UPS repair? (uninterruptible power supply)

[Divarin]

Hi all. I have a UPS that I've had since the 2010's (ish).  I have replaced the battery in it once but even with the new battery it doesn't work.

The UPS is a Powmax DataSafe 650VA.  The battery is a 12 volt 7Ah sealed lead-acid battery.

Using a multimeter I was able to verify it is feeding voltage to the battery and I let it sit to charge overnight.  The next day I tried to test it out putting a computer on the UPS then pulling the plug.  It utterly failed to do anything.  The LEDs on the front indicate that the battery is dead.  Putting the multimeter on the battery terminals it read a little over 13 volts.

What would be the best way to get started in trying to find the fault in this thing? 

[PaulAm]

One thing to do is to open it up and look for any damaged components.  I had a little UPS that behaved similarly to yours and found one IC in the battery charging section had blown itself to pieces.  Replacing that got it working again.

[Divarin]

One thing to do is to open it up and look for any damaged components.  I had a little UPS that behaved similarly to yours and found one IC in the battery charging section had blown itself to pieces.  Replacing that got it working again.

Thanks, I did open it up in order to measure the battery voltage.  I gave it a good look over and didn't see any obvious issues visually.
Yesterday (about 24 hours since I had tried to test it the first time) I switched it on (not plugged in) just so I can try to follow the current from the battery.  but now it wouldn't even light up the "battery low" LED, but the battery is still measuring about 12.8 volts  Since it is a drop from the last time I measured it I'm assuming something is consuming that power but still a 12 volt battery measuring above 12 volts you'd expect some signs of life.

I'm sure if I plug it in again and let it charge up a little more then it'll do what it did before when I pull the plug: LEDs still lit but indicating low power and no output on the AC sockets.

Well I'll futz with it a bit more and see if I can find anything but I might end up just scrapping it for parts.

[AaronD]

As a consumer device, there are enough variations in the circuitry between brands and models, that we can't really give any specific advice without seeing the gritty details.  As in, probably a reverse-engineered schematic.  Only you can do that, and there's a pretty good chance that it makes the problem glaringly obvious.

Based on the behavior, I would guess (just a guess) that perhaps the power stuff is perfectly fine, but the meter is bad.  Since the inverter controller thinks the battery is completely discharged, it refuses to kill it by over-discharge.  The charger still works, either because it's a separate module or because a unified power controller is still allowed to charge a dead battery (I'd sure hope so!), but it never gets a confirmation that the battery has enough juice in it to power a load.

Just a guess, could be completely wrong.

---

Edit: A chip can look perfectly fine and still not work.  ESD for a classic example.  (ElectroStatic Discharge)  I think Dave has a video about that somewhere back in the archives, where he tests some anti-static bags and kills a couple of chips in the process.  No difference at all in appearance, but they clearly don't work anymore.

I've also seen electron-scanning microscope images of ESD-killed chips, and that really is what it takes to see the damage visually.  At that scale, you can see a hole where there used to be a handful of transistors, but zoom out even a little bit, and it gets lost in the "rainbow shine" of the rest of the chip.  The plastic package and the leads are completely unaffected.

[james_s]

I'd look closely for cracked solder joints, broken wires or visibly damaged components. I wouldn't invest too much time in it though unless there is something special about it, UPS's get discarded all the time needing nothing more than a replacement battery. Since you have a good battery already (actually do confirm it is good, I've had a bad one right out of the box once) you should be able to find another similar unit and put the good battery in it.

[Divarin]

Yeah this is kind of what I figured.  I looked over the board pretty well and didn't see anything obvious.  There might be bad solder joints but it's unlikely.  If I remember correctly the thing stopped working at some point (it's hard to know when since you don't know until the power goes out), so I replaced the battery.  I think that even after replacing the battery it still didn't work so I kind of gave up on it and just stuck it in storage.

I guess all I can do now is take a closer look at solder joints and test that the battery is actually able to supply power under a load and not just on a multimeter.  Other than than I'll probably just strip out any spare parts I think might be useful and toss what's left over.

[strawberry]

battery is bad
check battery voltage under load

[Divarin]

battery is bad
check battery voltage under load

I think you're right.
I decided to use a portable CRT tv which will run off of a 12 volt DC power supply.  So I let the UPS top off the battery while I worked on making a cable that would connect the battery to the TV.  Then I disconnected the battery from the UPS and measured the voltage, just above 13 volts.  I also tested my cable with my bench power supply, works fine.
When I hooked up the battery to the TV the power LED would come on and some ... interesting ... noises would come out of the speaker but not enough power to bring up the CRT.
I measured the voltage on the battery again, (TV off) just above 12 volts.  Then I measured the voltage on the battery with the TV on, just above 6 volts.

So the battery is dead, the question is did it die because the UPS failed to charge it?  No I was able to charge it, or at least it appeared to be charging it.  I guess I'll replace the battery and see if it kills another one or (more likely) it'll work fine for years.

[james_s]

I think you just got a dud battery, I've had that happen once or twice myself, they're really cheap and margins are thin, and they're easily damaged in storage or transport so sometimes you just get a bad one.

[AaronD]

I think you just got a dud battery, I've had that happen once or twice myself, they're really cheap and margins are thin, and they're easily damaged in storage or transport so sometimes you just get a bad one.

I once had a UPS fall off a cart.  Battery was toast.  Wasn't much of a fall either.  Fortunately, I had a pile of not-dead-yet batteries that someone picked up from a hospital and gave me, so it wasn't that hard to recover.
(hospitals replace their life-support batteries on a set schedule, *before* they die, so that's what my pile was)

And we had a single-cylinder garden tractor (vibration!!!) when I was a kid, that would routinely kill the starter battery.

So yes, they are fragile.

[mleyden]

I repaired an APC UPS last year of similar vintage. It was continuously beeping... it was the capacitors.

[Alex_Baker]

Should there be a problem with the charging circuit, electrolytic capacitors is what I would check first, bulging caps is an obvious problem, but others could still be bad.

I tore down a UPS that I got for free because it did not work, there were bulging caps in the charging section so the battery just went flat. The UPSs that I have worked on have all been built down to a price and are practically disposable, which is sad really.   

[Divarin]

Well here's an update. Picked up a new battery, tested it with my CRT TV funky cable system (just a baseline to make sure this new battery does supply the required load), then installed it back into the UPS. Tested this on a CRT computer monitor, seemed good so I figured I'm set.

Set this up to provide backup power for my PC and one of the 3 monitors, not too heavy a load, should at least span the 1 sec power outages I've been getting almost weekly here in NE Ohio.  Well just now (about 2 days after installing the UPS) had yet another 1 second power outage.  The UPS did nothing, just died instantly.

So the UPS definitely has a fault. 

[Alex_Baker]

Maybe test the battery again to see if the UPS drained it over the last week? perhaps the charging circuit has a fault that is not charging the battery but is draining it instead. Otherwise something in the inverter part of the UPS is likely not working.

[Divarin]

Maybe test the battery again to see if the UPS drained it over the last week? perhaps the charging circuit has a fault that is not charging the battery but is draining it instead. Otherwise something in the inverter part of the UPS is likely not working.

very strange indeed.

So I took the UPS down to the basement to experiment, opened it up and checked the battery voltage.  It's good (13.something volts).
Before taking the battery out I tried hooking that same CRT monitor up to it, and when I killed the power to the UPS it kicked in.
I figured, maybe my PC is pulling more juice and I just need to see what this UPS can actually handle.  So I hooked up a computer (an old Amstrad PPC, and a CRT oscilloscope).  It handled this load just fine.  I noticed that the beeping is faster the higher the load.

I know a crt monitor, a computer, and a crt oscilloscope is a decent load but I don't know how much so I figured I'd try to connect a known load.  I had some 150 watt lights handy (for ... reasons) and plugged in two.  I figured 300 watts should be plenty. there's no way my PC and one LCD monitor was pulling more than that when it failed the other day.

It handled it.  It was certainly unhappy about it (fast beeping and the number of LEDs on the front panel went down to just 1) but it did keep power going.

After all this testing the battery was still above 13 volts.

*shrug*  So I brought it back and hooked it up to my PC and one of the monitors as before.  This time I added a kill-o-watt inline (going to the UPS) so I can measure the current draw.  Unfortunately this kill-o-watt doesn't show the immediate current only the aggregate over time (kwh) so I don't have a read on that just yet.

Anyway so now I wanted to test this out but didn't want windows files getting corrupted by power outages so I went into CMOS setup and sat there, pulled the plug and it shut off.  The UPS failed with no attempt to keep power going at all.  Then I tried again, but this time instead of pulling the plug I hit the switch on the power strip.  This time it kept power going, beeping going off.  It seems that the power draw at this point was fairly minimal, about the same as just the CRT monitor I was trying before (based on the fact that only one LED went out and the speed of the beeps).

I tried this experiment 3 more times and all had the same result.

This is the same way I was testing in the basement, switching the power switch on a power strip rather than physically pulling the plug. 
I suppose it's possible there could be some way for it to detect the difference.  It can't be going by the ground because in my office there isn't one (my house was built in 1940 and a lot of the outlets are still ungrounded, sucks but that's what I have to work with at the moment).

Anyway that still doesn't explain the failure when I had an actual power outage the other day, where it was still physically plugged in but there's no voltage (what's the difference really?)

So maybe it's just a coincidence and the fault is intermittent? 

[DavidAlfa]

Measuring floating/unloaded voltage says almost nothing, only it's not shorted.
Do it when removing the mains, probably drops to 8V or less.
In any case, dropping below 11V is a good indicator of high internal resistance, typically happening in old batteries that have ran through thousands of charging cycles, the electrodes warp and deform, their effective surface gets lower, and less surface equals less current flowing. Also, after 10 years the electrolyte might have dried up.
If it's the usual 12V 7Ah battery, those usually can handle 200-250VA, perhabs 300 at most.

300W would draw +30A (There're lots of power losses in the inverter) from that small battery, and it's definitely not going to happen if it has 10 years on its back.

[james_s]

One thing I can tell you is I have *never* seen a UPS that acted any differently when the plug is pulled, there would be absolutely no reason to detect that. I've unplugged them many times to reroute cords and whatnot and mine always pick up the load. I think you have either an intermittent fault or a weak battery.