Homemade non-inverting UPS/PSU idea

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flogge

[H]ard|Gawd
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So I've been trying to figure out why 1kw+ UPSes are so freaking expensive and I came up with an idea. I'm not sure if it's cost effective but we'll see what we can come up with.

The problem is that any stand-alone UPS converts AC power to DC, stores it in a battery and then inverts back to AC when it goes to battery power. The PSU uses AC power and then converts it back to DC. I'm trying to get around these extra conversions and make something with the benefits of a "double conversion" UPS without the severe efficiency penalty.

I found some places that sell ATX PSUs that live on 12, 24, or 48V DC power. They are very expensive for their wattage from the little that I've seen. $200 for a 300W unit. So we get one of these little guys to give us the 3.3V and 5V and so-forth. http://www.powerstream.com/DC_PC.htm

This gives us the amps we need for the various voltages, what we are lacking here is the 12V rail capacity. Wait, our battery is 12V, it's just not stable enough on the voltage to run PC components. If we had something like this: http://www.powerstream.com/DC_PC.htm to run straight off the battery into all of the other 12V loads would we get the stable and clean power we need? Would be nice if we could get about 80W units so each one could power a 6-pin PCIe connector. Parts list to make a 1kw UPS/PSU:

1kw/80amp 12-14V Power supply(solar or wind power possible)
deep cycle or SLA battery
300W 12V input PSU $200
12V DC-DC converter brick(s), preferably 75W or better
maybe some caps to stabilize the voltage a little more.

So now we have a full-time battery backup system so there is no lag from line power to battery power. Any ideas on some of the components I'm looking for?
 
Is ~$200 really that expensive?
http://www.newegg.com/Product/Product.aspx?Item=N82E16842102048
It may be worth noting that for a computer psu with active pfc, the VA rating of a UPS is the actual power that can be used by the psu (the wattages given by ups mfg's assume a power factor of 0.6).


If you really wanted to simplify the number of conversions necessary, you'd have to design your own battery management/charging system to place between the computer psu and the computer itself, as well as the necessary dc-dc converters for 12v -> 3.3V/5V.
 
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I thought about trying to use a battery UPS on the DC side of a normal 110V PSU. The problem is that you still have to switch from having AC power to battery power and you have to control all of that. With the above system you are isolating the PSU from the AC line by passing it all across a battery. So in the case of a power failure all that happens is you stop charging the battery and it begins to drain. The 12V PSU never sees a difference until the battery dies. You could probably use the circuitry from a cheap low-power UPS that senses battery failing in order to get an auto-shutdown in the event of an extended outage.
 
More and more datacenters are switching to DC-input PSUs for the individual systems with a single or a few centralized AC-DC converters. As it turns out, even without the whole UPS thing it's more efficient than having each system's PSU do its own AC-DC conversion.

Telecom equipment has been running on 48VDC for decades already. There are ready to be use PC PSUs which accept 12VDC input, BTW. Just don't expect them to be that cheap.

Alternatively you could design your own PSU to generate the 3.3V, 5V(SB), (-)12V rails. DC-DC conversion is a lot easier than AC-DC, so it's attainable even without an EE degree.
 
-48VDC (telco standard used in many datacenters) is probably your best bet. This lowers the overall current on the DC side of the equation which can get really carried away (100A continuous for this 1kw PC). Old telco float chargers for -48VDC battery banks should also be easy to come by in surplus.

So put -48VDC supplies in your PCs, get 48VDC worth of lead-acid or gel-cell batteries, and hook a -48VDC float charger up for supply.

It's not a new idea, the telcos have been doing it for a hundred years. All major network equipment and servers have -48VDC power supply options.
 
Sounds workable. I ended buying one of those 2200VA Power Sentry UPSs for $120 for my current rig but I might try something like that on a future build. If I ever get a wind or solar system set up I might integrate that also.
 
flogge said:
Sounds workable. I ended buying one of those 2200VA Power Sentry UPSs for $120 for my current rig but I might try something like that on a future build. If I ever get a wind or solar system set up I might integrate that also.
Click to expand...

The problem will be monitors and stuff that do not have -48VDC power supplies available. There are 100% duty cycle, reasonable efficiency inverters made into the 10s of kW range. These are very heavy, uncommon, and used, again, in data center or central office environments where some AC is needed in a -48VDC plant.

I considered doing a "whole home" backup system with surplus parts using this scenario. There would actually be a lot of load shedding but I was looking at enough to keep some lights, fridge/freezer, furnace fan, that sort of thing operational. Although the gear may have been free it would have been incredibly heavy and bulky. Eventually I'll get a natural gas whole home generator and transfer switch...

You get something like these:1 2

Build a 48VDC bank of batteries: 1 (or just about anything of lead-acid chemistry which charges by constant VOLTAGE, not constant CURRENT - not NiCd, NiMH, Li, LiP)

-48VDC power supplies for your computers: (having a hell of a time finding links for standard ATX)

Inverter for anything else not natively 48VDC: 1 2 3

None of this stuff will be cheap, but you might find surplus around. There's also a bunch of 24VDC stuff, so if you can only find ATX power supplies in 24VDC that might be the way to go. You won't save any money, your currents will double, your wire size will increase, and you'll still need the same number of batteries, just both in series and parallel.
 
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A number of years ago I built a total DC PSU for computer use. I had access to fresh Edison Accumulators, 12v batteries known for their 20 year + lifespan even under abusive conditions.

The Edison batteries were different in that they were 1.2 volts per cell vs. today's lead acid battery at 2.0 volts per cell. Some of these units made in 1903 are still in use today at RR crossings to control the gates and lights.

By center tapping and cell taping I was able to get close enough ATX spec so any voltage regulation needed could be done by the motherboard. The mother boards already have voltage dividers and regulation built right in. The really best part was I had over 100 AMPs on tap for every voltage. They were charged by an Edison Trickle or Float charger I ran on a timer.

If you are into green, these are perhaps the greenest batteries ever produced, no lead.

Typical of our economy the lead acid battery took over because they are so cheap you just toss them out at their end of life. Were I able to switch to wind, water, solar and run a 12v home this would be the only way to go for long term cost effectiveness.

We stopped making the Edison unit here in 1972 (figures) and the only known source today is...China. Go figure one more time.

http://en.wikipedia.org/wiki/Nickel-iron_battery#Present
 
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