Inverter to UPS - Blowing?

alkitalkit Registered Users Posts: 6

My office is set up that 2 computers and router are connected to a UPS (650VA, offline). The UPS is then connected to the AC wall socket.
I measured the total wattage of everything connected. It ranges between 200-250W.

As we had a power outage the other day, I did the following, and these were my results:
1) Connected a 500W cheapo inverter to 105Ah deep cycle battery. Then plugged the UPS into the inverter. There were sparks coming from the battery positive terminal, and then the inverter voltmeter went from 13V down to 0V.
2) Then connected a 720W Quantum inverter (larger, in plastic box with cooling fan). As soon as I connected the main plug, inverter made a slight pop, and fan went dead. Inverter will not power on at all now.
3) Then connected a 500W inverter (car style, without built-in charger) to the UPS. It powered on my setup without an issue for 5 minutes. I then turned off the inverter and turned it on again. There were sparks at the battery, and inverter made a long beeeeep.

I can no longer use any of these inverters, as they all do not work now.

I just want to know where I went wrong?

The only thing I can think of is the fact that I connected all of these to a UPS.
Could it be that either the UPS:
1) Needs power from pure sincewave inverter only?
2) Possibly has a large amount of surge wattage which could have blown the inverters? (Also find this hard to believe as it is a 650VA ups (360W), so shouldn't be able to surge that high, even when in charging mode).

Alternatively, is there something else I could have done wrong? I just want to make sure I do it correctly next time so that I don't blow anything again (costing $$...)

Thanks in advance for the help!


  • BB.BB. Super Moderators, Administrators Posts: 32,601 admin
    Welcome to the forum Alkit, and very sorry it is under these circumstances...

    My first guess... Many UPS inverters and less expensive DC to AC inverters are MSW type (modified square/sine wave) type inverters. This inverters do not have a transformer isolated output.

    Because of this, if your AC power is ground referenced, a standard for North American power (I think you are in South Africa) and if your battery bank is also grounded (typically negative ground, but positive ground would also have the problem), the combination of having an AC Neutral to Earth Bond and a DC Battery to Earth Bond (i.e., common ground rod/water pipe/etc.)--This creates a dead short through the MSW inverter and will generally let the magic smoke out (and draw way to much current in the AC and DC wiring).

    Take a look at the Inverter and UPS manuals. See if they mention anything about ground bonding/referencing (many manual are very unclear or skip the discussion completely).

    Can you tell me bit about your DC and AC wiring? Is the battery bank ground referenced and one of the AC legs also ground referenced? And are these all MSW type inverters? (most TSW/PSW true/pure sine wave output inverters have isolated outputs and ground referencing the Battery Bus and AC Bus is not an issue).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • alkitalkit Registered Users Posts: 6
    Hi Bill,
    Thanks so much for the response.
    It's actually been a fun journey. Just recently started looking into UPSs and Inverters as our country is starting to have daily electricity cuts.
    I've learnt tons over the past few weeks. (MSW vs PSW, lead acid and traction batteries, inverter vs UPS, watts, amps etc etc.). Really a good learning experience.

    Sorry for my ignorance, but what do you mean by ground referenced?

    Do you mean the ground wire in the plug?
    If so, this is totally disconnected as battery goes to inverter, invverter to wattmeter, which only plugs in with a 2 pin plug to the UPS. So nothing receives the 3rd pin grounded.
    Sorry if I totally missed what you meant?
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    If you are seeing sparks at the battery terminals while operating, then you have a poor connection. Yes, things can surge many times their rated watts. I suggest trying the computers and router direct to your inverter, without using the UPS.

    I am available for custom hardware/firmware development

  • alkitalkit Registered Users Posts: 6
    I just unplugged all the devices from the UPS, and plugged the UPS into my wattmeter.
    I don't see any surge. I just see a constant pull of 27.3W whilst charging up the battery.
    I have also attached the wattmeter to all the equipment and turned it on and off. I don't see it going higher than 250W on the wattmeter (this is with the UPS connected).

    What else could the issue be?
  • BB.BB. Super Moderators, Administrators Posts: 32,601 admin
    I have to write this quick--Heading out of the house.

    A TSW/PSW inverter usually has a full isolation transformer. Only AC current passes through a transformer. Any DC offset is "blocked" by the transformer.

    MSW inverters usually use inductors to "buck" voltage up from 12/24/48 VDC to 120/230/240 VAC. If there is a DC offset voltage, it can cause (a lot) of DC current to flow through the inductors and the H Bridge switching FETs (transistors).

    If you have the DC battery negative Bus connected to building/ground rod/cold water pipe. And have the AC neutral Bonded to the same ground/rod/pipes, this effectively creates a DC Current short circuit path through the the MSW inverter. And will usually "toast" the AC inverter (whether you have fuses/breakers or not).

    Grounding DC Battery negative bus, and one leg of the AC neutral (white wire in US, is the "brown wire" in other countries?) is a standard safety practice in North America... And somewhat variable in other countries.

    That is my guess from what you describe.

    It is true that a solid battery connection should not spark/arc--Unless the terminal/cable ends melt from over current (and no fusing/breakers).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    Unfortunately, the typical watt meter isn't fast enough to show surges.

    I am available for custom hardware/firmware development

  • josephrusselljosephrussell Registered Users Posts: 1
    First of all, sorry about your inverters. Like others, me too guess that the comparatively cheaper modified sine wave inverters might be a reason for such an incident, but connecting a UPS to an inverter itself is not a great practice. You could have used a pure sine wave inverter with built in UPS functionalities. I have one of those Staticon Inverters with UPS functionalities and they are good. They are a little more expensive, but would have done a better job handling your load.
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    ...but connecting a UPS to an inverter itself is not a great practice.

    Not necessarily, as long as the upstream inverter/UPS is PSW. MSW to MSW would be bad, yes. I have several small MSW UPSs connected downstream of my big UPS, to ensure my DVRs don't go down. No issues after years like that.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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