Available in red, green or blue, these little LED voltmeters are the quick and easy way to add voltage readout at various points in your project. This meter has a third wire for the measured voltage allowing for a greater range of measurement.
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Apparently referring to a meter powered by the circuit.
I use handheld or bench instruments powered by AC or battery. They can measure millivolt to 1000V.
If you were trying to measure one cell precisely, wouldn't want voltage drop at contact resistance to affect reading, or or variable power supply voltage, so use an instrument.
Avoiding contact resistance would need 4-wire not 3-wire. And it still loads the circuit.
I guess these circuit powered panel meters might be installed in a system to monitor it.
Monitoring a 12V battery, you're in the voltage range of a panel meter and accuracy isn't that important. 2-wire would be fine. If you get 3-wire, then you can move it from battery to measuring other voltages (but only relative to negative.) That could be useful to measure connections between cells.