APC Smart-UPS 750XL

[pussycat]

I replaced the batteries after unit indicated batteries need to be replaced, I ordered 2 12v 18ah batteries wired in series and connected to unit red to + black to -. When I attempt to turn it on there are no lights and will not power up. I removed batteries and slow charged each one to 13.6v and there is 26v to unit connector. I have tried Reset Button, and Brain Dead method, nothing works.  Any help would be greatly appreciated.

[Teken]

When the RBC was assembled was the fuse inspected for continuity? When you connect both ends of the Anderson connector do you measure 26 ~ 27.XX VDC on the  unit side?!?

What is the input line voltage at the wall outlet when measured with a True RMS multimeter?

Have you pressed the rear fuse button a few times to insure it hasn’t popped out? If this is a euro model is the battery disconnect connector fine and fully seated?

Brain Dead: Complete the applicable brain dead reset as outlined in this APC Resource Link 3 times:

https://www.se.com/us/en/faqs/FA156611/

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[pussycat]

RBC assembly - fuse checked for continuity and passed measured with Fluke 87 True RMS Multimeter.

Anderson connector 26-47 VDC on unit side connector measured with Fluke 87 True RMS Multimeter

Input line voltage 120 VAC measured with Fluke 87 True RMS Multimeter

Rear fuse button pressed multiple times before, during and after battery connected

Battery disconnect fully seated and upon inspection looks OK

Brain Dead procedure completed 5 times, still no response of any kind, NO LIGHTS, NO FAN NO POWER

Note: When I re-join the Anderson connector I sometimes get a small pop sound

UNIT still unresponsive, any other ideas?

[Teken]

Your system is a 24 VDC UPS as such when the RBC is fully charged the float DC voltage will range from 27 ~ 27.6X depending upon operating temperature.

If the measured DC value is / was 26 ~ 47 VDC there’s a serious problem!

As a sanity check (Disconnect) and measure only the Anderson connector on the unit side and report the DC output voltage.

My expectation is it would range from 27.X ~ 28.X.

Next disconnect and measure the RBC and document the current charged voltage. Next allow the RBC to dissipate the surface charge by sitting for a minimum of 72 hours than measure the resting voltage.

The expected resting voltage will either be the same or drop (0.1 ~ 0.3) VDC. If the voltage say is 12.9X and after 72 hours measures 12.6 ~ 12.7X report this for each single cell.

Repeat the test with the RBC connected to the system and measure the DC voltage at the battery terminals and Anderson connector.

Both DC values should the same if not report your findings.

NOTE: A small spark is typically normal because one of two scenarios are present. The charge capacitors are not fully charged as such a small spark is seen. To avoid this always ensure the unit is plugged into AC Mains for a minimum of 30 seconds then connect the Anderson connector.

The second reason is there is no bleeding resister present as such the unit will always make a small spark regardless if the unit is connected to AC Mains.

Caution: It should be noted (IF) the spark is very large this would indicate a dead short in the charging circuit. As such never leave the RBC connected to the system as it will cause the RBC to short out and cause a run away condition.

If you really measured 47 VDC that’s a serious problem.

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[pussycat]

Teken, the 47v was a typo should have been 26.47v

Re-ran tests results as follows

Day 1 no load completely unplugged - July 24, 2026 Cell 1 = 12.87v Cell 2 12.96v Anderson connector 25.83v 

Day 3 no load completely unplugged - July 27, 2026 Cell 1 = 12.90v Cell 2 13.00v Anderson connector 25.90v

Day 4 no load completely unplugged - July 28, 2026 Cell 1 = 12.87v Cell 2 13.00v Anderson connector 25.90v

Tried reconnecting to unit, still no response, dead as a door nail.

Is there an internal fuse that might be blown?

[Teken]

Please upload a photo of the front and back of the UPS while plugged into AC Mains along with a clear photo of the RBC top and the fuse assembly.

I would like to confirm the exact APC UPS SKU. If this device supports (Cold Start) remove AC Mains from the wall outlet and follow the steps outlined in this APC Resource Link:

https://www.se.com/us/en/faqs/FA156611/

Connect the unit to a computer running APC Power Chute Software. Let me know if the computer detects the connection from the UPS.

NOTE: Depending upon specific model & production year the system will have several fuses on the main board. Some are ATC style vs others depending upon where in the circuit are through hole / SMD fuses.

The PCB will be marked with a (F) next to any component that is a fuse. The fuses are designed to break open during a high current draw that exceeds the design limits.

You’ll need a multimeter set to continuity to verify any fuse besides any obvious blown ones which will have all kinds of burnt debris.

It should also be noted if there was a (Surge / Spike) this protection circuit can be activated / damaged which includes the following in the SPD Circuit: Resistors, MOVS, Diodes, and rare cases GDT’s.

It goes without saying remove AC Mains & RBC from the system and press the power button for 10 seconds. Even then do not allow yourself to touch any of the high voltage capacitors.

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[pussycat]

Teken, please see attached photos.

I cannot het the APC Power Chute software as it is discontinued.

However, when I connected my laptop to the USB port on the UPS there was no connection detected, probably because the UPS unit is DEAD.

I will look at the internal fuses in a couple of days, unless you have something else for me to try

       

[Teken]

You may use this APC Resource Link to download the latest copy of Power Chute Software (PCSS):

https://www.se.com/us/en/download/document/SPD-PCSS_WIN_EN/

As you noted the older version (PCBE) has been deprecated and EOL. If for whatever reason the latest PCSS application doesn’t work or detect your legacy Smart-UPS.

You may contact APC Technical Support to see if they can provide you the last version of PCBE.

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