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Tricky Question Regarding Ups System


jbrain

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I'm in need of UPS system, and notice that a 1500 VA 900 W will set me back around 15.000 Bht.

For that money I get a line interactive, modified sinewave with back up of around 15 - 20 minutes.

I understand that it means that it can deliver around 900 Watt for around 15 - 20 minutes, right?

The system uses 2 x 7Ah batteries so 14 Ah in total.

Then I consider that for about 9000 Bht I can purchase a half decent 12V 2000 W inverter with charger and ups function.

Add to that a deep cycle lead acid battery 12 V 200 Ah/20 at 6500 Bht and I reach also 15.000 Bht total.

If a 14 Ah UPS is capable of carrying a 900 W load for 15 minutes, I assume that a 200 Ah battery will carry the same load for 210 minutes or 3.5 Hours, right?

Then I start reading on the web about inverters and how to convert Amps and Voltages and Watts etc., and discover here

http://www.batteryweb.com/pdf/inverter_battery_sizing_faq.pdf

and here

http://www.edaboard.com/thread247423.html#post1058598

http://www.edaboard.com/thread247423.html#post1058630

that a 200 Ah battery would last me also only a few minutes with a 1000 W load.

Is there something I misread or what am I missing ?

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Assuming the output of your 2 batteries is 12Volt the your 14AH batteries can provide (MAXIMUM) 14*12 =168 watts over 1 hour, or another way your 900W load will only last 168/900*60 = 11 Minutes. Thus the claims of the UPS supplier do not match up. If the 12V batteries are serial and not parallel then you will get 24V but only at 7Ah so the total capacity will be the same 168W/hour

NOTE the above sums quote the maximum possible.

You also have to take in to account the efficiency of the inverter (assume 90%),

The fact that the computer power is rated in KVA and not watts so another 80% (Link)

and finally the quoted amp/hour capacity of the batteries will decrease as the load increases.

Putting 900W on to the UPS you mention will probably only give you about 5 minutes.

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Assuming the output of your 2 batteries is 12Volt the your 14AH batteries can provide (MAXIMUM) 14*12 =168 watts over 1 hour, or another way your 900W load will only last 168/900*60 = 11 Minutes. Thus the claims of the UPS supplier do not match up. If the 12V batteries are serial and not parallel then you will get 24V but only at 7Ah so the total capacity will be the same 168W/hour

NOTE the above sums quote the maximum possible.

You also have to take in to account the efficiency of the inverter (assume 90%),

The fact that the computer power is rated in KVA and not watts so another 80% (Link)

and finally the quoted amp/hour capacity of the batteries will decrease as the load increases.

Putting 900W on to the UPS you mention will probably only give you about 5 minutes.

Thanks for your response,I understand that the UPS supplier's spec might be overrated.

The real intend of my thread however was that if a 14 Ah UPS can deliver 900W for a certain amount of time,then a 200Ah deep cycle battery should be able to deliver the same power 14 times longer. e.g. more than 2 hours taking your calculation in consideration

The links provided in the OP however claim the opposite.

I recall that Moderator Crossy had a thread on here where he has 240 Ah ( 6 x 40 Ah ) in sealed lead acid batteries attached to a 2000W inverter and that he can survive a night on it if neccessary.And that aren't even deep cycle batteries.

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The main purpose is to have a 24/7 server, a home computer including modem/router,a security system including cctv, PABX system attached to it.These all together will consume close to 900W I suppose.

There are also some internal lights attached,but these can always be switched of if needed.

As the power may fail in these areas for several hours in line, those 900 W should be covered for a few hours.

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Get enough back up to hold your system up until an auto start gen. can warm up and take the load. 5 minutes. or longer if you want a manual start up system. This means you can keep the beer cold too.

Edited by RKASA
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Point is that you're not always home when the power fails, or is enforced to fail e.g. If some burglars cut the power from my transformer to the house.

If the power is down I don't have a server and router/ modem, hence no cctv or security system.

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However I thank you for keeping this thread in the attention, I think we're getting off topic as I have no intent of installing a generator.

The OP was why a an inverter with a 200 Ah deep cycle battery can't keep up a 900W power much longer than a UPS with 14 Ah batteries installed.

I would have hoped that Crossy had joined in , maybe someone can wake him biggrin.png , as I have no doubts that when he wrote somewhere on this forum that he keeps his house powered up all night on a similar system, he was talking about more than a single bulb usage.

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I would love to hear personal experiences creating home UPS systems with deep cycle or ordinary car batteries.

Please use Watt hour or kWh as stored energy unit as Ah does not mean anything if the voltage of the batteries is not known.

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Nothing to stop you attaching a car battery to a UPS.

Then it can last a looooooong time.

Not so according to the links I provided in the OP, a UPS is basically an inverter with a faster switch time.

Most UPS units will also no have enough charging power to charge a car battery.

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Allright I have no idea about Watt hours as it are all layman's terms for me, all I know is that I would like to keep approx 900 W power alive for a few hours using a 12v 2000w inverter using a battery EB 160 with specs described in attached pdf.

Energy requirements:

2 hours * 900W = 1.8kWh

Battery capasity: 160Ah/12V = 1.920kWh

Battery discharge (max current): 33.5A/12V = 402W

In this case the Amp-hour is a valid point.. so I have to admit my earlier mistake :)

You'll need at least 3 of these batteries to get 1206W power out of the batteries as your power requirement was 900W. 2 batteries would only give 804W in theory.

This would give you 5760kWh which on 900W consumption lasts for 6.5 hours.

Now if this deep cycle battery would be designed to be discharge to 50% of it's capacity, you'd get a bit more than 3 hours of continuous power. Let's assume that the charge controller, cables etc reduce 20% of that, you'll still be on the few hours limit.

I was too lazy to check all the details, so I would be happy if someone wiser than I am would check the calculations :)

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Ok, Ok I'm here smile.png

Yes we do have a big inverter/UPS and 6x40AH batteries, but we won't survive all night on full load. We were managing 12 hours or so during the floods running a fan on low and firing up the LED lights to use the loo.

The batteries are getting a bit tired (the heavy use during the flood finished them off, really need to get new ones) so we are currently good for about an hour at 800W (our standing technology load), but it does give enough time to go and crank up the genset.

Most consumer UPS's will either supply their rated load (VA) OR they will maintain the stated run-time (minutes) they will not do both together. If you look at the manufacturers data it should give run-time at a particular load.

Adding massively bigger batteries will indeed extend the available run-time, but be aware that the built in charger may not be able to charge the bigger batteries in a reasonable time (or even at all). Additionally, the UPS electronics is designed to a price and may overheat if run for longer than the design period.

To our OP, 14AH @ 12V is about 168WH, so at 800W you'll get a maximum of 0.2 hours, probably more like 0.1 hours (6 minutes). Replacing the standard batteries with a 200AH @ 12V is 2400WH, at 800W that's a maximum of about 3 hours, a realistic run time of 1.5 hours without destroying the batteries.

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I prefer to give every device his own UPS, so one for the server, one for the CCTV, one for the PC modem and router and one for the PABX.

If you give every device his own UPS you can calculate how long do you want each device to run without electricity.

If you have one UPS for everything it needs to be big (expensive) and you create a 'single point of failure'!

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I will add to the above, that my desktop PC has it's own UPS that shuts it down gracefully when the batteries get low.

The big beast it aimed at keeping the lights on, the fish tanks bubbling and the TVs viewing during brief power failures although it does power the home theatre PC.

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Ok, Ok I'm here smile.png

Yes we do have a big inverter/UPS and 6x40AH batteries, but we won't survive all night on full load. We were managing 12 hours or so during the floods running a fan on low and firing up the LED lights to use the loo.

The batteries are getting a bit tired (the heavy use during the flood finished them off, really need to get new ones) so we are currently good for about an hour at 800W (our standing technology load), but it does give enough time to go and crank up the genset.

Most consumer UPS's will either supply their rated load (VA) OR they will maintain the stated run-time (minutes) they will not do both together. If you look at the manufacturers data it should give run-time at a particular load.

Adding massively bigger batteries will indeed extend the available run-time, but be aware that the built in charger may not be able to charge the bigger batteries in a reasonable time (or even at all). Additionally, the UPS electronics is designed to a price and may overheat if run for longer than the design period.

To our OP, 14AH @ 12V is about 168WH, so at 800W you'll get a maximum of 0.2 hours, probably more like 0.1 hours (6 minutes). Replacing the standard batteries with a 200AH @ 12V is 2400WH, at 800W that's a maximum of about 3 hours, a realistic run time of 1.5 hours without destroying the batteries.

Crossy, thanks for joining in and explaining the situation as i seemingly overrated it with my " all night long ".

Now in your last line you say, adding 200Ah @ 12V batteries to the UPS it would last me 800W for 3 hours, as they are deep cycle I guess they are difficult to destroy.

So I take it that using an inverter instead of the UPS would bare the same results, is that correct?

So my question is why the links in the OP claim that I would need at least 427 Ah to achieve that, as it seems that the formula's they use to calculate are correct.

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The issue is that you never can get all the energy out of a lead-acid battery, if you work on about 1/2 the rated capacity then you're not far off even with deep-cycle batts.

You'll get about the same results from a unit like mine which is a UPS with external batteries.

To be honest, if you need more than a couple of hours a baby genset is cheaper in the long run.

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The issue is that you never can get all the energy out of a lead-acid battery, if you work on about 1/2 the rated capacity then you're not far off even with deep-cycle batts.

You'll get about the same results from a unit like mine which is a UPS with external batteries.

To be honest, if you need more than a couple of hours a baby genset is cheaper in the long run.

Thanks again Crossy.

My main concern is that as stated in the links in the OP, that a 200Ah battery can only deliver the needed 66 Amp to provide a 800 Watt load for a very limited time, they talk about minutes, as such a battery is only supposed to deliver 10 Amp for 20 hours.What is your experience with that?

Is there a way to start up a genset automatically when not at home without losing PC power, maybe in combination with a small UPS unit.

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A consumer UPS has been designed to keep the attached device up and running for a 'few minutes' so you can shut it down properly.

It's not designed to operate for hours and hours, then you need to install a genset.

Agree, but an inverter should be able to hold power for as long as you can provide enough battery power.

So I try to find out how much battery power I would need to keep an 800W load alive for say 3 hours.

Theoretically a 200Ah should suffice, but according to the links in the OP, it would last only for minutes in real life.

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In a perfect world 200Ah (12V) at 800W would last 3 hours, but in reality you will get less than half that. So the estimate of 400Ah is about right.

You're not helped by the fact that as you increase the discharge rate the available capacity goes down sad.png

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You can try this calculator.

http://powerstuff.co...ize-calculator/

That's indeed a handy calculator, but my question still stands.

A 200 Ah batterry will be able according to the battery spec from the manufactures to deliver 200 Ah/20 = 10 A

Or 160 Ah/5 = 32 A

To provide 800W I would need 66A. so how long would the battery be able to deliver that load.

Then to add to the discussion, if I use 4 x 50Ah/5 batteries in serie in combination with a 48V inverter, I would need only to provide 16 A to power the same 800W.

Would that mean that with this kind of setup my batteries would last much longer?

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I have a UPS rated at 600W for my home theatre system.

With a 120Ah battery it lasts about 3 hours.

Allright, but how much load you put on it during those 3 hours.

I still try to figure out, and I hope someone can answer that, if a setup of the same 200 Ah batteries but split over 2x100Ah or 4x50Ah batteries in series so I can use a 24V or 48V inverter will give advantages in time the load of 800W can be provided.

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You can try this calculator.

http://powerstuff.co...ize-calculator/

I think I found a calculator that answers all my questions.

While continiously searching the web regarding this subject for the past 6-7 hours I was finally confronted with the Peukert law.

Something I had never heard about, and probably most normal people haven't heard about.

http://www.batterystuff.com/kb/tools/peukert-s-law-a-nerds-attempt-to-explain-battery-capacity.html

In short, this law determines how long a lead acid battery can deliver a specific load.

Lucky for this dumb they even had a calculator, so one doesn't have to combine all the formula's

http://www.batterystuff.com/kb/tools/calculator-for-load-specific-run-time.html

Anyway, thanks all for your contributions, but I guess this answers all my questions.

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