UPS Batteries calculation

[faresos]

I'm working on a data center project, the requirment to have 500KVA @480V UPS to back up the center from power outage. My questions are:
1. what method of calculation to be used to determine the number of batteries that will be needed to provide 8 minuits of power?

2.My understanding we will be using 240 cell @ 2V. Is this means each battery will have 240 cell? 240V X 2 = 480V which means they are all connected in Parallel

3. How we determine the capacity of the largest battery to be charged in amps-hours?

4. How we determine the maximum obtainable amps from the charger?

....This is all new to me, any hint to do the calc would be very appreciated

Thank you,

Sam

 

[zbang]

I'm working on a data center project, the requirment to have 500KVA @480V UPS to back up the center from power outage. My questions are:
1. what method of calculation to be used to determine the number of batteries that will be needed to provide 8 minuits of power?

Is that really 500KVA or 500KW? (Just checking)
Start by assuming maybe 90% conversion effeciency of the UPS, apply that to the projected load, so for 500KVA output, assume at least 550kva input. At 480vdc, that would be 1150a (approx). Since 8 minutes is about 15% of an hour, theoretically you only need 200 AH. On the other hand, most 200AH batteries won't discharge at that rate... and survive. (If my math is right.)

2.My understanding we will be using 240 cell @ 2V. Is this means each battery will have 240 cell? 240V X 2 = 480V which means they are all connected in Parallel

Series- 240 cells * 2vdc/cell. More likely, it'll be multiple serial/parallel stacks.

3. How we determine the capacity of the largest battery to be charged in amps-hours?

4. How we determine the maximum obtainable amps from the charger?

You don't?

When all is said and done, sizing of UPSs is usually done by the manufacturers. You tell them what your loads and times are, and they'll engineer a package of UPS & batteries for that usage. At that power level, it's a custom job. They'll have info on their web sites. Try
http://www.liebert.com/
http://www.mgeups.com/
or even http://www.apcc.com/

Don't forget provisions to cool the UPS. Letting the HVAC fall during genset startup is one thing, letting it fall for 10 minutes is another.

 

[petersonra]

I'm working on a data center project, the requirment to have 500KVA @480V UPS to back up the center from power outage. My questions are:
1. what method of calculation to be used to determine the number of batteries that will be needed to provide 8 minuits of power?

Depends a lot on what your criteria is for battery life. If you are willing to accept fewer charging cycles between replacing the batteries, you can get by with fewer batteries. Normally you would not want to discharge them past about 50%.

2.My understanding we will be using 240 cell @ 2V. Is this means each battery will have 240 cell? 240V X 2 = 480V which means they are all connected in Parallel

These will be connected to an inverter that will change DC to AC.

You will need to provide 500 KVA at 480V. The voltage is sort of irrelevant at this stage. Lets say it is 500kW for the time being (worst case). To get 500kW out of a 2V battery, that would amount to 125,000 amps, which is impractical. Lets say we chose 240 cells @ 2V (in series) to give us 480VDC on the battery side. That would mean we needed to supply a little over 1000A out of the battery stack. Eight minutes is about 13% of an hour so you would need a battery storage capacity of 138 AH. This would probably not work in real life as the battery can't operate at full power as it gets discharged, but you get the idea.

This calculation is very rough and does not include a lot of other factors, but gives you a vague idea of what you are up against.

Zbangs 200AH number is probably pretty close.

3. How we determine the capacity of the largest battery to be charged in amps-hours?

They all get charged at the same rate. It is based on how fast you can charge the batteries without damaging them.

4. How we determine the maximum obtainable amps from the charger?

The manufacturer will tell you what the battery charger is rated at.

Zbang is probably right that a small 200AH battery would not discharge that fast and survive very long. This is a design that should be done by a pro.

 

[27hillcrest]

The manufacture of the UPS will have that info on hand I would contact them.
Are you planning on building the battery string yourself? Are you suppling the UPS?

 

[bonding jumper]

Go to www.liebert.com and download all the product documentation for the 600 series UPSs. These are kind of the standard. You can get all your data from that, when you are buying a box that costs 150,000+ they will do some legwork for you. When you get serious about it, call them up and they will get you whatever info you like. Tell them you need 8 mintues on a 500kW/555kVA UPS and they will get you the dimensions, number of battery cabinets, etc. Their documentation will list a MCA, minimum circuit ampacity, a reccomended fuse size etc. Don't worry about doing battery calcs, the 480V AC isn't eqivalent to DC battery voltage. The batteries will operate around 540 DC and there's a curve where they drain down to 510 very quickly, then hold from 510 to 490 for a long time, then they drop off a cliff and drop quickly. IMO 8 minutes is a very short time, 15 is standard, but if you don't have room in the location for the extra strings then you have to sacrifise. Batteries life expectancy is only 5 years, so if you get 8 minutes of battery, you will have 8 minutes from the start of the units life, but after 3-4 years that 8 minutes could be 2 or 3 or 4 minutes, depending if you get a battery maintenance contract.

 

[kingpb]

I assume thats 500KVA @ 480Vac 3-phase? Which equates to 600A/ph. You need it for 8 minutes, so your duty cycle is 1041A @480Vdc, for 8 minutes.

Your inverter will need to take the Vdc and convert it to 480Vac, 3ph. Using a 480Vdc battery, you will need Qty. 240 x 2Vdc/cell in series, with each one having the capability of 1041 amps. You have some efficiency losses in the inverter, so you will need a little more to keep the ampacity up. Typically, you only want to discharge to 1.75Vdc/cell.

A rough calc says you will need approx. nominal 700AH - 8hr capacity. using a GNB Gel Cell Battery (no eye wash or special room required) it takes a #3-100A15 module which is 6 volts each, so to get to 480V you will need 80 of them. They can stack 8 high, so you would have 10 stacks.

The foot print is 26"+ deep, by 22"+wide (each) therefore you will need a battery room of approx 25 ft long, 11-12 ft wide, to allow space for other equipment and working clearances.

The long and short of it is that this is a very complicated process and extremely critical to get it right. I would take the advice of others and start working with a manufacturers rep ASAP. They will probably try to sell you a little larger system then is required, but in this case, bigger is better. To small and your screwed!!!!

My preference is a flywheel to ride through the power loss while an DEG starts and takes over the load. Cost is less, and in my opinion more reliable and longer life. Batteries deteriorate very quickly, as Bonding Jumper pointed out.