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Checkng Smart UPS Inverter funcion

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smallfreak:
I have acquired a cheap Smart UPS 700 that has been sold because "batteries are old". Sure enough it moans about replacing battery.

It took about 2 hours to load the batteries. At least the BCHARGE status went from 10% to 100% in that time. Just to check general function I placed a small load on it (~8%) and removed the mains cord. The device made some clicking for about a second and then it was off, indicating "battery fault". Voltage on the battery clamps dropped to about 21.6V for a short time. It does operate in online/filter mode.

Just to be on the safe side I want to check whether the inverter IS OK and it's really the battery that is faulty. Is it OK to clamp a 27.6V power supply to the UPS instead of the battery or additionally? At least that's the voltage of the pack after getting "fully charged".

Do I risk to blow the bench power supply due some current backflow? This one is no premium gadget too, but one of the cheap Chinese 300W 30V parts.

Vovk_Z:
Yes, there is a risk to blow a regulated power supply because of backflow (if it is not designed to work as a charger too). But it became safe to add a series diode (I use 5-10-20 A Shottky diode). So you'll not have backfeed from the UPS charger.

The only typical problem with most (APC) UPSs - 22 uF electrolytes became dry instead of being wet (and some larger ones too). After that the charger floating mode voltage rises up and batteries die fast. So, if UPS is 10 or more years old I would check all electrolytes and possibly most of them need recap.

Vovk_Z:
You may check batteries (and invertor) without a load too. The UPS itself eats about 1-2 Amperes DC from batteries.

smallfreak:
Quote from: Vovk_Z on Yesterday at 04:42:13 pm
--- Quote ---You may check batteries (and invertor) without a load too. The UPS itself eats about 1-2 Amperes DC from batteries.

--- End quote ---

Unfortunately, no as the UPS does shut off within two seconds even without load. This may be because of a dead battery OR because of a bad inverter.


Quote from: Vovk_Z on Yesterday at 04:29:48 pm
--- Quote ---The only typical problem with most (APC) UPSs - 22 uF electrolytes became dry instead of being wet (and some larger ones too). After that the charger floating mode voltage rises up and batteries die fast. So, if UPS is 10 or more years old I would check all electrolytes and possibly most of them need recap.

--- End quote ---


So when the charging voltage ends at 27.6V then for each of the two batteries that's just the recommended 13.8V for "regulated power/standby" use - as is noted on the batteries. The capacitors might not be a problem for now.

I checked with the support line for my power supply, but the answer was rather inconclusive  |O:


--- Quote ---Under normal use, the charging voltage should not be higher than the rated charging voltage of the battery is no problem.
--- End quote ---

A quick check with an 8R power resistor told me that each of the (single) batteries is able to deliver 1.2A which is quite low and the voltage drops from 12.6 just below 10V and slowly falling even with such a moderate load. I captured a voltage profile on the combines 24V battery connector during the "APC self-test" without any load.



This pattern is done twice before the UPS declares the battery bad.

Dos anyone know whether this big drop is to be expected?

Vovk_Z:
All syptoms are pretty simple and clear I guess. You just have dead batteries, as UPS say. Charging voltage is fine too, so UPS (now) is fine.
Those old dead batteries may be useful if you need to test other UPS or else until they are totally dead. I mean, they are useful because they can't deliver large current so they are 'more safe' then new ones for testing something.

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