The volt meter gauge is similar in function to a fuel pressure gauge. However, it doesn't measure the fluid in psi, it displays the electrical system's pressure levels in volts instead. All that is necessary for this gauge is to tap into a pre-existing circuit of the vehicle. The truck's fuel or electricity doesn't have to make a trip through the gauge itself. All you have to do is hook it up to a fused ignition on/off switch. You don't even have to modify the circuit that is used to recharge the battery.
More on Voltmeter Gauges - Diesel Trucks & More
The voltmeter will stand on its own: a circuit which measures the performance of the charging system. The volt meter will alert you in the event of any alternator-system failure or damage, displaying the low voltage level that continues to fall as your battery discharges. It will also shoot up from its normal zone in the event of an overcharging.