APC UPS Repair Project - Overvoltage on battery

[CallumA]

Hi!

I acquired an APC SUA1000RMi2U from eBay for a very low price in hope that the only issue would be a dead capacitor (which was leaking electrolyte on the board).

I took out the board, removed the capacitor, cleaned the electrolyte, put in a new capacitor to no avail (looks to just be a filtering cap for the battery).

The UPS will work fine on battery if I connect "simple" things such as an incandescent light bulb (40W) however if I connect a computer LCD it will immediately over volt and shut down (with over voltage indicated on the front panel LEDs). Same result with even an LED light bulb (only 6W, died). This happens with the devices on their own or with the 40W incandescent.

I recorded a video of the UPS working just on battery with the incandescent bulb and then I plug the LCD in to the back:

My next guess would be to replace all the MOSFETs but since they're all screwed to a heatsink (some sandwiched between two heatsinks) which are in turn soldered on to the board I thought I would check that this could be the issue and if not, does anyone have an idea what it could be?

Thanks in advance!
Callum

[johansen]

i have a 1400va of the same.

if it was that large 40 volt capacitor next to the H bridge that leaked out, that doesn't surprise me but the fact that it still works means the fault lies downstream in the output voltage sensing network.
i'm not familiar with these things so i can't comment further.

[CallumA]

i have a 1400va of the same.

if it was that large 40 volt capacitor next to the H bridge that leaked out, that doesn't surprise me but the fact that it still works means the fault lies downstream in the output voltage sensing network.
i'm not familiar with these things so i can't comment further.

Yep, that's the one.

The voltage sensing network? Is that supposed to provide feedback to keep the voltage constant? The output does definitely over volt (you can see as the light flashes brighter when a device is plugged in) momentarily.

Thanks

[EEVPiobee]

And I'm pretty sure you checked for the obvious: "Is the battery is still good" right?

(Just saying, I personally do these kind of stupid mistakes myself)

[CallumA]

Hm, I thought I replied to this before. Obviously not.

My multimeter doesn't update very frequently however I didn't see the voltage fall below 24v whilst reproducing the issue (the pack is supposed to be 24v).