APC SMT 750i replace battery warning despite battery replacement

[rsdu]

Hi there,

Before Christmas I bought a used APC SMT 750i with a "Replace Battery" warning. So I bought a new battery, installed it, ran a self-test, all fine. Ready to start Christmas, I left it without load.

When I returned after festive season, it showed a SOC of ~35% and a "Replace Battery" warning (including the beeping). I was unable to perform a new self test to reset the error because of low SOC. So I hooked the battery to an external charger and charged both packs. Then I reinstalled it (SOC shown ~70%) and was able to perform a new self test to inform the unit about the "swapped" battery. It successfully completed but the unit immeadtly after showed "Replace Battery" warning again.

Any ideas on how to further troubleshoot this? Do I have a faulty unit?

Many thanks for any ideas!

Marvin

[Teken]

You can try the brain dead procedure which I have appended a link: https://www.apc.com/us/en/faqs/FA156611/

In the interim recharge the battery pack until they are 100% and let them sit for at least 24 hours. Measure the battery pack while at rest with nothing connected and relay what the resting voltage is.

For a single 12 volt battery it should be hovering around 13.6 ~ 13.8 VDC once charged. 

If the battery shows 12.80 - 12.90 VDC (which is considered full) only after 24 hours this indicates the cell are / is less than ideal. Reardless, once the battery pack is fully charged reconnect the pack into the UPS but leave the pack outside.

Take a multimeter (DMM) and measure the voltage once the UPS is connected to 120 VAC. Power on the UPS once it completes its POST (Self Test) use your multimeter and confirm the UPS is charging the battery pack. If say the battery pack is 12.XX / 24.XX prior to inserting the cells and it doesn't rise to say 14.XX / 28.XX there's something wrong with the charging circuit.

Two batteries in series is 24 volts DC, while floating it will be 27.XX, during charging it will be 28.XX VDC. As an aside what brand and (AH) are these batteries - how old?

[rsdu]

Hi Teken,

Many thanks for taking the time to look into my issue 🙂

That's what I did and measured:

1. Performed Brain Dead procedure as per given link.
2. Loaded both batteries with external charger. Voltage 13,7V after 100% charge.
3. Let both batteries rest for ~36hrs. Voltage 13,2V
4. Insert batteries to UPS (left UPS off): Voltage 27,4V
5. Turned on UPS
6. Selftest completed successfully, but replace battery warning right after.
7. UPS shows Battery Voltage of 27,3V and SOC of 32%.

The battery pack is from Battery Direct, model TBC48-BD1 with 7,0 Ah bought in December 2023.

I hope that helps in troubleshooting my issue. Many thanks for any advice!

Regards

Marvin

[Teken · Accepted solution]

I did not see in your reply that once the battery was installed and the UPS was turned on. That you measured the battery to see (confirm) the UPS is indeed trying to recharge the cells. If the charger is working it should be above 28 VDC. 

Please do so and relay what you observe.

One thing I didn’t see mentioned is if you tried to connect the battery pack while the UPS was powered by AC Mains. Larger systems allow hot swapping the battery cartridge / pack while the UPS is operating. Normally when a pack is inserted the system will prompt to confirm a new cartridge is in place. This is followed by asking for the installation month / year. This can also be done using Power Chute Business / Home software ad well.

If you have the ability to tell the UPS a new battery pack is in place and also the installation date. This is important step to complete as this resets internal counters and also tells the UPS to use charging voltage / current (lookup tables) to manage charging the cells.

If the UPS has just positive / negative battery terminals you connect to. They are not hot swappable so use caution if you try to connect the pack while the unit is connected to AC mains. 

Let me know if your UPS has the ability to reset / tell the unit a new battery pack is in place. If it does great and reset / update that information.

The next step is to complete a battery calibration. This requires a 100% charged battery pack with a (minimum) 30% attached load. Unplug the unit from AC Mains and track how long it operates for. Let the UPS shut down and then remove any loads. Let it recharge and see what the UPS displays as SOC.

[rsdu · Accepted solution]

Again, Teken, many thanks for your reply!

I, indeed, missed part of your instructions and now performed the following based on your further instructions:

1. Removed the battery pack and 230V.
2. Reconnected 230V and started the USV. It indicated that the battery is missing.
3. Reconnected the battery with the USV turned on.
4. Confirmed the new battery and replacement date in the menu.
5. Performed a calibration test -> Success! SOC at the end ~10%
6. Waited for the SOC to raise to >70%.
7. Performed a self-test -> Fail with "overvoltage on output" error
8. Restarted the unit and performed the self-test again but with some load -> success!

With some load the unit now behaves as expected. Many thanks for your support!

[Teken]

That’s really great to hear! 👍 May I ask how much load was on the output when the over voltage error message occurred? Just wanted to clarify any informational steps I provided that may have led you astray.

Rock On . . . 🍺

[rsdu]

There was no load connected when the over voltage error occured during the self-test.

Side observation: Current load is ~55W, as soon as I remove 230V and therewith force the unit to battery mode, the indicated load increases to ~230W. This still gives me about two minutes of runtime, which is fine for me. However, I expected a bit more ...

[rsdu]

For the folks googling this thread: There is a follow-up story https://community.se.com/t5/APC-UPS-for-Home-and-Office-Forum/APC-SMT-750i-replace-battery-warning-d...