Here's a cool-looking battery tester for your car´s battery and alternator. As the input voltage rises, the LED´s light up from left to right.
With engine off your battery should be above 12V and below 13V. If below 12.2V yellow LED´s light up, if above 12.2V green led´s light up.
With engine on, a regular alternator should keep the tension levels between 12V to 14.2V, so if above 14.2V the orange LED´s light up.
If your car has start/stop technology or a battery management system, then it's ok to be orange because your alternator may push the tension levels up to 16.3V depending on the battery health and the total load of the system.
Above 17V the red LED´s light up and you should really look into it because there may be a problem on your tension regulator.
Try to increase the amplitude or DC level of the input and see what happens. Don't be afraid to burn the circuit, it's just simulation :)
