[dcb]

Yes a plain old voltmeter (perhaps not enough drain there to worry about a pushbutton) can also be used in such a situation, but it won't sound an alarm. A comparator and a piezo can, so you don't have to keep a close eye on the volt gauge.

so you calibrate this for 9.5 volts and when it beeps you need to restart, or apply power to the alternator field, or take some other corrective action.