Electronics > Repair

UPS repair? (uninterruptible power supply)

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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:

--- Quote from: PaulAm on May 06, 2022, 02:25:58 am ---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.

--- End quote ---

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.

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