I guess it depends on what you are trying to monitor. Voltage @ the battery will tell you the overall condition of storage in very small increments (difficult to see on the gauge)and you would need a relay or switch to disconnect the gauge. The voltage output of the generator or alt. can be important in terms of are you adding to the electrical system; however if the voltage does not make it back to the battery then all is for not. The easy way is to watch the voltage @ the switched side of the key or ignition. Looking @ voltage going to the ignition will allow you to see the effects of the charging system. Just after start (low voltage), increase RPM and watch the charging system overcome low voltage.