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How do APC UPS's detect low battery condition?

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I got an old APC Smart UPS that was tossed out apparently because the owner thought it was broken because it would only run for a few seconds. Being as old as it is, the battery was shot. I put a new battery in and it runs to manufacture's specified run time. Although there seems to be a problem with the low battery indicator and software's runtime estimate.

The unit is rated to run 650VA 400W.

The interface port is connected to a test computer and a simulated load of 200W(computer power supply with resistive load to simulate computer like load)applied under which condition manufacture claims to give 15min runtime.

The low battery alarm goes off only one minute into battery operation and tells the computer that is connected to the interface to turn off, but the simulated load continued to run for 20 minutes, well beyond the spec.

Other than not being able to tell when the battery is actually low, this is not a problem when used as a dumb UPS. However, it is a major problem when I use it with a management software, because it thinks the battery is low and signals the computer to shut down almost immediately after power goes out with 90% of battery capacity remaining.

I don't understand how the UPS is detecting low battery. The software estimates 7 min of run time(which is way off). The estimate changes with load, so it's measuring the amount of load some how.

Below are the test results of continuously monitoring battery voltage at the circuit board:

13.95V float voltage at 25°C ambient (isn't this kind of high? I wonder this is why the installed battery was dead as a brick only after 3 yrs)

Low voltage alarm came on about a minute into battery operation and the battery voltage was around 12.2V, very health reading for a battery under load.

UPS cuts off at 10.3V, seems about right.

Voltage drop at the terminal was already eliminated since readings were taking at the board. I suspected there's something wrong with the voltage detection, although a reasonable cut off votlage seems to imply that internal voltage measurement is working fine.

So, I pulled the board out to see if I could somehow change the low voltage alarm threshold. There is no adjustment trimmer of any kind.

The possible causes I can think of are: Something was programmed incorrectly through interface, firmware is retarded, out of tolerance passive components or failing semiconductor parts.

Anyone have any idea what might be wrong with it and how to fix it?

Reply to
AC/DCdude17
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It's based on the battery's capacity during the last calibration run. You need to force the UPS do a calibration run, where it runs the load off battery until the battery's nearly flat and then switches back to mains. It will remember the energy the battery supplied. The figure it currently has will have been set when the previous battery was in the unit.

What I didn't ever work out was how you reset the battery fitted date, but that wasn't used for anything important.

Reply to
Andrew Gabriel

The float voltage does seem high, and the cut-off voltage (10.3) is too low. Check the battery manufacturer's specs for the "care and feeding" of their battery. The load you are using may have an affect, too - I don't know. A PC uses a switching power supply - that might make a difference to the UPS. It will draw current in spurts while the resistive load draws continuously.

Reply to
ehsjr

See

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and look down near the end for the answer to "Q: I replaced the battery in my APC Smart-UPS and now it thinks the battery is low all the time. How do you fix this?"

Matt Roberds

Reply to
mroberds

In article , AC/DCdude17 writes

Run "inititate run time calibration" in APC's Powerchute software. This then stores the run time in the UPS.

It's slightly high, but I wouldn't expect it to cause a problem. Is this at the board or the battery terminals?

Reply to
Mike Tomlinson

Battery terminals

Reply to
AC/DCdude17

ACTUALLY BOTTOM POSTERS ARE THE MOST ANNOYING. Who the hell wants to scroll down to the bottom to read the attachments on every post. That is the cause of about 90% if carpal injuries.

Low voltage detection is quite a problem sometimes. Particularly when you have 3 phase power.

On one UPS I built for some very sensitive equipment that operated on 3 phase this was a problem that we had to overcome. The Voltage itself was not as important as their not be a transient on the input.

I built what I called a slow switch.

The power to the unit was supplied by the 3 phase mains. But in parrelel with that voltage was a regulated power supply coming off a battery . The regulated voltage was set to about 0.05 volt above the DC voltage from the mains. So no current was drawn from the battery.

In the event of the input voltage off the 6 rectifiers dropping, for any reason below a set voltage even for an a microsecond the slow switch would be activated. It would then begin to supply some of the load current. If the voltage remained at the proper point, inother words, it was just a drop out transient, then the switch would shut off and the normal current path would be used.

But if the voltage dropped out, the regulated supply would supply the current to the load. Now the current was in the order of 25 or so amps and the switch could not handle that kind of power for long. As the peak came off the battery the regulator pass current would slowly satureate and raise the voltage to the load and reduce the power dissipation in the pass transistor.

When the power came back on, the supply would detect this and the regulator would again come on and slowly drop the output voltage till it was equal to the supply voltage and then the pass transistor would cut off again.

It worked quite well.

I believe it was used on a test set for the SR 71 power supply which we also designed. That was a rather interesting supply. It had to operate at 100,000 feet and 85 degrees c. This was also before Silicon transtors.

Reply to
bushbadee

In article , bushbadee writes

OK, fine. If you won't post properly, I won't be reading it. Bye.

Reply to
Mike Tomlinson

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