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Discussion in 'Troubleshooting and Repair' started by JMW, Jan 28, 2016.

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  1. JMW


    Jan 30, 2012
    I have a UPS from one of the big box stores. 625 VA. The battery has failed and I was using an 100 AH battery as the source. Lasts much, much longer. At any rate I was moving it and the battery, and touched the battery terminals and received a jolt. Turns out the battery has common mode voltage of 70 VAC on both +and- terminals measured to ground. I have no continuity from either battery terminal to either side of the AC power cord (120VAC) but typically around 128 or so. (Puerto Rico power company). I have heard of this issue on some UPS when used on floating ground A/C systems (primarily military vessels). Those UPS's are marine rated as under certain conditions 180VAC can be applied to the unit it there is a partial AC failure. I see no burned components and don't see any indication of magic smoke being released.
    Thanks for any help
    Jim W
  2. Gryd3


    Jun 25, 2014
    I would call that normal unfortunately, and a good reason to not touch exposed conductors on a powered circuit.
    To be sure, we would need to see a wiring diagram. A UPS can be designed to isolate mains from the rest of the circuit or it can be designed not to...
    I should also mention that testing for continuity between the battery and AC outlet won't give you all the answers... the connection could be in a normally open state controlled by a FET, or Relay. Again, we can't know for sure until we look at it.

    (The Eaton 1500VA Units I have at home remain disconnected from mains and operate off their battery while they power up and confirm line conditions... then they connect to mains... So measuring in this instance would show an open circuit. I admit to not knowing if my UPS LV side is completely isolated from the HV side though...)
  3. JMW


    Jan 30, 2012
    Thank you. You may be correct, as for a schematic, I doubt it. An update, when in UPS mode it only generates 97VAC. Fine for my use, and the voltage appears on the battery during UPS mode as well as when connected to the mains. I will mess with it in the future.
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