie system that was $100,000, I wonder how that compares to a battery bank....
A Koenigsegg Regera lists for $1.9 million but that doesn't mean all cars cost $2 million.

The term "grid tie inverter" covers a lot of real estate. Some people have enough generating capacity that they can produce more than their home consumes, at least part of the time. And some electrical utilities will buy excess/unneeded power from their customers. In those cases they aren't using a GTI just to inject power into their home's circuit but also into the power grid. Which takes a much more robust and sophisticated device. You can get a GTI big enough to feed a PC for around a hundred bucks.

...The bank of batteries wired to a bunch of servers seems like maybe an interesting idea. I know Google Servers are all already tied into a battery backup which go right in the server....
All server rooms use UPSs. Some UPSs (known as "online" or "dual-conversion") power the servers from the batteries all the time. It's mostly the cheap, consumer-grade UPSs that only switch to battery power when the commercial supply is interrupted.

...I wonder why it has never been done before especially for people with server rooms.
I'm sure it's being done somewhere. The reason it isn't more widespread is because commercial electricity is substantially cheaper than quote-unquote renewables, and commercial power doesn't crap out every time the sun goes behind a cloud or the wind dies.
Old 11-05-17, 12:11 PM
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Honestly, you're going to have to decide on a trade-off between ease/cost and efficiency.

If you want efficiency, then you'll need to build or buy a computer power supply that runs directly off a 12V DC supply. Then you'd buy a DC transfer switch, which would monitor the battery voltage and auto-switch to a 12V DC power converter (power supply) that you'll also need to buy. This will turn your 120VAC into 12VDC. It needs to be always on, so you'll want to research something that has really low standby-current, so it's not just wasting power when it's not being used to power the computer.

If you want easy (what I'd do), I'd just use a standard PC power supply that takes 120VAC. Then I'd buy an inverter (get a true sine-wave inverter) that is rated high enough to power your PC (plus monitor and whatever else you need), and I'd also buy a transfer switch. You hook the battery-powered inverter up to the primary line of the transfer switch and the house-power up to the secondary. Pretty much all inverters have a low-power cut-off system. When your batteries get below a certain voltage, it turns off the inverter to protect your batteries.

One caveat with this, is that the transfer switch will want to switch back to the primary soon after it has power available. You could run into issues early in the morning, when the batteries are slowly charging at a rate lower than what the computer needs. The transfer switch will be on house-power and a few seconds after the batteries are charged enough to turn the inverter on, the transfer switch will flip, draining your batteries faster than they can charge which will turn the inverter back off, flipping the transfer switch back to shore power, and repeat until the sun is solid enough to charge faster than the PC uses... This won't be a major issue if the solar array is large enough.

At that point.... if your array is sufficiently large, you might as well bite the bullet and get a grid-tie inverter as suggested above.

Another potential issue of a transfer switch is that it's only rated for certain number of switches. Since it has to switch the power HOT, they wear out. If you're switching twice a day, it might last for several years. If it's switching dozens of times a day, you might only get a few months out of it.

Good luck!

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