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I'm building a battery box from a 75 AH deep cycle AGM battery, I have installed a voltmeter, but for some reason it doesn't display the correct value.

I measured the volt with a multimeter directly to the battery, and it reads around 12.70V-12.72V, when connecting the voltmeter it reads 12.5V(it could also be 12.59V, it reads only 1 digit after the period, so I can't really know), I have also connected a voltmeter I have for a 12v car socket I connected, which also reads 12.5V, so I don't think the voltmeter is faulty, but somehow doesn't display the correct value.

I connected the multimeter to the wires that connects the voltmeter and it displays 12.7V, so I really don't know what else I can try, I also tried connecting it directly to the voltmeter(the wires runs through a fuse box for each component), still 12.5V.

Does anyone know what I can try or why it happens?

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    \$\begingroup\$ What's the rated accuracy of your voltmeter? What about the multimeter? \$\endgroup\$ – Hearth Oct 25 '18 at 15:22
  • \$\begingroup\$ Connect multimeter, take a reading, then connect voltmeter on the fly and see changes in multimeter reading. Do you have voltmeter model? The only idea so far that voltmeter loads the battery, and battery is not able to keep up to even tiny current flowing through voltmeter. What is the health of the battery? \$\endgroup\$ – Anonymous Oct 25 '18 at 15:22
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    \$\begingroup\$ you can often adjust voltmeters with a pot on the back. \$\endgroup\$ – dandavis Oct 25 '18 at 15:23
  • \$\begingroup\$ why it happens: one+ of the meters is inaccurate. Weight something on 10 different digital scales; you'll get some variation as well, usually Gaussian. Same with resistor values too, or basically anything analog... \$\endgroup\$ – dandavis Oct 25 '18 at 15:40
  • \$\begingroup\$ What is "correct value"? A good approximation would be if you measure the battery with a newly calibrated 6.5-digit Keithley benchtop DMM or something of this sort. If you have some junk "voltmeter" with 2-digits or even handheld DMM with 3.5 digits, you can get any number, usually with 2LSB of full scale. \$\endgroup\$ – Ale..chenski Oct 25 '18 at 16:21

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