I have a 10-year old APC UPS. Naturally the 12V SLAs were dead, so I bought 8 new ones of similar specs.

After swapping out the batteries the unit starts and charges fine, but when I try and run a load on it then pull the line input power, the UPS shuts down and lights up the red X "battery failed" light while pip-pip-pipping. The load goes down hard at this point.

On inspection, the original batteries used the F2 connector (pictured on the right), and my replacements use the smaller F1 connector (on the left)

enter image description here

Question Is it likely that using the wrong connector with the original F2 wiring is limiting the current that can be drawn? The battery back reads about 52 volts (its 48v nominal with 12V x4 x2 so a total of eight 5Ah batteries) or is my UPS simply broken ?

  • I don't have the resources to buy 8 more batteries on the off-chance this is the root cause.

  • Should I recreate the battery pack internal cabling with F1 spade connectors - would that help? Or do something else to improve connectivity?

  • UPS is an APC 2RU rackmount job rated at 2.2kVA, and has a network management card, so its not a cheap and nasty home-grade device.

  • I've changed batteries in over a dozen different UPSs in the last couple decades, so while not an every day thing I haven't made any significat mistakes. Size range from 275VA up to a 6kVA emerson monster.

  • Data sheet for the batteries I used

  • \$\begingroup\$ Your batteries should have a discharge rate quoted for them? I would assume it will be less as those connectors are going to take less current draw as you say. Presuming your getting good electrical connection and ups is functional you should only get an error when you trigger the ups, if its a current issue, not the ups itself. If that's the case stick a multimeter on the pack voltage and see if it sags when you try to use the ups feature. It sounds like current as you have suggested and the batteries are unsuitable :( \$\endgroup\$ Commented Jul 28, 2017 at 11:08
  • \$\begingroup\$ Wrong connectors, alone, shouldn't cause big problems. But they could be a clue that other parts are designed for lower currents. Do you have datasheets or specifications for the batteries? \$\endgroup\$
    – Jack B
    Commented Jul 28, 2017 at 11:09
  • \$\begingroup\$ And does it happen with just a tiny load like a desk fan? \$\endgroup\$ Commented Jul 28, 2017 at 11:13
  • \$\begingroup\$ @JackB I have found a data sheet on the supplier's website, and added it to the question. It doesn't mean a lot to me, not sure if its typical for a SLA battery. \$\endgroup\$
    – Criggie
    Commented Jul 28, 2017 at 11:23
  • \$\begingroup\$ @Rendeverance Excellent thought - I will try that and update question in the next 24 hours. I do know that the UPS is happy to run without mains power with no load at all. I was trying to run 5 servers and 2 switches, which sounds like a lot but this currently runs off a 1500 APC UPS without bother. \$\endgroup\$
    – Criggie
    Commented Jul 28, 2017 at 11:25

4 Answers 4


Those batteries are not really suitable

The datasheet for the batteries does not give a maximum discarge rate or current draw, but does indicate that the capacity is cut in half at 8 amps. At 12A, the voltage drops to 10V after only 8 minutes.

At full load, a 2.2kVA UPS will draw 45A from a 48V battery bank. Since you have two parallel sets of four batteries in series, that is 23A per battery. Drawing that much current will cause the battery voltage to drop a lot. The UPS then detects the voltage drop, and decides the batteries must have failed.

All is not lost though - if you keep the current draw low, then the voltage will probably stay up, and the UPS is more likely to work. What load you can manage will depend on details of the UPS which we don't have, so you'll just have to go by trial and error.

  • 1
    \$\begingroup\$ I'm accepting this answer because it tells me exactly what's wrong with the batteries and what to look for next time. \$\endgroup\$
    – Criggie
    Commented Jul 31, 2017 at 19:48

As per our comments i thought i would leave a more comprehensive answer: The connectors themselves should not cause the issue - but if enough current was requested they could overheat. Since the batteries have smaller connectors, it is likely they have lower maximum and continuous discharge ratings, which in your own case it seems indicative that you are exceeding this with your UPS.

Rebuilding the packs with larger connectors is very unlikely to aid any benefit as the discharge rating will be most certainly set by the internal resistance of the cells themselves.

If the UPS works with a light load but dies on application of a heavy load it is likely the voltage of the bank is sagging under load, which you should be able to observe on a multimeter when you activate the UPS.

If this is the case the UPS is not lost, but until you can upgrade the cells, or add another bank in parallel to increase current capacity, you may have to attach the UPS to a more suitable load (i.e. Attach less too it)

I know its not what you wanted to hear but I would say its about all you can do in this scenario.

If you decide to run with a load that doesn't drop the UPS like a stone, i would run it for a while and see if the connectors or batteries are getting hot, just in case. But otherwise you should be fine.

Update : You have added the datasheets, yeah - they are unsuitable. Try a light load or you may need to change / add another bank in parallel later on.


Putting a 0.250" Faston on a 0.187" tab is not automatically a problem. BUT, there also are two different thickness of the tabs. The 0.250" tab almost certainly is 0.031" thick, while the 0.187" tab might be thinner. This could lead to very light contact causing the localized heating already mentioned, or even intermittent contact that confuses the control system.


This answer from AnalogKid explains the real issue here. Your voltage observation is spot on for a APC 2.2KVA system in each sled. You have (4) parallel batteries in series 4 other batteries, hence 48v (~52v). The issue is heat. When you use F1/T1 vs F2/T2 you're asking for trouble. Any one of those 16 total (pos/neg) connections is an opportunity to be loose, i.e., arcing and added heat. Add that single loose connection to one or two other loose connectors and things heat up pretty quickly. I have had engineers mistakenly replace the incorrect connector type batteries and the sleds heat up to over 140deg F. They are rated for 104 deg F. The batteries, crack, and leak within months if not weeks of deployment. You will get a noxious small from the sulfereric acid and know.


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