Custom long run time/capacity UPS solution?

[Red Squirrel]

I've been wanting to setup a backup solution that can last for as long as I can by adding more batteries. For my budget I'm looking for around 3-5 hours or so (probably like 2 100ah batteries, my load is less than 500w) but I want the ability to expand in the future.

I've looked at inverter-chargers and it's hard to find a decently priced one that flips over fast enough (<10ms) and produces true sine wave. Finding a decent used APC UPS is also hard and expensive because of shipping. Lot of the home geared UPSes have no fans and arn't meant to run for an extended time so I don't want to use those, it has to be a commercial grade unit. They tend to cost close to 1k even used because of shipping. People on ebay list them under 200 bucks or so than jack up the shipping to like 900 bucks.

I've thought of a rectifier/inverter setup but it gets more complicated, having to know what to set the float voltage to etc... and battery life is probably not as good.

Then I started thinking, why not run everything off a power supply that powers an inverter, and have a relay that swaps the inverter's source to battery when the power goes out? This is what I came up with:





So basically, it's a PSU powering an inverter and the inverter powering the loads. This is not really as efficient as a typical UPS, but it does have the benefit of more or less isolating the equipment from the mains as well so probably less chance of lightning damage to equipment if the AC line is hit. I would still use surge protectors though.

Now the battery would be hooked up to a trickle charger and be isolated from the circuit by a relay that is turned off as long as there is power applied to it. It could be direct, or a 12v adapter plugged into the mains. Either way, the idea is that if the mains goes out, that relay turns on, switching the battery power over to the inverter. This would require a very fast high amperage relay but a benefit here is that the psu may have a bit of juice left over to carry over the load. So I'm thinking if the relay is like <10ms or less it should be ok. Could always add a capacitor bank.

The nice thing with this setup would be the ability to add multiple psus and maybe even multiple inverters for redundancy. Could also have stuff running directly off the DC side. Think this idea could work? It would be way cheaper and more versatile than trying to find a proper inverter-charger. Decent qualtiy inverters are fairly cheap, even the 3000w ones, so this idea is very expandable. Then the only thing I really need is the small accessories like fuses and the relay, and a cheap charger, it does not even need to be able to handle the full load or charge the batteries at turbo speeds. It would be trickle charging most of the time.

Could also expand this to a 48v setup if I ever go bigger. For now I'd stick to 1000w or so capacity though. My current load is around 500w.

 

[tynopik]

http://www.dansdata.com/upsupgrade.htm
http://revision3.com/systm/hackedups
http://forums.anandtech.com/showthread.php?t=2026013

Lot of the home geared UPSes have no fans and arn't meant to run for an extended time so I don't want to use those

this is addressed in one of the links, but if you use one that's rated much higher than your anticipated load, it shouldn't be a problem

i imagine something like this (which includes a fan) should be fine

 

[Red Squirrel]

I'm playing around with a UPS now, but this was more a different twist to what I had posted before. Seems this idea would be more scalable if it works. Ever try finding a 5000watt UPS? What about a 5000watt inverter? The 5000w inverter (even pure sine) will cost 1/10 of the UPS.

 

[Red Squirrel]

I may take a different approach. Think this could work?



I just need a relay that is very fast, like <10ms. Only danger is every single little flicker will make it flip back and forth. Then again, guess that's what I want.

I found a 3 way relay in ironicly, an old UPS, which I'll test with. I will most likely need a solid state relay for what I want though. Do they make 3 way solid state relays? Basically if there's power it contacts to one side, and if there's no power it contacts to the other side.

 

[Rifter]

Just buy a generator, seriously. The whisper quit ones they have out now are less than 65db under load, you could run it inside no problems if you vent the exhaust.

 

[Red Squirrel]

That's an investment for later. I want to get one of those big whole house ones from Generac. But for now I want to stick with batteries.

I was also testing out switching on the AC side, and my idea wont work unless I can find a DPST two way relay that is ultra fast. The neutral of the inverter is not synced with ground so I can't pass through the neutral like in my diagram. I think I'll stick to switching at the DC side as it's more forgiving as the psu will have some power left while the switch process occurs.

Idealy I'd like to just skip the relay stuff and have a 100a charger constantly charging while the inverter runs constantly off batteries, but finding a 100a charger is like trying to find Atlantis.

 

[Red Squirrel]

I gave in, and bought this:

http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=251062015408

I was trying to save money by rigging something myself but it would have ended up costing just as much and potentially not be as reliable. Finding the parts was not really easy either without guessing games especially when I found out a single basic 3 way relay would not work for AC due to the neutrals being different potential on the inverter. At least this product is made for what I want and has 16.6ms transfer time which should be good enough. I've seen some computer power supplies with a hold time of 30ms so worse case scenario if my servers can't handle it I'll just buy a new psu for it and ensure it has the hold time documented and that it's higher. I wanted 1kw but have to compromise, my current load is 350 so this is still double of what I currently use.

 

[hoorah]

Just buy a generator, seriously. The whisper quit ones they have out now are less than 65db under load, you could run it inside no problems if you vent the exhaust.

Running a generator indoors under any conditions is a terrible idea.

 

[Red Squirrel]

Running a generator indoors under any conditions is a terrible idea.

Was thinking that too, but I suppose if you can locate the intake and exhaust and isolate them to two pipes going outside it could work. I don't know if it would be to code though.

 

[Rifter]

Running a generator indoors under any conditions is a terrible idea.

The honda whisper series can be run indoors as long as you vent the exhaust, we have a 6500w unit at work for our backup generator and its located inside the electrical room upstairs and doesnt cause any issues being run inside. Have had it running for 12 hours at a time no issues.

 

[Cholo1313]

Hi Red Squirrel have you project works successfully? planning to follow your project to connect battery on my modem so that no interruption on internet.

 

[Cholo1313]

Was thinking that too, but I suppose if you can locate the intake and exhaust and isolate them to two pipes going outside it could work. I don't know if it would be to code though.

Hi Red Squirrel have you project works successfully? planning to follow your project to connect battery on my modem so that no interruption on internet.

 

[Red Squirrel]

I ended up going with a Trippe Lite inverter-charger. It's essentially a UPS, but with connections to put your own batteries. It was cheaper than buying a used UPS on Ebay, due to the shipping cost of used UPSes. (some sellers were asking like $600-$800 for shipping alone).

So basically what I got is a battery bank split up in two for redundancy (two separate switches and two separate fuses so I can isolate them for maintenance) and then the main PDU is plugged into that UPS.

Older pics but this is basically the setup, it's just not fully complete in some of the pics:




The amp meter portion of that is useless - the numbers jump all over the place, measuring current off a shunt is a very tricky thing to do due to picking up noise after the very tiny voltage change is amplified to ADC levels. I need to revisit it at some point.


There's two batteries per shelf. idealy I'd like to put 4 per shelf but I would need to make some custom plastic trays so I can fit them all properly. Been toying with even moving the batteries to a separate room in a battery box that is vented as it's own system. Basically could get away with small ABS pipes and a small HRV unit to try to recapture the heat so they don't freeze. Right now my whole setup is just free air vented, but I do plan to enclose the walls eventually and make it an actual room.

The DC side is kind of ugly, I want to redo it one day. Been toying with going solar too, so I'll probably do all of this as a single project. Would be 48vdc dual conversion with solar added to the mix.






Overall view of racks:

 

[Cholo1313]

Good job there looks like you have 2 banks which lasts longer what you think about this simple circuit? Basic UPS do you think might work? worried about the battery would overcharge and not being used by a battery tender. http://www.equestionanswers.com/circuits/ups.php

 

[Red Squirrel]

That circuit is a half decent representation of a basic low capacity DC only UPS but it's not good for a 120v application. Now you could have an inverter at the end of that then it would be a sorta dual conversion UPS but there is still a switch over time.

Basically how a typical UPS works is that it just sends the AC directly to the loads and has circuitry to detect that the AC is present inside the UPS. It also has circuitry to power a trickle charger for the battery. 13.5v per 12v battery is typical. Lot of them are 24v, some are 48v. So as long as AC is present the power is just sent straight to the output... well there is some surge protection and stuff too but we'll forget that for now.

Now if power goes out, the circuitry will detect that and flip some relays so that the output is now connected to the output of the inverter, which is connected to the batteries. Of course the issue with this style UPS is there is a short delay (10-16ms) between switch over. I've seen some computers not like that and shut down anyway.

Personally if building a UPS what I would do is have the power come in, bridge rectified and filtered to a DC voltage at the line level. Then have an oscillator turn it on/off very fast like 100khz with some beefy mosfets. That will then allow to send it through a small transformer (vs the big one you'd need if you kept it 60hz). This provides isolation and ability to step the voltage down to various windings. I'd step it down to like 80. The output of the transformer would then go to a SMPS circuit that steps it down to say, 54 volts regulated. Regulated is important here. That is the float voltage for a 48v battery and critical to not over/under charge it. Then you have a 48v battery connected directly to that output, and then you have a 48v to 120vac inverter also hooked directly. This is basically a dual conversion UPS and has no switch time or relays to worry about, but you need parts that are beefy enough to handle whatever load you have. The reason why consumer ones arn't like this is that it's cheaper to make them standby. The advantage to this setup though is it allows you to build redundancy into the system. This is how telcos do it. They call the SMPSes "rectifiers" which is basically pluggable AC-DC converters. If any fail the others just keep going. In telecom the equipment just runs straight off DC but there are some inverters too and they'll be redundant as well. For a home setup I would just have multiple inverters for various circuits. Anything that is super important can just have dual PSUs and plug into two separate inverters.

I'm no EE so take any of this with a grain of salt but should provide you some half decent theory. Read up on SMPS design to go more in depth. (been reading up a lot on that myself but far from an expert)