A while back, I bought a secondhand Liebert GXT2-2000RT120 uninterruptible power supply (UPS) on eBay. The GXT2 is a series of online UPSes, meaning that the output power always comes from the inverters off the battery bank; it doesn’t switch from utility power to battery power like an offline UPS. Besides eliminating any possible switchover glitches, online UPSes always deliver conditioned power at a constant voltage. The 2000RT120 is a 2000VA unit with 120V output — large enough to power all my servers for a good little while.
The batteries were due for replacement and the seller removed them to save on shipping costs. I got a UPS with a set of wires and no instructions on how to connect them.
Also one of the wires was compromised … but since it appears to be a ground wire, I figured no big deal if it shorts out against the cage. KIDDING!
Yesterday I figured out the wiring, installed batteries, and got the UPS set up in my server rack.
I’ve bought replacement UPS batteries from Digi-Key before, measuring the size I needed and using their parametric search to find sealed lead-acid batteries with the same size and highest capacity. Digi-Key’s replacements for the Liebert UPS seemed a bit pricey this time, though. Using the power of Google, I found a set of replacement batteries from BatterySpec for only $72 plus about $20 shipping.
They processed my order quickly — I ordered after the close of business on a Friday night, they shipped Monday afternoon, and my package was waiting for me Friday when I got home.
At first I thought they had provided fully-insulated battery terminals that I’d have to disassemble to connect the fully-insulated spade terminals on my UPS wiring, but those are just insulating caps for shipping.
The first thing I did was repair the ground wire. The wiring harness is a bit odd — it uses two connectors, I assume for current-carrying capacity — but that makes it easy to see how long the ground wire should have been. I stretched everything out and the remaining three wires were all in agreement about how long the cut wire should be.
The original wires are very supple 10-gauge with relatively fine strands. The closest thing I had was stranded 10-gauge left over from an electrical install; it’s not as supple, but it’ll do. I cut it to fit, lap-soldered the connections, and covered them with a double dose of heatshrink tubing.
The next order of business was figuring out how to wire this up, and the wires themselves provided the clues. The two sockets are wired in parallel with each other, so they had to connect at the ends of whatever parallel-series chain the batteries might form. The wires didn’t include any Y cables besides the sockets, so the batteries apparently needed to be connected in a single 48V series chain.
The cage is sized for the batteries to be laid down on edge, but it was easier to figure out the wiring with the batteries standing up. The socketed cables were long enough and had bends in them for the positive connector to reach between the nearest two batteries and the negative connector to reach between the furthest two.
Everything else fell into place like assembling a jigsaw puzzle without looking at the box — the batteries will tip together in pairs with their terminals at opposite edges of the cage, the short wires connect the two end pairs into series chains, and the long wire connects the two chains.
I connected all the wires with the batteries still upright, stopping before the last couple of connections to test carefully with the voltmeter and make sure I hadn’t overlooked anything that was going to short out.
With this much current available, any short circuit would cause a lot of heat very quickly; so knowing I wouldn’t be able to unplug red-hot wires, I had a wirecutter handy in case of extreme emergency. Fortunately it wasn’t needed and everything went together as expected. I measured just over 48V at the sockets from red to black, so all looked well.
I laid the batteries down and tucked the wires away into the grooves at the sides of the cage. I don’t know how it was done originally, but I had to route the black wire through the bottom race and the red through the top in order to be able to close the cage.
All back together and buttoned up.
You can see a nick in a red wire’s insulation, but it doesn’t go through to copper. Because it’s above the Y in the cable, it would have required cutting and resoldering the cable to cover with heatshrink. I didn’t think that was worth the effort, and I don’t put much stock in electrical tape. I guess I could go back and cover it with a larger heatshrink that encloses both of the red wires together.
The battery cage slid perfectly into the bay.
Connecting it for the first time was a bit unnerving — what if the seller was less than honest and took the UPS out of service because of a severe malfunction? Seeing no evidence of fire or overheating in the battery cage or bay and trusting Liebert to engineer adequate safety systems into their UPS, I took the plunge and connected the cables.
The wires did not immediately get warm. Good so far.
The next order of business (and the next unnerving step) was to plug the UPS into AC power.
That’s a 5-20P 20A plug. I don’t have any 5-20R receptacles in my basement.
By fiddling with the pushbuttons on the panel, I somehow got the UPS to turn on while on battery power and the indicator lights looked okay. I had to reference the manual to figure out how to turn it off — press the Standby button twice for about a second each time.
Well, I already want to get an electrician in to run a new service entrance and install a new master breaker box. For now I’ll probably just do something inadvisable with an extension cord.
The seller didn’t include the plastic front bezel, and I’d really like to find one.