Bunch'a Questions regarding UPS/Inverters for Solar use.

Discussion in 'Power Electronics' started by bhvm, Jun 26, 2016.

  1. bhvm

    Thread Starter Active Member

    Jul 16, 2009
    Dear all,

    I am trying to work out a small Solar backup unit. I got the solar part right,I can now charge 2x12v 150AH tall tubular batteries at about 10~11A (In parallel) Using a state of the art MPPT solar charge controllers. Now what remains is the UPS/Inverter part. Please provide guidance on the following questions-

    1> I have 2 UPS options to use. Both are rated at 800VA. Made by Same manufacturer- Microtek (reputed manufacturer in our area). The Only difference is , One unit runs on 24v (2 batteries in series) and other is 12v Unit.
    Surprisingly, the 12v unit is beefier & heavier. Does it mean that 24v units can use smaller transformer for the similar VA ratings? What is the underlying reason. I have modified Both units with fans for added cooling.

    2> For this application, I am leaning towards the 24v Unit. As Charging batteries with 24v config appears more efficient. Is that correct?

    I have connected the 24v 800va UPS for the moment.

    3> There is a slight shock/ Stray current at battery terminals when UPS is ON. Is this normal?
    The shocking is not present in 12v Unit.

    4> The Lights connected to UPS (On battery mode) Flicker a bit when there is hardly any other load on the UPS. I am using Seoul LED modules with appropriate Drivers & Heatsinks. The Flickering Vanishes when some additional load (Eg a Fan) is started on UPS. What is causing this behavior?

  2. tcmtech

    Distinguished Member

    Nov 4, 2013
    More than likely the 24 volt UPS is designed on an autotransformer winding configuration (pretty common design on small UPS units) so the DC input is connected to the AC output.

    The other issue is run time. Typically with small self-contained units, they are designed to run at their rated output only for the time it takes for their battery to go dead which is about 10 - 15 minutes.

    When modified for use with a larger external battery system or constant DC supply most won't handle running much over 1/3 to 1/2 their rated power for extended periods without burning up so keep that in mind.
    bhvm likes this.
  3. bhvm

    Thread Starter Active Member

    Jul 16, 2009
    Dear RC,
    thanks for the reply.
    Can you please explain what is this autotransformer config? How does it work? Does it harm the battery?

    I would not worry about runtimes. The Loads on this UPS is mostly in range of 75~100w (i've designed my house to be efficient) with Peak loads of only 300w.
    Also, the UPS is modified with extra fans & heatsinks. The UPS was sold to me as "EXTENDED RUNTIME" model. Probably means It can handle it. And it went through the first 24 hours just fine. Voltage in Morning is 25v. Now jumping to 27v with sun coming up.

    Can you guide a bit on Points 1,2 & 4?