Dell K792N UPS- Battery Over Voltage

[Generalkrause]

Two weeks ago, I managed to get a really good deal on this 1920 watt ups. It took about three days to arrive, and I wasn't able to use it until a week later, when the batteries were delivered. This ups is, from the outside, in decent condition. The only physical damage is a dent on the detachable front bezel. Upon powering up the ups for the first time, I got a very strange error.
Battery Over voltage. A pretty self-explanatory error. But, the issue is the batteries are indeed at the right voltage. I have confirmed this using two multi meters.
I delved deeper into the issue. In the measurements menu on the ups, this appeared.


Just, what? To clarify, this ups uses six 12 volt 9 ah batteries. The total input would be 72 volts. I am no electrician, but I'm pretty sure that its impossible for these batteries to do this.
I decided to rule out the batteries once and for all. I removed two of the six batteries, bringing the true voltage down to 48 volts. It still read the same thing: 90.3 volts and 24%.
I started up the UPS and removed the batteries entirely, and just to be sure, cleared all errors and checked the dynamic readings. The same thing: 90.3 volts and 24%
Confused, I decided to open up the UPS and see if something came unplugged in shipping. While all components seem intact and all wires seem fine. There was rust in many different places. Most of it was just on the chassis and not on the electronics.

Overall look at the inside


Closeup of the rearmost transformer and the back of the chassis


Closeup of the front, showing some rust on the chassis.


Closeup of the control board

Of these images, the control board is the one that concerns me the most. While the control board itself seems fine, the connections between the mainboard and the control board have some damage on it. I am unsure if it is corrosion or dust. Also, two components just below it, one in the exact center of the picture and another component just below that, have a lot of rust on its connectors. I am still unsure if the issue is due to physical damage or logical error. Does anybody have any ideas, suggestions or experience with these units?

Note:
The rest of the UPS seems to behave normally. All functions work on the menus, I was even able to flash the firmware on the ups via USB cable.

I have been unable to find any documents that go into detail on the physical layout of this ups. Only a manual of how to install it and maintain it.

The seller has high ratings from over 200 people. I was doubting this was a scam. And was reassured when I contacted them about this broken UPS and they decided to send another ups as sending this back would be a major hassle. Unfortunately, the unit they are sending is only a 700 watt unit, far less than I would like.

If all else fails I may see local repair businesses, but I am doing this first in order to avoid the expenses.

Any help would be much appreciated.

Link to the Manual for the Dell K792N:
https://www.manualslib.com/products/Dell-K792n-195388.html

[drussell]

You are getting this error with the UPS plugged into the wall, right? 

Sounds like bad batteries and possibly a battery charger voltage regulation fault.

Does the UPS run and provide output power from the batteries properly when it is unplugged from the wall?

[Generalkrause]

This error appears regardless if it is plugged into the wall or not. But, the UPS disables all output receptacles when this error occurs. Upon unplugging utility power, The UPS beeps for about five seconds and then shuts off. When the other UPS comes I will have a platform to test the batteries and their full capability. As for now, though, my only available way to test the batteries is with a voltmeter, which reads each independent battery at their appropriate 12 volts.

[drussell]

This error appears regardless if it is plugged into the wall or not.

I find that extremely unlikely since it will never see 90+ volts from the battery bank without a charger connected.

If that is actually true, then there is something wrong with the battery bank voltage level sensing circuitry.

But, the UPS disables all output receptacles when this error occurs.

Uhhh..  I think you misunderstand me...  Do NOT plug the UPS into the wall.  have it completely powered down to start, then connect the battery bank.  Turn the UPS on.  It should fire up and provide proper output voltage without any issues, running entirely from battery power.  You should be able to plug in at least three 500w halogen worklamps simultaneously or boil some water in a 1500w kettle with it in that state if it is operating correctly.

Does this occur?

What is the actual open circuit voltage of your battery bank?  By your explanation, I expect those batteries may be well past their useful life and totally, completely useless, though you will need to test further to be sure where the problem is.

Unless you know the batteries are definitely still good, when purchasing a UPS, I would probably always just buy the UPS rather than bothering to ship old, dead, useless cells for the lead scrap value.  

Brand new, decent quality batteries for that thing can't be more than what, $150 total?

When I set up my big-guns, I usually use full size car batteries instead, which does increase the battery cost significantly but the trade off is typically a 6-8 hour runtime. 

Upon unplugging utility power, The UPS beeps for about five seconds and then shuts off.

I'm asking about the behavior on BATTERY power, not plugged in to mains power at all.  You need to know if your charger circuitry is screwed or if you just have useless batteries that aren't drawing any current to charge and fooling the battery charger as if no load was connected.  I have an old Tycor 48v that I've replaced a transistor in the charger a couple of times.  It seems to die after a few years when the cells get weak.  I suppose I really should figure out why it does that, it is a ctually a pretty robustly made ol' unit.

When the other UPS comes I will have a platform to test the batteries and their full capability. As for now, though, my only available way to test the batteries is with a voltmeter, which reads each independent battery at their appropriate 12 volts.

You have no kind of 12 volt loads?  An old car headlamp sealed beam or anything at all to see if those batteries have any useful life at all?   

I find that surprising...  There seems to always be something at hand to use for a load. 

[Generalkrause]

The UPS does not power on with the batteries connected and the utility power unplugged. Holding down the power button, and another power on method(I heard this from a friend), holding down the arrow keys for three seconds, does nothing. These batteries were purchased new. The batteries were purchased separately from the UPS. Also, plugging in utility power without the batteries plugged in does not power up the UPS either. This leads me to believe that the UPS requires both to power on, and it may be intended to behave like this. Open circuit voltage of the battery bank is just a little more than 72 volts. I am guessing that the voltage level circuitry is messed up. Do you think this is something that a local electronics repair shop can fix, or is it more complicated than that?

[johansen]

if that vertically mounted control board can be removed from the main board, then i would pull it out and wash it. distilled water and alcohol only.

If not, then i would consider washing the entire thing. toss it in a dishwasher without soap perhaps.

[Armadillo]

if that vertically mounted control board can be removed from the main board, then i would pull it out and wash it. distilled water and alcohol only.

If not, then i would consider washing the entire thing. toss it in a dishwasher without soap perhaps.

+1;

Heavy Hose entire boards, scrub with brush and soap [glass cleaner], then hose entire boards again, then dry the boards for few days.

[Generalkrause]

Thank you for the feedback. I will clean the boards and see if that works