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It may be easier (and more practical) to build an automotive voltmeter. That was my first experiment with a 3914 IC when Radio Shack first carried them decades ago. Ten outputs to ten led's calibrated in steps of 1V (overall range 7V to 16V). I plug it into a cigarette lighter socket using a plug from my junk box or Radio Shack.

Tells me running V, idle V, cranking V, charging V. Response is instant and undamped as volt level droops momentarily starting the engine. Analog or digital meters cannot match that speed.