Battery Backup - Expected Runtimes

All batteries are list with AH(Amp Hour) Rating. 100 AH is 1 amp for 100 hrs or 5 amps for 20 hours. If you know what your load is and at what voltage is then you can compile the data to build the battery bank you need.

That post is correct, but I think the APC site was also trying to tell you something. The point of an UPS is to carry you through a momentary brownout or blackout, and allow you to shutdown gracefully in the case of an extended loss of power. If you need more time than a standard UPS will give you, what you want then is a standby generator that will automatically kick in after a few minutes of blackout, well within the time frame that your UPS can keep your IT equipment running.

Consider the battery that would keep a 250 Watt PC supply running for a half-hour. That is a 2 Amp current draw at 120 Vac, which means a 20 Amp draw from a 12 Vdc battery. And that's assuming perfect UPS efficiency. The UPS is likely about 90% efficient. So you need a 12 Vdc battery with a 10 Amp-hour rating. It doesn't work quite like that, but it's in the ball park.

Here's just such a battery:

http://www.batterymart.com/p-12v-10ah-sealed-lead-acid-battery.html

Say you have three such loads to maintain for the same time period. Now you need 60 Amp-hours. Here's that battery:

http://digikey.com/scripts/DkSearch/dksus.dll?Detail&name=522-1018-ND

Your off-the-shelf APC UPS is built around a certain battery size. If you need more amp-hours, you either need a larger UPS, you can jury rig some kind of parallel attachment of extra batteries to your UPS batteries (will never fly in today's safety conscious, regulated and litigious environment), or you can do as I suggested at the beginning: acquire a standby generator that ties into your mains distribution for ITE and provide back up power after lights have been out for a few minutes.

1) What is your usage?

2) How big is your Load?

3) How long do you need to back up?

Thank you for your responses and help, everyone--I am learning very much!

My company is basically set up like so: we have a main building with all of our servers and network equipment. We have three other buildings for different departments (accounting, marketing and PR, etc) and each building has a networking area in their basements. In each basement are switches attached to APC SmartUPS devices that feed our PoE phones and Access Points and so forth. These are what the administration is concerned about: they want our switches to remain operational and on battery backup for anywhere between 20 and 30 minutes.

In one of said areas we have a SmartUPS 1000XL providing battery backup to 2 Cisco switches. I've found a Power Usage calculator on Cisco's site that estimates a single switch with 15 PoE phones and 2 APs will use 218.3W of power. Another switch with 10 phones and 0 APs will use 129W of power with a total of 347.3W.

In looking into APC SmartUPS solutions I found a calculator that says a SmartUPS 1000XL should last for 33 minutes with a 400W draw. In our real world situation however it lasted for only 13 minutes. Would replacing the batteries be enough to increase the runtime 20 minutes? Or are their calculators potentially incorrect? I'm new at this so I'm taking the cheap way out and using their calculators--once I learn more I'd like to understand enough to do the calculations on my own, I just can't find Amp-Hour information on APC's site, only output Watts and VA.

Olimar,

The Calculator on the APC site would be correct for batteries up to approx 18-24 months old, especially if the UPS has not been setup to excersize the batteries.

The batteries in the APC UPSs vary in size according to the UPS and are either 12V or 24V for thier off the shelf packs, which is why in my original post I suggested to go that way.

Log on to the UPS Network Card and under [Smat-UPS XXXX] and then [Diagnostics] select [Initiate a UPS Diagnostic Function] then from the drop down select [Start Runtime Calibration]. Once the test is completed it will tell you how long Your batteries should last with the existing Load.

While you are in there, you can also set the [Self Test] function to a frequency that suites you and your environment 7 or 14 days. I personally run this every week.

The general design life cycle for these comparitively small batteries is approximately 5 years in this use, so if they are nearing this point, it may still be time to look at replacing them, depends on the criticality of your environment.

My last point for this entry is that I would agree with SB (I banged in a GA) that given the load, runtime requirement and variability of your load as described, you should look at getting an appropriately sized generator installed and set for Auto Start-up if the power is out for more than approx 60seconds. Just make sure you load said Generator up to at least 50% (i.e. don't go too big unless you have other uses for the power)

Regards,
Sapper.

p.s. Appolgies for mis-spelling your name in my first post.