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I am in the middle of diagnosing an issue where the case of my battery box (steel) is floating at a potential of 120V relative to the negative terminal of the battery (350V). I've started removing connections but in this process i've been probing a lot.. The battery is completely isolated by several layers from the box. It's modular so i've removed one of the modules and placed it 2 meteres away, on top of two wooden blocks and a towel on a bench.

How is it possible I can measure a solid voltage (In this case, roughly 10V) between a battery terminal and the battery box which is 2 meteres and multiple insulators away? I then started probing random metal surfaces in the area and all showed a voltage, what is happening here? How is the multimeter showing a solid voltage when there is no current path?

jsotola
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dos584
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  • Does this answer your question? [Measuring mains voltage with DMM and only one probe gives 100 volts!](https://electronics.stackexchange.com/questions/483928/measuring-mains-voltage-with-dmm-and-only-one-probe-gives-100-volts) – Finbarr Oct 16 '20 at 00:46
  • @Finbarr In relation to capacative coupling from mains, I don't think so because the voltage reading is DC and there is 0V AC reading. The Battery is split into roughly 35V modules so it's not at a high voltage DC either. – dos584 Oct 16 '20 at 01:50
  • Also connect a high resistance (high enough to be safe like 1 Megohm) between the two points, and see what effect that has on the voltage. If the voltage collapses to nothing (or 1V), it may just be insulation leakage. If it remains above (use your judgement but my guess is) 10V, that's a lot of leakage. Suspect insulation breakdown (salt water ingress? electrocuted and slowly carbonising ants? true story!) and nail it before it develops into a fire. –  Oct 16 '20 at 13:06

1 Answers1

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  • Voltage is a measure of how much energy a unit charge would gain/loose if it moved through the electric field between two points. Remember that 1V = 1 Joule per Coulomb.

But its just a measure of how much energy would be lost or gained. That potential exists weather or not you actually move any charges. So voltage can certainly exist without current.

  • Objects can charge up to all kinds of voltages via static electricity.

  • Nothing is ever "completely isolated". Insulators including the towel, wood blocks, the bench, and even air don't have infinite resistance. It may be very high but its not limitless.

user4574
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  • Makes sense, cheers. If this is the case, is it possible the 120V could come from the battery through the insulation? – dos584 Oct 16 '20 at 03:09
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    Possibly as long as very little current current is drawn (like pico-amps). Once you start drawing current, the voltage would quickly drop. There can also be electrical paths that go around the insulation, between the battery terminals and the measurement point. Accumulated dirt on the surfaces or condensation are examples. – user4574 Oct 16 '20 at 04:09