UPS

Orion

I'm looking for some thoughts on a project.
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My problem is I have 2 computers and several pieces of networking/cctv equipment·I need to have powered 24/7.· Currently·I have separate 600va UPS·for each computer and the·combined network/cctv gear (3 total).· This works ok, but 1 of the ups has failed with the others coming near the end of their lives.· My idea is to get two or three car batteries and connect them to inverters.· Power a SPDT relay·with·the AC mains and wire the load on the common.· Normally closed = AC mains, normally open = Inverter output.· Inverters will run 24/7 and prebuilt chargers will keep the batteries charged.· Granted once the "core" of this system works I would like to incorporate the stamp to control the switching and issue rs232 command to the pc's when battery level is low.
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What are your thoughts on the relay? Will·a relay·switch fast enough to keep the computers from shutting down?
After hacking open the failed ups it looks like a relay served this purpose before.·
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Any recommendations for charge/monitor ICs/circuits for lead acid batteries?
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I’m sure that there will be problems using this simple means of detecting power loss and switching to backup, but this is easy to fix with the stamp.

Jim McCorison

Orion,

I do marine wiring for a living, and one of the things I deal with are inverters. I am familiar with the marine inverter units and their installation and wiring. I have no experience in alternative energy or other land based units. So within those limits, this is professional advice, but worth what you paid for it.

DISCLAIMER: The following advice is based upon my experience with marine requirements and installations. This advise may not, and probably won't meet your local building / electrical codes. Consult with an experience electrician regarding proper installation.

To really do this right:

1) You will need a full sine wave inverter. While a Modified Sine Wave (MSW) inverter will probably be ok for many pieces of equipment, some will die an early and quick death on MSW. The cheapy inverters you see at stores like Costco or your local auto store are MSW units. You will want to get a good quality full sine wave unit.

Based upon my experiences, I would recommend an inverter / charger from Xantrex. The SW2512MC www.xantrex.com/web/id/44/p/1/pt/7/product.asp would be a good choice. It will provide up to 2500 va peak continuous. You don't want to run these at their rated power all the time, so doubling the actual requirements is a good guideline.

These units are a combination charger and inverter. When there is electricity they will charge the batteries, or maintain their charge as appropriate. They will also automatically pass through the incoming AC to the outgoing feed lines. When incoming AC drops, the inverter will automatically pick up and supply the AC power. In essence, they work just like your UPSs.

2) You probably should use AGM (absorbed glass mat) batteries. These are initially more expensive than lead acid batteries, but they have numerous advantages. The do not gas, or more accurately they should not be gassed. If they gas they are for all practical purposes dead. They will only gas if you use incorrect charger settings on them. They will withstand deep discharges far better than lead acid. Each discharge beyond 50% of a lead acid is another nail in its coffin. AGMs can readily tolerate discharges to 75% without doing them harm. AGM batteries will also accept much higher charge rates than lead acids, but for you installation this will not be an issue.

3) The size of inverters you are looking at will not have simple plugs in the back of them. They will have connections for supply and feed wires. You will need to wire them into a supply circuit and into a sub panel for the consumers.

4) Quantity of batteries: You will need to do an accurate assessment of the power consumption of your current configuration. You will also need to decide the maximum amount of time that wish to provide battery stand-by for. Ten calculate what you need for batteries. Let's assume that you are currently using 1,200va and want to provide for 4 hours of stand-by time, 1,200va at 120vac translates to 95 amps at 12.6vdc. But this inverter is 90% efficient at peak, but plan for 85%. So you'll need 110 amps dc for 4 hours, or 440 amp hours. A Lifeline 4D AGM battery provides 210 amp hours. Since you don't want to plan on discharging below 50%, you'll need 880 amp hours of capacity, or, at least 4 batteries. Obviously you'll need more batteries if you want to provide more backup time.

Costs:

The inverter charger will be around $2,500.
4 Lifeline 4D AGM batteries @ $325 = $1,300
Battery boxes: make your own out of plywood and fiberglass, or by them at $100/each
DC Wiring and fuses: Around $150 - $200
AC side: You'll have to determine

When you are done you will have a system that is almost as good as a 2,500va UPS. One big thing missing will be controlled shutdown for low battery voltage.

Hopes this helps in your thinking,
Jim

Orion

Thanks for the input. I plan on getting real-time current measurements to size everything on. I like the idea of the inverter/charger combo and not discouraged by the price. Cost of the equipment needing ups far exceeds the costs you have laid down. I would hate to have my current investments ruined by a rigged setup. However the total cost would be about the same as buying a commercial ups for what I require.· As far as an electrician that’s not a problem.· I have been in the commercial field for over ten years.

What about changing this setup a bit.

PC power supplies supply 12VDC and 5VDC. The remaining equipment is low voltage dc supplied by wall wart. What if I remove the PC power supplies and handle this without using AC at all? Supply the motherboards with what they need by constructing regulated power supplies and supplying off the batteries/charger. I realize things will have to be faked out to get the pcs to fire up without a true power supply. There should be plenty of info on the internet to over come this.

Any more ideas one way or the other?
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Post Edited (Orion) : 2/25/2005 4:15:44 AM GMT

paysonbadboy

Yikes! Time to spend some money.

However, I've been using $35 "auto store" inverters on two PC type computers for quite a while even though I do understand they are "dirty" the computer power supplies have been working ok.

In my work vehicle, I use a inverter to run a computer (using a PC type since my laptop died a yr ago) and I even charge 18volt cordless drill batteries all at the same time without problem.

But I guess I would not do that on computers with IMPORTANT duties.

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http://www.paysonarizona.net/

Orion

I have used pcs on inverters before also but not for extended periods though. No problems as of yet, other than CRTs start a little funny. On this setup, the pcs have no monitors anyway. I don't know how much I can trust cheap inverters though. While pc power supplies are cheap enough if something happens to the rest of the pc hardware/data it going to be costly.

Jim McCorison

Orion,

You would need to provide tightly regulated power supplies. It could certainly be done, but I think in the long run it would be more problematical. For instance the various wall warts provide a myriad of different voltages. Providing the DC levels to satisfy these would be a bear, and then, once you have it all done, you buy that new widget that requires 16.5 vdc which doesn't match up to anything else. I think if you're doing this seriously, which you appear to be, you need to do it in a robust and supportable manner. But that's just my cut on it.

Paysonbadboy,

Yes, a large number of devices work just fine on MSW. The inverter on our boat is MSW and our navigation computer and LCD monitor work just fine on it for days on end. Our Makita chargers also work fine. However DeWalt chargers burn up and die on MSW, or at least they used to. There is really no way of telling what will work and what won't other than word of mouth, or the smoke test method. For example, the Christmas light timer we had worked great until the power went off and we rolled to inverted power mode. Poof! Also, things with clocks (less likely to be an issue in Orion's case) may or may not run properly depending on how they are designed. In general, if you are planning to provide "clean" power for a variety of devices and are not tolerant of abnormalities, a true sine wave inverter is the way to go.

Another thing to keep in mind is that inductive loads will consume more current when running off MSW than true sine wave. An example of this is the pancake compressor we use to provide air pressure for our horns. On shore power it worked just fine with a 15 amp breaker. On the inverter it popped the breaker if it ran more than about 30 seconds. Fortunately I had run 12 gauge wire up to it so I was able to swap a 20 amp breaker instead. Ran fine after that.

Jim

Orion

You may be right Jim, I haven't checked yet to see what a range of voltages I will require. Maybe I will have to call the manufactures of the pcs and see what they have to say about MSW. For everyone info this setup is in my home, so support / electrical inspections are "flexible". Cost isn't a huge problem, however I would rather spend the money on more computing hardware than the ups.

Gadgetman

Just wondering...

Those old UPSes you have...

What's wrong with them?
Usually when these fail it's the batteries that is rotten, but that can usually be changed.

On another note, are they inline or offline models?
(The inline models always feed from the batteries, while the offline model passes the AC through until it detects a drop, then it switches the inverter online.)
----
Oops....
didn't read the first post well enough...

I wouldn't trust an UPS with a relay further than I can throw it...
Relays are SLOW to switch, and while the caps in the PSU might cover for it for that brief moment, the power surge to make up for that again.... isn't nice...

For that same reason, offline UPS will ever show up at any of the server rooms where I rule...
(Server PSU's are more fragile than you would think.... )

Post Edited (Gadgetman) : 2/25/2005 7:19:43 PM GMT

Jim McCorison

Orion,

The PC power supplies most likely won't be an issue, unless your running boxes built as cheaply as they possibly can. While I certainly won't say that all PC will run off MSW, I've yet to hear of anybody having one that didn't. You're more likely to run into trouble on the more cheaply built wall warts.

In my original reply, I didn't stress the DC wiring side of this. If you go ahead with this, follow the manufacturer's recommendations to the letter. You will need 4/0 cable, don't schrimp. You will need properly made up connections. If you don't have the proper crimping tools and can't borrow one, prepare all the cables and bring them to somebody who can crimp them properly. The installation instructions will require a T class fuse. Don't get cheap in it. The batteries you will be using can discharge a heck of a lot of amps into a short. The critical specification on these is amps of interupt capacity (AIC). A fuse or breaker with insufficient AIC can actually weld themselves in a shorted condition. The AIC of a T class fuse is 20,000 amps. You'll pay around $50 for the fuse holder and $30 for the fuses.

When installing, the number one installation error by DIY'ers, and surprisingly enough professionals as well, is to omit the third heavy grounding cable from the chassis of the inverter to the ground terminal of the battery bank. This cable should be the same size, or at worst one size smaller (i.e. 2/0), than the main positive and neutral cables. On the surface it doesn't make sense. But it's reason for existance is if the inverter shorts internally to the grounded or case components, you need a high amperage ground line back to the battery bank to create a high amperage short and blow the fuse.

Anyway, if you persue this further and need more advice, please feel free to PM me.

Jim

Orion

Thanks both of you for your insight.