Why do I need a in car Voltmeter?
A voltmeter requiring just two light-gauge wires to an ignition switched source and earth will tell you lots about your charging system and give advanced warnings of trouble
Under normal circumstances it is the charging circuit that is supplying all the electrical loads, even at idle in the case of an alternator, as well as trickle-charging the battery once the cranking losses have been replaced. Ordinarily a voltmeter will show about 14v, reducing towards 12.8v as the current load goes up and gets towards the maximum capacity of the alternator. When the current load exceeds the output of the alternator the voltage will drop below 12.8v and the battery will then be supplying part of the load, and hence discharging.
This could happen for a number of reasons including having added some high-current loads but not uprated the alternator, or the alternator is failing, or it could just be some iffy connections somewhere. In all cases a voltmeter will indicate these problems - also the problem of overcharging - much sooner and clearer than an ammeter will.
Some Voltmeters are marked on the rear +/- and some are not this is bacause the are not sensitive to which terminal goes where they are just heating a strip and indicating the position on a scale.
You will then either have 2 wires in a insulated plastic bulb holder one to a sideligh powered connection and earth, or a single wire bulb and these usually fit in the casing which is metal and a earth wire goes from one mounting thumb screw to earth.
I fit a lot of in car voltmeters for the reason above and recommend everyone has one installed as it gives advanced warnings of electrical problems associated with classic cars including Minis