UPS drops voltage in battery mode

Hi there,
since this is my first post in this forum, I want to introduce myself first.

I am working as a foreign Community Developer and in Disaster relief in the small mountainous country of Lesotho, in southern Africa.
I am a qualified electrician, have limited equipment to test onside and very little knowledge in electronics.

Now to my current challenge:
To keep a system critical fan in the sewage system running during the common power outages I want to use an old PSU (Netstar XT700). The UPS was long time not used and the original battery is finished but I used another 12V/7Ah battery (fully charged) for testing.

When the UPS is connected to the main power, I have proper 230V on the outlet. But as soon as I disconnect main and the UPS switches to battery power, the voltage at the outlet drops down to 168V. I don't think that is meant to be....

Can anyone help me where to look first and what could be broken? What can be the component that regulates the voltage? How to measure and test component? Your advice is much appreciated.

Many thanks and greetings,
Mosuto

The first thing I would check is the voltage across the 12V battery when that is put under load - if it's not up to supplying the current needed, the voltage will "sag" badly.

You specifically need a high-current, deep-discharge battery such as one for an electric vehicle, not such as a motorcycle battery.
At 350W out, the load on the battery will be 35A or more.

Does the fan work OK?, it might be your meter reading incorrectly if it's not a true sinewave UPS.

How big is the fan, and what power does it take?.

rjenkinsgb said:

The first thing I would check is the voltage across the 12V battery when that is put under load - if it's not up to supplying the current needed, the voltage will "sag" badly.

You specifically need a high-current, deep-discharge battery such as one for an electric vehicle, not such as a motorcycle battery.
At 350W out, the load on the battery will be 35A or more.

Click to expand...

Hi rjenkinsgb,
the UPS is designed to work with a 12V/7,5Ah battery. But i connected it to a 12V/12Ah battery since I only had this one available.
The battery shows 12.1V when the UPS switches over to Battery mode. But with no load connected to the outlet. Even then it drops the outlet voltage to 168V.

The fan is a small fan with only 20W.

The fan

Nigel Goodwin said:

Does the fan work OK?, it might be your meter reading incorrectly if it's not a true sinewave UPS.

How big is the fan, and what power does it take?.

Click to expand...

Hi Nigel,
thanks for your response.

Its a quite small fan with 20 Watt. Its the kind of fans you can use for a Desktop Computer case.
And so far the fan was not connect to the UPS. I just tested the UPS before I want to put it in place.
The fan works fine so far with 240V main supply.

I don't think its a true sinuswave UPS. How could I verify my voltmeter reading?

Since it is only an ordinary fan it should not be a problem it it is not a proper sinus wave, right?

Mosuto said:

The fan


Hi Nigel,
thanks for your response.

Its a quite small fan with 20 Watt. Its the kind of fans you can use for a Desktop Computer case.
And so far the fan was not connect to the UPS. I just tested the UPS before I want to put it in place.
The fan works fine so far with 240V main supply.

I don't think its a true sinuswave UPS. How could I verify my voltmeter reading?

Since it is only an ordinary fan it should not be a problem it it is not a proper sinus wave, right?

Click to expand...

If you've not tried it, then presumably you're testing the UPS with no load? - it very likely won't work properly like that, supplies don't like no load, so I wouldn't be surprised if the output reads wrongly.

Stick the fan on it and see if it works OK, the voltage 'might' even read correctly then as well - although 20W isn't much of a load for it.

Why not connect a 230 Volt 60 W incandescent lamp (filament bulb) instead of a fan?

Ramussons said:

Why not connect a 230 Volt 60 W incandescent lamp (filament bulb) instead of a fan?

Click to expand...

60W bulbs were always my 'go to' for dummy loads, but don't really exist any more - presumably pretty well everyone has long since replaced them by much more efficient lighting?, and they were removed from sale long ago in the civilised world.

So, i tested the output voltage with different loads. And it seems the higher the load, the higher the voltage goes... Strange....

I used a 1kW heater and the voltage came up to 192V. Can this be the result of the sinus wave being more in steps than a real wave?

But however, encouraged by the testing I just connected the fan to the UPS and it seems to work. Even with 168V....

Thanks to your advices and support!
Much appreciated.

Greetings from "lockdown" Lesotho.
The Mosuto

Mosuto said:

So, i tested the output voltage with different loads. And it seems the higher the load, the higher the voltage goes... Strange....

I used a 1kW heater and the voltage came up to 192V. Can this be the result of the sinus wave being more in steps than a real wave?

But however, encouraged by the testing I just connected the fan to the UPS and it seems to work. Even with 168V....

Thanks to your advices and support!
Much appreciated.

Greetings from "lockdown" Lesotho.
The Mosuto

Click to expand...

As I said before, your meter won't be giving the correct reading, the VAST majority of meters give an average reading on AC, scaled to read as RMS assuming a pure sine wave. To get a more accurate reading you need a true RMS meter, much more common now than they once were, but still fairly rare.

Your mistake though was trying to measure it in the first place, you should have just stuck the fan on it and happily used it