APC UPS Refurbish and Expansion Project

I had an old APC UPS that needed new batteries. While I was locating new batteries, I decided to see how hard it would be to add extra batteries.

• Battery Replacement

The APC unit that I have is so old that it was manufactured before APC started making replaceable batteries. As a result I could not get the replacement from APC. APC has a real nice replacement policy that includes exact replacements and a 2 year extension of the manufacturers warranty; I highly recommend getting batteries from APC when possible.

I found many suitable suppliers for replacement batteries, but finally settled on Battery Specialists for no particular reason.

The battery turned out to be fairly difficult to remove.
More to come once the battery arrives..........

• UPS Marketing Strategy

Wow, now I know more about the marketing of UPSs. Did you know that there is no simple way to determine the "backup time" at rated load? So a 350W UPS might have 2 hours of backup time or it might have 1.5 minutes. The batteries are a big part of the cost so the cheaper the unit the shorter the backup time.
Oh, and the marketing groups all market the "VA" rating which is always much larger than the watt rating. Older computer power supplies ran at a power factor of about .7, so the UPS vendors designed the UPS to have a watt rating of about 70% of the VA rating. Now days most power supplies have a power factor much closer to 1 so the watt rating becomes the real limiting factor. For a UPS that does not have a watt rating on the name plate it is only safe to assume that the watt rating is about 70% of the VA rating. Of course everyone on the planet measures equipment load in watts. So a UPS is advertised using a load metric that most people do not understand and does not typically indicate the full load backup time. Talking about a truth in advertising issue!

• Battery Expansion

I determined that the current battery would only last 15 minutes under the full 350W load. How did I figure that out? Well first I had to figure out that the UPS was actually a 350W UPS, because the nameplate indicates 520VA. That was simple 520VA * .7 = 364W, so call it 350W. I know that there are (2) 6V 12Ah batteries inside in series for 12V. After several late night hours and some extrapolation of chart data on the APC website, I finally determined that the APC runtime numbers can be calculated from the installed battery Ah rating using a conversion factor of 1.632. This number represents a unit conversion from VA to Watts, the efficiency of the power inverter, and the effeciency of the battery at high currents. Knowing that the VA to Watts factor is .7, I calculated the combined inverter and battery efficiencies are ~87%. So for a 12Ah 12V battery the run time in minutes T = (12*12)/(load*1.632) * 60. If the load is 350W then the run time is 15.1 minutes.

If I want the UPS to run for one hour (60 min) at 350W then the total battery Ah at 12V must be: (60min/60min) * (350W*1.632)/12V = 47.6Ah.

• Battery Selection:

Now to select a battery and figure out how to safely connect it to the UPS. I found a couple of people that used automotive lead acid batteries. That is a bad idea because the charge termination voltage for those batteries is different than the charge termination voltage for sealed lead acid batteries. I figure that I can safely use any modern sealed lead acid battery. Those can be purchased at dozens of places including batteryspecialist.com.

• Connectors, Fuses, etc.:

The battery needs to be connected using a good connector, suitable wire, and a fuse. While the batteries that I choose to use may be able to deliver a whole lot of current, the UPS is current limited to 350W output so the input current will have a well bounded upper end. I placed a fuse at the batteries with a current rating that matched the UPS input current. Placing the fuse at the battery allowed for smaller wires and connectors. At 350W the UPS has an input current I = (350W/12V)/87% = 33.52A. Applying a 20% safety factor yields 40.23A. I selected 40A fuses and 45A connectors. I found 45A connectors manufactured by Anderson Power Products. This ham site has a good description of the Anderson Power Products PowerPole connectors. I ordered the connectors from Allied Electronics.

Qty Price Allied APP Desc
1 1.38 803-0148 1470G1 - PP45 4 Pole snap in receptacle
2 0.61 803-0152 1399G1 - PP45 Short space key, red
2 0.26 803-0158 110G9 - PP45 Snap-in receptacle retaining pin

2 0.36 803-0100 1327G6 - PP45 Housing Black
2 0.36 803-0108 1327 - PP45 Housing Red
4 0.45 803-5522 261G2LPBK - PP45 Contact 10AWG wire
2 0.40 803-0164 110G16 - Retaining pin, 1 row of housings

This provides a panel mount on the UPS and a pigtail coming out of the battery enclosure. I thought about having panel mounts on both boxes with a "jumper" in between but that would nearly double the cost of the connectors and add more assembly time.