I'm talking about batteries installed in UPS. They are always fully charged and works only for some seconds/minutes after a black-out. Depending on the black-out duration, they can discharge almost completely. After black-out, the battery is recharged again with a simple/complex recharger.
After 1-2 years, the battery can't make its job well: immediately after a black-out, the PC switch off. So I understand the battery must be changed.
Is it possible to have a warning (battery fault) *before* the next black-out event?
It isn't simple. A fault battery usually shows a full voltage in open circuit (no load), but it can't generate "current" for a certain time.
You have to test the battery under load. Large UPSes do this sort of self test automatically. Small ones aren't reliable anyways, so it just doesn't matter if they work or not past 90 days or whatever the warranty is.
A man comes by with a hot stick and volt meter and tests the voltage of every battery in our ~480 volt battery locker a couple times a year. They can identify any suspect battery by voltage alone under load and then replace it. If more than some low percentage of total batteries are bad they all get replaced.
Yes. Many UPS's have a battery test feature where they run on battery for a short period of time, during which the battery voltage is monitored. The charger then extrapolates the runtime from the initial voltage and the voltage at the end of the test run. This feature is common in the higher end APC UPS's. I have about 4 assorted UPS's in the office, and I don't know how many at customers offices. For various reasons, I prefer APC SmartUPS which uses the above battery self test feature. I can usually tell if there's going to be a problem as the PowerChute software will complain if the battery shows any indication of a capacity loss. There's also a battery test on power on. (Note: APC BackUPS do not have this feature).
Despite this battery test feature, I do preemptive replacements of customers UPS batteries running mission critical servers and devices approximately every 3 years. Much of my personal collection of half dead batteries came from such replacements.
Everything looks simple in the beginning of a project and not so simple as the deadline approaches.
The battery test described above uses the (typically) 24VDC to 117VAC inverter as a load. You would not want to run a load test with the typical UPS load because a dead battery would drop the load almost instantly. Punching the test button from the front panel or associated software should not dump the load during the test.
Are you perhaps thinking of adding such a feature to an existing UPS? That's possible but not very easy unless you have control over the microprocessor that's running the UPS.
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Well, to begin with, my UPS has held up each time for a decent period (never timed it), more than long enough to finish my (mostly internet) work, shut down browser, shut down OS, and turn off (external switch after UPS)computer. Shutdown time of OS alone can be as long as 5 minutes.
And i have numerous other powered equipment on the UPS: printer, Comcast modem, telephone, computer monitors (2).
**NOTE i got the UPS used and i finally had to replace the battery after FIVE YEARS of use. It has been another 5 years with that new replacement and there is no sign of a problem.
AFAIK all of the modern UPSes have that monitor scheme you allude to, with RS-232 or other communications link to the computer. Plus software that will safely shut down the computer for you.
Then again, my UPS is not a cheap design POG; it is made by APC.
A proper UPS will switch off the load when the battery cells are discharged to a voltage of 0.875/cell. Lower than this will damage the cell.
After black-out, the battery is recharged again with a
A simple charger might not always monitor the voltage well enough to prevent gassing.
Not too fast. Many times those sealed batteries are nothing more than 'wet' batteries with a pressure regulating valve and some catalytic component that helps recombining the H2 and O2 into water. After 1 - 2 years it's quite possible that the valves have had to open in order to release the pressure due to gassing so many times, that most of the water has escaped from the acid. Only one cell needs to be in that condition in order for the whole battery to seem to have failed. Removing the sticker that says: "NEVER REMOVE THIS STICKER" will sometimes reveal the unscrewable (I mean: you can unscrew them) caps through which you can check the liquid level and, if needed, refill with clean distilled water.
Coincidentally I was in the same situation, although not with a UPS, just
2 weeks ago and was able to continue using the battery.
If it is a gel battery, even then you could try to revive the battery in this way. However, after this operation do not return the gel battery to its preferred position, which is laying on its side with the plates horizontal, because after opening the cells, the liquid might leak out this way.
Periodically remove the grid voltage in an uncritical situation to test whether the battery keeps going for a sufficiently long time.
From your answers, I understand the *only* method to detect a fault battery on the field is to switch the battery on a load for some time, monitoring its voltage.
If the voltage doesn't go down too much, the battery could be considered ok.
Now some other questions arise.
Starting from the nominal battery voltage (12Vdc), its capacity (7Ah) and its typical load range (0.5-2.0A), how to find:
- frequency of tests (one time a day?)
- dummy load (the current during the test, 100mA?)
- how long the test (1 minute?)
- when the battery could be declared good (V > 13.0?)
Well, I think the ultimate test is the capacity test: Fully charge the battery (whatever that may mean--some say 13.7 V for a valve regulated and 14.4 V for a 'wet' one) and then discharge until 10.5 V, not lower, while counting the electrons flying through the wire. Then check with the advertised Ah rating.
If this is 'mission critical', then I won't comment.
Otherwise I think you don't need to do the capacity test too often.
A full battery will show around 12.5 V at rest and at around 20 deg. C.
If you draw 1%, i.e. 4 A during one minute, and check the voltage after
10 minutes, I think it should have come back to 12.5 V. You could do this daily.
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