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There's an Android battery meter application for enthusiasts (control freaks actually) that displays the remaining charge percentage and the current voltage. A typical phone will use a Li-Ion battery that always has its voltage in 3-4.2 volts range.

Now at some point the battery voltage is 3,789 volts which is 3789 millivolts.

Should this be displayed as "3,789 V" or as "3789 mV"?

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3 Answers

The battery state is given in volts, so I would argue that if you are going to show the voltage, you should use the same units. Additionally, the IEEE standard is to show the units of the most significant figure, so you would show it as 3,789 V or 3.789 V depending on the region.

You should be able to pick up from the regional settings which decimal separator to use.

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Well if you display it as mV, at least you don't have to worry about whether to use a comma or a point as your decimal mark! (Since I would have read that as 3 thousand volts!)

However, apart from the internationalization aspect, the question arises - At what point do you determine that V becomes more appropriate than mV?

For example why is 3789 mV ok and 9873 mv less ok.

Most handheld voltmeters I have come across have ranges like 200mV to 2000mV; 2V to 20V; 20V-200V, (see DCV section of the dial on the digital voltmeter below) and actually I would usually align with that - ie, if it's equivalent to a 4 digit number starting with a 1 then it's ok to have 1234 mV or 1.234 V, but once you tip over 2000, it's better displayed as 2.345 V rather than 2345 mV.

A similar thing happens in speech - we might typically say 'eighteen hundred metres* but less likely to say 'twenty eight hundred metres'.

So if you can skirt the issues of decimal mark confusion, I would suggest 3.789 V by virtue of convention and picking 2.0 V as that unit-changing tipping point.

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Whether a value should be expressed as volts or millivolts depends in some measure on the range of expected values. If something will very seldom read below 1.00 volts, using millivolts would seem odd; by contrast, if something would often read below 0.1 volts but seldom above 1.1, using mV consistently would seem better than using volts for all values, or switching between volts and mV. If values could range from 0.01 volts to 100 volts, switching at 1.00 would probably be best. –  supercat Jan 16 at 21:27
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This has not much to do with actual usability. The real deal behind power meters is to tell the user if there is enough power (until next charging, typically in the evening, at least in my case). Everything else is escessive. But for control freaks, well, I think using the mV will give more freaky look and feel, so I would stick to it.

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